Clusterin in renal tissue: preferential localization with the terminal complement complex and immunoglobulin deposits in glomeruli

A decline of protective apolipoprotein J and complement factor H concomitant with increase in C5a 3 months after cardiac surgery-Evidence of long-term complement perturbations

Background

Heart surgery results in complement activation with the potential for collateral end-organ damage, especially if the protective elements (complement factor H, Apolipoprotein J) are inadequate. Here, we have investigated if peri-operative stress results in an imbalance between complement activation and its protective mechanisms up to 3 months after heart surgery.

Methods

101 patients scheduled for non-emergent cardiac surgery donated blood before the procedure (t_baseline), and 24 h (t_24h ), 7 days (t_7d ) and 3 months (t_3m ) after. Complement activation was measured as a serum level of soluble activated component 5 (sC5a) and soluble terminal complement complex (sTCC). Simultaneously, protective complement factor H (CfH), and apolipoprotein J (ApoJ) were measured. Inflammatory responses were quantified using C-reactive protein (CRP) and interleukin-6 (IL-6). Details regarding anesthesia, intensive care unit (ICU) stay, pre-existing conditions, the incidence of postoperative complications, and mortality were collected from medical records.

Results

C5a declined at t_24h to rebound at t_7d and t_3m . sTCC was significantly depressed at t_24h and returned to baseline at later time points. In contrast, CfH and ApoJ were depressed at t_3m . Milieu of complement factors aligned along two longitudinal patterns:cluster#1 (C5a/sTTC continuously increasing and CfH/ApoJ preserved at t_baseline) and cluster#2 (transient sC5a/sTTC increase and progressive decline of CfH). Most patients belonged to cluster #1 at t_24h (68%), t_7d (74%) and t_3m (72%). sTCC correlated with APACHE_1h (r ² =-0.25; p < 0.031) and APACHE_24h (r ² = 0.27; p < 0.049). IL-6 correlated with C5a (r ² =-0.28; p < 0.042) and sTTC (r ² =-0.28; p < 0.015). Peri-operative administration of acetaminophen and aspirin altered the complement elements. Prolonged hospital stay correlated with elevated C5a [t (78) = 2.03; p = 0.048] and sTTC serum levels [U (73) = 2.07; p = 0.037]. Patients with stroke had a decreased serum level of C5a at t_7d and t_3m.

Conclusion

There is a significant decrease in complement protective factors 3 months after cardiac surgery, while C5a seems to be slightly elevated, suggesting that cardiac surgery affects complement milieu long into recovery.

The Most Promising Biomarkers of Allogeneic Kidney Transplant Rejection

Clinical transplantology is a constantly evolving field of medicine. Kidney transplantation has become standard clinical practice, and it has a significant impact on reducing mortality and improving the quality of life of patients. Allogenic transplantation induces an immune response, which may lead to the rejection of the transplanted organ. The gold standard for evaluating rejection of the transplanted kidney by the recipient's organism is a biopsy of this organ. However, due to the high invasiveness of this procedure, alternative diagnostic methods are being sought. Therefore, the biomarkers may play an essential predictive role in transplant rejection. A review of the most promising biomarkers for early diagnosis and prognosis prediction of allogenic kidney transplant rejection summarizes novel data on neutrophil gelatinase-associated lipocalin (NGAL), kidney injury molecule-1 (KIM-1), C-X-C motif chemokine 10 (CXCL-10), cystatin C (CysC), osteopontin (OPN), and clusterin (CLU) and analyses the dynamics of changes of the biomarkers mentioned above in kidney diseases and the mechanism of rejection of the transplanted kidney.

Deposition of the Membrane Attack Complex in Healthy and Diseased Human Kidneys

The membrane attack complex-also known as C5b-9-is the end-product of the classical, lectin, and alternative complement pathways. It is thought to play an important role in the pathogenesis of various kidney diseases by causing cellular injury and tissue inflammation, resulting in sclerosis and fibrosis. These deleterious effects are, consequently, targeted in the development of novel therapies that inhibit the formation of C5b-9, such as eculizumab. To clarify how C5b-9 contributes to kidney disease and to predict which patients benefit from such therapy, knowledge on deposition of C