C5b-9 increases albumin permeability of isolated glomeruli in vitro

MiR-130a-5p prevents angiotensin II-induced podocyte apoptosis by modulating M-type phospholipase A2 receptor

Podocyte apoptosis is considered as the important element that promotes the development and progress of membranous nephropathy (MN). Unfortunately, the underlying mechanism of podocytes apoptosis in MN remains elusive. We compared the renal expressions of miR-130a-5p and M-type phospholipase A2 receptor (PLA2R) between MN patients (n = 30) and 30 controls by qRT-PCR and western blot, respectively. The podocyte damage model in vitro was established by angiotensin II (Ang II, 100 nmol/L) exposure for 24 h. Interaction between miR-130a-5p and PLA2R was determined using dual-luciferase reporter gene assay. MN mice were induced by intravenous injection of cBSA. In this study, miR-130a-5p expression was significantly decreased both in the renal biopsy specimens from MN patients and podocyte cell line AB8/13 following stimulation of Ang II. Overexpressed miR-130a-5p in AB8/13 cells significantly attenuated the Ang II induced-apoptosis in vitro. In contrast, down-regulated miR-130a-5p induced podocyte apoptosis. PLA2R was identified as the target of miR-130a-5p in AB8/13 cells. And up-regulated or down-regulated PLA2R could obviously attenuate the effect of miR-130a-5p overexpression or knockdown on the apoptosis of AB8/13 cells. Furthermore, it was also observed that overexpressed miR-130a-5p by miR-130a-5p agomir could obviously alleviate renal injury in MN mice. In conclusion, decreased miR-130a-5p was contributed to the pathological mechanism of MN through increasing PLA2R expression, which induced podocyte apoptosis.

Antibodies to protein tyrosine phosphatase receptor type O (PTPro) increase glomerular albumin permeability (P(alb))

Glomerular capillary filtration barrier characteristics are determined in part by the slit-pore junctions of glomerular podocytes. Protein tyrosine phosphatase receptor-O (PTPro) is a transmembrane protein expressed on the apical surface of podocyte foot processes. Tyrosine phosphorylation of podocyte proteins including nephrin may control the filtration barrier. To determine whether PTPro activity is required to maintain glomerular macromolecular permeability, albumin permeability (P(alb)) was studied after incubation of glomeruli from normal animals with a series of monoclonal (mAb) and polyclonal antibodies. Reagents included mAbs to rabbit and rat PTPro and polyclonal rabbit immune IgG to rat PTPro. mAb 4C3, specific to the amino acid core of PTPro, decreased its phosphatase activity and increased P(alb) of rabbit glomeruli in a time- and concentration-dependent manner. In contrast, mAb P8E7 did not diminish phosphatase activity and did not alter P(alb). Preincubation of 4C3 with PTPro extracellular domain fusion protein blocked glomerular binding and abolished permeability activity. In parallel experiments, P(alb) of rat glomeruli was increased by two mAbs (1B4 and 1D1) or by polyclonal anti-rat PTPro. We conclude that PTPro interaction with specific antibodies acutely increases P(alb). The identity of the normal ligand for PTPro and of its substrate, as well as the mechanism by which phosphatase activity of this receptor affects the filtration barrier, remain to be determined.

Induction of passive Heymann nephritis in complement component 6-deficient PVG rats

Passive Heymann nephritis (PHN), a model of human membranous nephritis, is induced in susceptible rat strains by injection of heterologous antisera to rat renal tubular Ag extract. PHN is currently considered the archetypal complement-dependent form of nephritis, with the proteinuria resulting from sublytic glomerular epithelial cell injury induced by the complement membrane attack complex (MAC) of C5b-9. This study examined whether C6 and MAC are essential to the development of proteinuria in PHN by comparing the effect of injection of anti-Fx1A antisera into PVG rats deficient in C6 (PVG/C6(-)) and normal PVG rats (PVG/c). PVG/c and PVG/C6(-) rats developed similar levels of proteinuria at 3, 7, 14, and 28 days following injection of antisera. Isolated whole glomeruli showed similar deposition of rat Ig and C3 staining in PVG/c and PVG/C6(-) rats. C9 deposition was abundant in PVG/c but was not detected in PVG/C6(-) glomeruli, indicating C5b-9/MAC had not formed in PVG/C6(-) rats. There was also no difference in the glomerular cellular infiltrate of T cells and macrophages nor the size of glomerular basement membrane deposits measured on electron micrographs. To examine whether T cells effect injury, rats were depleted of CD8+ T cells which did not affect proteinuria in the early heterologous phase but prevented the increase in proteinuria associated with the later autologous phase. These studies showed proteinuria in PHN occurs without MAC and that other mechanisms, such as immune complex size, early complement components, CD4+ and CD8+ T cells, disrupt glomerular integrity and lead to proteinuria.

Spontaneous activation of the NF-κB signaling pathway in isolated normal glomeruli

In this report, we describe that NF-κB is spontaneously activated in isolated, normal glomeruli. Ex vivo incubation of isolated rat glomeruli triggered expression of a NF-κB-dependent gene, monocyte chemoattractant protein-1 (MCP-1), in parallel with downregulation of IκBα and IκBβ proteins and activation of the p65 NF-κB subunit. The induction of MCP-1 was also observed in mesangial cells coincubated with isolated glomeruli or exposed to media conditioned by isolated glomeruli (GCM), which was abrogated by inhibition of NF-κB. The activation of NF-κB by glomerulus-derived factors was confirmed using reporter mesangial cells that produce secreted alkaline phosphatase (SEAP) under the control of the κB enhancer element. When the reporter cells were adoptively transferred into normal glomeruli, expression of SEAP mRNA and activity of SEAP were also upregulated in the explanted glomeruli. The molecular weight of factors responsible for activation of NF-κB was >50 kDa, and TNF-α was identified as one of glomerulus-derived activators. To examine upstream events involved, we focused on MAP kinases that are spontaneously activated in explanted glomeruli. Selective suppression of ERK or p38 MAP kinase significantly attenuated activation of NF-κB in mesangial cells triggered by coculture with isolated glomeruli. Interestingly, the suppressive effects by MAP kinase inhibitors were not observed in mesangial cells treated with GCM. These data suggested that NF-κB was spontaneously activated in explanted glomeruli via autocrine/paracrine factors including TNF-α and that the production of NF-κB activators by glomeruli was, at least in part, through MAP kinase pathways.

Mechanisms of immune-deposit formation and the mediation of immune renal injury

The passive trapping of preformed immune complexes is responsible for some forms of glomerulonephritis that are associated with mesangial or subendothelial deposits. The biochemical characteristics of circulating antigens play important roles in determining the biologic activity of immune complexes in these cases. Examples of circulating immune complex diseases include the classic acute and chronic serum sickness models in rabbits, and human lupus nephritis. Immune deposits also form "in situ". In situ immune deposit formation may occur at subepithelial, subendothelial, and mesangial sites. In situ immune-complex formation has been most frequently studied in the Heymann nephritis models of membranous nephropathy with subepithelial immune deposits. While the autoantigenic target in Heymann nephritis has been identified as megalin, the pathogenic antigenic target in human membranous nephropathy had been unknown until the recent identification of neutral endopeptidase as one target. It is likely that there is no universal antigen in human membranous nephropathy. Immune complexes can damage glomerular structures by attracting circulating inflammatory cells or activating resident glomerular cells to release vasoactive substances, cytokines, and activators of coagulation. However, the principal mediator of immune complex-mediated glomerular injury is the complement system, especially C5b-9 membrane attack complex formation. C5b-9 inserts in sublytic quantities into the membranes of glomerular cells, where it produces cell activation, converting normal cells into resident inflammatory effector cells that cause injury. Excessive activation of the complement system is normally prevented by a series of circulating and cell-bound complement regulatory proteins. Genetic deficiencies or mutations of these proteins can lead to the spontaneous development of glomerular disease. The identification of specific antigens in human disease may lead to the development of fundamental therapies. Particularly promising future therapeutic approaches include selective immunosuppression and interference in complement activation and C5b-9-mediated cell injury.

Glomerulonephritis

The term glomerulonephritis encompasses a range of immune-mediated disorders that cause inflammation within the glomerulus and other compartments of the kidney. Studies with animal models have shown the crucial interaction between bone-marrow-derived inflammatory cells and cells intrinsic to the kidney that is both fundamental and unique to the pathogenesis of glomerulonephritis. The mechanisms of interaction between these cells and the mediators of their coordinated response to inflammation are being elucidated. Despite these pathophysiological advances, treatments for glomerulonephritis remain non-specific, hazardous, and only partly successful. Glomerulonephritis therefore remains a common cause of end-stage kidney failure worldwide. Molecule-specific approaches offer hope for more effective and safer treatments in the future.

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.

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 microg/mg creatinine in the WT controls (p > 0.1). After NTS, albuminuria in CD59a(-/-) and WT mice (186 +/- 154 and 183 +/- 137 microg/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 micro g/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.

Chronic progression of tubulointerstitial damage in proteinuric renal disease is mediated by complement activation: a therapeutic role for complement inhibitors?

The mechanisms by which increased urinary protein concentrations lead to nephrotoxic injury are certain to be multifactorial and involve complex interactions between numerous pathways of cellular damage mediated by both cellular and humoral pathways. These may include a major role for the podocyte in glomerular diseases leading to chronic renal failure, the loss of microvascular endothelium, the albumin-induced upregulation of renal cytokines and growth factors that promote tubulointerstitial injury by inflammation and fibrogenesis, and the role of complement-mediated tubulointerstitial injury due to proteinuria. This review will focus on the last mechanism, and emphasize recent studies implicating a primary role for activation of complement in proteinuric urine as the principal mediator of tubulointerstitial damage and progressive renal disease in various experimental animal models of nephrosis. It will be our contention that intraluminal activation of the terminal complement cascade leading to the formation of the C5b-9 membrane attack complex is the principal mediator of chronic progressive interstitial damage and progressive renal failure irrespective of the type of primary glomerular injury. This paradigm has important implications for the potential therapeutic role of complement inhibitors that are currently being developed.

Permeability factors in focal segmental glomerulosclerosis

The pathologic diagnosis of focal segmental glomerulosclerosis (FSGS) is associated with a syndrome of steroid-resistant nephrotic syndrome and progressive renal insufficiency. The incidence of FSGS has increased in recent years. Known causes of FSGS include genetic abnormalities, viral infections, decreased nephron number, and hyperperfusion/hyperfiltration. The etiology is unknown in the majority of cases. FSGS recurs after initial renal transplantation in as many as 30% to 50% of patients. Recent studies have verified the hypothesis that plasma of patients with FSGS contains a factor or factors that increase permeability of glomerular capillaries and cause proteinuria after injection into rats. Patients who experience posttransplant recurrence of FSGS and those with rapidly progressive disease exhibit this activity. Permeability activity has been verified in functional assays and defined by measurement of albumin permeability (P(alb)) or glomerular volume variation (GVV). Permeability activity is decreased by plasmapheresis or immunoadsorption and can be recovered from discarded plasma or eluate from adsorption materials. Studies from our laboratory indicate that permeability activity is carried by small, highly glycosylated, hydrophobic protein(s)/peptide(s). Normal plasma contains substances capable of blocking or inactivating the FSGS permeability factor. Pharmacologic agents including cyclosporine, indomethacin, and derivatives of Trypterigium wilfordii also block permeability activity in vitro. The observation that permeability activity can be blocked by diverse agents raises hope that specific therapy may be designed for FSGS. Future investigations will permit identification of the active FSGS permeability factor, of mechanisms that initiate and perpetuate proteinuria, and of interventions to prevent renal failure in native kidneys and recurrence of disease in renal allografts.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Induction of metallothionein-I protects glomeruli from superoxide-mediated increase in albumin permeability

Metallothioneins (MT) are low-molecular-weight, heat-stable, cysteine-rich proteins with four isoforms. MT-I and MT-II are ubiquitous and are induced by oxidative, physical, and chemical stress. MT-I is an efficient scavenger of superoxide (*O2) and hydroxyl ion (OH(-)). We have demonstrated that *O2 and hypohalous acid can cause an increase in glomerular albumin permeability (P(alb)) in vitro. The purpose of this study was to document the protective effect of MT gene product on the *O2-mediated increase in P(alb). Glomeruli from Sprague-Dawley rats in 4% BSA medium were incubated for 4 hr at 37 degrees C in duplicate tubes. Each set contained glomeruli alone or with 5 microM Cd(++), 0.3 mM Spermine-NONOate (NO donor), 0.3 mM Sulfo-NONOate (nitrous oxide donor), 0.6 mM SNP (nonspecific NO donor) and SNP + carboxy-PTIO (10 mg/ml). After incubation, one set of tubes was used to isolate total RNA for the measurement of the mRNA levels of MT-I by reverse transcriptase polymerase chain reaction (RT-PCR). Duplicate tubes were incubated for an additional 10 min with 10 nM of *O2, and P(alb) was measured using video microscopy. RT-PCR of total RNA from Cd(++) and Spermine-NONOate treated glomeruli revealed a 2-fold induction of MT-I expression at the mRNA level. *O2 caused a significant increase in P(alb) (0.8 +/- 0.06 vs. control 0.0 +/- 0.12, P < 0.05) and induction of MT-I in glomeruli by Cd(++) or by Spermine-NONOate blocked this effect (0.21 +/- 0.12 and 0.24 +/- 0.19, respectively, P < 0.05 vs. *O2). In contrast, Sulfo-NONOate and SNP did not induce mRNA for MT-I in glomeruli and did not provide protection against *O2-mediated increase in P(alb.) We conclude that MT-I gene products may play an important role in protecting the glomerular filtration barrier from the injury induced by reactive oxygen species in immune and/or nonimmune renal diseases.

Early detection of radiation-induced glomerular injury by albumin permeability assay

Renal irradiation leads predictably to glomerular vascular injury, cell lysis, matrix accumulation, sclerosis and loss of renal function. The immediate effects of renal irradiation that may be associated with glomerular pathology and proteinuria are not clear in the human disease or its rat model. We hypothesized that radiation-induced injury causes immediate and subtle alterations in glomerular physiology independent of the neurohumoral and hemodynamic regulatory mechanisms. We employed a sensitive in vitro functional assay of glomerular albumin permeability (P(alb)) to demonstrate radiation-induced damage to the glomerular filtration barrier immediately after total-body irradiation of rats. In blinded experiments, control rats were sham-treated, and experimental rats received 9.5 Gy X rays. Rats were killed humanely at 1 h to 9 weeks after irradiation and glomeruli were isolated. In parallel experiments, glomeruli were isolated from normal rats and irradiated in vitro. The change in glomerular capillary permeability due to an experimental oncotic gradient was determined using videomicroscopy and P(alb) was calculated. Results show that in vivo or in vitro irradiation of glomeruli caused an increased P(alb) at 1 h. Increased P(alb) was observed up to 3 weeks after irradiation. Glomeruli from mice irradiated with 9.5 or 19.0 Gy X rays did not show increased P(alb) at 1 h postirradiation. We conclude that glomerular protein permeability of irradiated rats increases in a dose-dependent manner immediately after irradiation and that it appears to be independent of hemodynamic or systemic influences.

Components of normal serum block the focal segmental glomerulosclerosis factor activity in vitro

BACKGROUND

Sera from some patients with focal segmental glomerulosclerosis (FSGS) increase glomerular albumin permeability (P(alb)) in vitro. The hypothesis that a component of normal serum can protect the glomerular permeability barrier was tested using sera from FSGS patients, normal individuals, and several mammalian and avian species.

METHODS

In most experiments, isolated rat glomeruli were incubated in medium containing FSGS serum known to increase P(alb) in vitro, normal serum, or both active FSGS and normal serum. In other experiments, fractions of normal serum and serum from other vertebrate species were incubated with active FSGS serum. P(alb) was calculated from glomerular capillary expansion in response to an oncotic gradient. To enrich the blocking activity, normal pooled human plasma was subjected to various biochemical manipulations.

RESULTS

Normal human serum prevented the increase in P(alb) (active FSGS sera, 0.77 +/- 0.12; active FSGS sera:normal serum, 1:1 mix, 0.06 +/- 0.30, P < 0.001). Protection diminished as the concentration of normal serum was decreased. Specific fractions of human serum, including human albumin and immunoglobulin fractions, were not protective. Blocking activity was present in 80% ammonium sulfate precipitate and certain fractions from size-exclusion chromatography of normal pooled human plasma. Normal serum from each of the vertebrate species tested also prevented the increase in P(alb). Preincubation with normal serum was protective during subsequent incubation with FSGS serum, but normal serum was not protective after preincubation with FSGS serum.

CONCLUSIONS

We conclude that a factor or factors in normal serum block the permeability effect of active FSGS sera. This phenomenon may account for variability in proteinuria among patients with FSGS and may explain inconsistent proteinuria following injection of FSGS sera into experimental animals. Characterization of the protective substance(s) and the mechanism by which the increase in permeability is blocked may provide insight into the pathogenesis of FSGS.

"The FSGS factor:" enrichment and in vivo effect of activity from focal segmental glomerulosclerosis plasma

A circulating causative factor has been postulated in focal segmental glomerulosclerosis (FSGS). It has been shown that serum or plasma from some FSGS increases glomerular albumin permeability (Palb) in vitro. Palb greater than 0.5 (i.e., FS activity) is associated with recurrence after transplantation. Specimens from 15 FSGS patients were studied to document the presence of a permeability factor, to isolate this factor, to characterize its biochemical properties, and to show its effect in vivo. Total lipids were extracted by chloroform/methanol (2: 1); FS activity was absent from total lipid extract. Chylomicrons and lipoproteins were removed from the plasma with dextran sulfate, followed by sequential precipitation of proteins at 50 and 70% ammonium sulfate saturation. FS activity was retained in the 70% ammonium sulfate supernatant and exhibited a 100-fold purification. FS activity was lost after heating at 100 degrees C for 10 min or after protease digestion. Under nondenaturing conditions, electrophoresis of the FSGS 70% supernatant showed a prominent low molecular weight band that was not evident in the 70% supernatant from normal plasma. Dialysis and centrifugation-based membrane ultrafiltration of the FSGS factor indicated a molecular size between 30 and 50 kD. Injection of the 70% FSGS supernatant into rats caused a threefold increase in urine protein in collections from 6 to 24 h after injection. No increase in proteinuria occurred in rats injected with 70% supernatant from normal individuals. It is concluded that the FSGS factor is a low molecular weight protein with the potential to increase Palb in vitro and to cause proteinuria in vivo.

Spontaneous apoptosis of podocytes in explanted glomeruli. Technical note

Despite the extensive use of isolated glomeruli for kidney research, little is understood about whether and how isolation and explantation affect the structure and function of the glomerulus. In this report, we investigate the incidence of apoptosis in explanted, normal rat glomeruli. By using ladder detection assay, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling and histological analysis, the present study provides evidence that significant number of resident cells, especially podocytes, spontaneously undergo apoptosis immediately after explantation of glomeruli.

Cultured human glomerular mesangial cells express the C5a receptor

BACKGROUND

While the association of complement activation and glomerulonephritis has been recognized for decades, the pathogenic mechanisms of complement-mediated glomerular damage are incompletely understood. Expression of the C5a receptor in the kidney suggests that C5a could play a direct role in initiating or promoting glomerulonephritis.

METHODS

Expression of the C5a receptor by cultured human glomerular mesangial cells (HGMC) was examined by immunofluorescence, by FACS analysis and by reverse transcriptase-polymerase chain reaction (RT-PCR). Potential mitogenic effects were examined by analysis of neutral red dye uptake after treatment with recombinant C5a (rC5a). The production of cytokines [interleukin-1 (IL-1), interleukin-8 (IL-8), and monocyte chemoattractant protein-1 (MCP-1)] and growth factors [transforming growth factor-beta (TGF-beta) and platelet-derived growth factor (PDGF-AB)] by mesangial cells stimulated with rC5a was examined by ELISA of cell culture supernatants.

RESULTS

Expression of the C5a receptor by the cultured HGMC was demonstrated by both immunofluorescence and FACS. The presence of mRNA encoding the receptor was confirmed by RT-PCR. Treatment of HGMC in vitro with rC5a resulted in mild cellular proliferation. No IL-1 was detected despite stimulation with up to 100 nM rC5a. Concentrations of IL-8 and TGF-beta did not increase beyond basal levels in control samples at any level of stimulation. Mean MCP-1 concentrations and PDGF-AB concentrations increased by 40% and 70% above control values 48 hours post-stimulation (P = 0.01 and P = 0.003, respectively).

CONCLUSIONS

These data indicate that the C5a receptor is expressed on HGMC in vitro, and may play a role in mediating glomerular injury by promoting cellular proliferation and the production of cytokines and growth factors.

Renal microvascular injury induced by antibody to glomerular endothelial cells is mediated by C5b-9

We have recently developed a model of thrombotic microangiopathy with injury to the glomerular endothelial cell (GEN) induced by heterologous antibody to rat GEN. In addition to GEN injury rats developed glomerular platelet aggregation and fibrin deposition, acute renal failure, and acute tubular necrosis with interstitial inflammation. To study the role of complement in mediating this lesion, we induced the disease in normal complement PVG rats and measured the effects of generalized complement depletion with cobra venom factor (CVF) and of selective C6 deficiency using genetically C6 deficient PVG animals. Complement sufficient rats developed severe endothelial injury accompanied by platelet aggregation, fibrin deposition, decrease in endothelial cells assessed by antibody staining in the glomerulus, and macrophage infiltration. These changes were associated with marked reduction in renal function. These features were either absent or markedly diminished in complement depleted or C6 deficient rats. This demonstrates that C5b-9, the terminal product of activation of the complement cascade, plays an important role in the pathogenesis of this immune renal microvascular endothelial injury model. Thus, the complement system may play a pathogenic role in renal microvascular diseases such as thrombotic microangiopathy.

Cyclosporine protects glomeruli from FSGS factor via an increase in glomerular cAMP

Cyclosporine (CsA) administration to patients with recurrent focal segmental glomerulosclerosis (FSGS) after transplantation results in remission of proteinuria. We have shown that sera from patients with recurrent FSGS can increase the glomerular albumin permeability (Palbumin) and that increase in glomerular cAMP levels can alter the permeability characteristics of glomeruli in vitro. The purpose of this study was to determine if the increased glomerular levels of cAMP were related to the protective effects of CsA on an increase in Palbumin by FSGS sera. Glomeruli from Sprague-Dawley rats following intraperitoneal administration of CsA (25 mg/kg/day), cremophore (25 mg/kg/day), or saline for 5 days were incubated with 1:50 dilution of serum from three FSGS patients or with pooled normal human serum prior to calculation of Palbumin. Glomerular cAMP was measured by radioimmunoassay. Glomerular ultrastructural changes were assessed by transmission electron microscopy (TEM). Serum from three FSGS patients markedly increased Palbumin of glomeruli from saline or cremophore treated rats (saline, 0.68+/-0.08; 0.72+/-0.07; 0.70+/-0.07; and cremophore, 0.79+/-0.05; 0.81+/-0.02; 0.79+/-0.01; n=25 glomeruli in each group). In contrast Palbumin of glomeruli from CsA treated rats was not increased by any of the three FSGS sera tested (0.03+/-0.02; 0.04+/-0.05; 0.02+/-0.07, n=25 glomeruli in each group). Glomerular cAMP (pmol/mg of protein) increased 5 fold in CsA treated rats (328+/-26; 5 rats) compared with cremophore or saline treated rats (87+/-24 and 65+/-23, P<0.01; 5 rats in each group). The glomerular basement membrane appeared to be thickened and the lamina densa had an irregular appearance after treatment with CsA. No ultrastructural changes of glomerular epithelial or endothelial cells were evident. We conclude that CsA may have a direct protective effect on the glomerular filtration barrier in FSGS. We postulate that increased levels of glomerular cAMP by CsA may play an important role in protecting the glomerular Palbumin effect of the FSGS factor and may contribute to remission of proteinuria in FSGS patients.

Beneficial effects of systemic immunoglobulin in experimental membranous nephropathy

To test the hypothesis that systemic administration of immunoglobulin might reduce glomerular injury in membranous nephropathy through mechanisms involving inhibition of complement activation, we studied the passive Heymann nephritis (PHN) model of membranous nephropathy in rats. The daily administration of immunoglobulin goat IgG (600 mg/kg i.p.) reduced proteinuria by 52%. Quantitative immunohistochemical analysis showed that the glomerular deposition of C3c, an indicator of ongoing complement attack, and of C5b-9 was significantly decreased in the immunoglobulin treated group, while deposition of anti-Fx1A was not affected. Electron microscopic analysis demonstrated that the extent of subepithelial immune complexes did not appreciably differ between treated and control animals. Systemic complement levels were not altered by immunoglobulin treatment. These data suggest that the reduction in proteinuria that resulted from systemic immunoglobulin administration was mediated by modifying the effect of complement induced glomerular injury. This interpretation was further supported by in vitro data that documented a significant reduction in C5b-9 induced glomerular epithelial cell lysis in the presence of both goat and rat IgG. These results indicate that systemic administration of immunoglobulin can substantially reduce ongoing complement activation in the glomerulus in PHN rats and that this effect is associated with a significant reduction in glomerular injury.

Transfected CD59 protects mesangial cells from injury induced by antibody and complement

CD59 is a complement regulatory protein on the glomerular cells that inhibits C5b-9 assembly and insertion. We employed an overexpression strategy to determine the functional significance of CD59 in mesangial cells. We made a CD59 expression vector tagged with FLAG utilizing site-directed mutagenesis and PCR, which allows transfected CD59 to be distinguished from the constitutively expressed protein. In stable clones, overexpressed CD59 was clearly detected immunocytochemically both by anti-FLAG and anti-CD59 antibody in a granular pattern. The overexpression of CD59 was also confirmed by Western blotting. To determine if overexpression of CD59 by mesangial cells protected these cells from C5b-9 attack, we performed complement-mediated cell lysis assays. CD59-transfected mesangial cells demonstrated marked resistance to complement-mediated cell lysis which was reversed in the presence of antibody to CD59. We also investigated the role of CD59 in protecting cells from the effects of membrane insertion of sublytic quantities of C5b-9. Overexpressed CD59 suppressed production of superoxide, one of the inflammatory mediators induced by sublytic C5b-9 attack. These results demonstrate directly that transfected CD59 functions as a potent protector of mesangial cells against both lytic and sublytic attack by C5b-9. CD59 may be an important regulator of complement-mediated disease in the glomerular mesangium.

Circulating factor associated with increased glomerular permeability to albumin in recurrent focal segmental glomerulosclerosis

BACKGROUND

Heavy proteinuria and progressive renal injury recur after transplantation in up to 40 percent of patients with renal failure caused by idiopathic focal segmental glomerulosclerosis. A circulating factor may be responsible for this recurrence.

METHODS

To determine whether patients with focal segmental glomerulosclerosis have a circulating factor capable of causing glomerular injury, we tested serum samples from 100 patients with the disorder in an in vitro assay of glomerular permeability to albumin. Of the 56 patients who had undergone renal transplantation, 33 had recurrences. Sixty-four patients, many of whom had undergone transplantation, were being treated with dialysis. Thirty-one patients with other renal diseases and nine normal subjects were also studied.

RESULTS

The 33 patients with recurrent focal segmental glomerulosclerosis after transplantation had a higher mean (+/-SE) value for permeability to albumin (0.47+/-0.06) than the normal subjects (0.06+/-0.07) or the patients who did not have recurrences (0.14+/-0.06). After plasmapheresis in six patients with recurrences, the permeability was reduced (from 0.79+/-0.06 to 0.10+/-0.05, P = 0.008), and proteinuria was significantly decreased. Patients with corticosteroid-sensitive nephrotic syndrome or with membranous nephropathy after transplantation had low levels of serum activity. The circulating factor bound to protein A and hydrophobic-interaction columns and had an apparent molecular mass of about 50 kd.

CONCLUSIONS

A circulating factor found in some patients with focal segmental glomerulosclerosis is associated with recurrent disease after renal transplantation and may be responsible for initiating the renal injury.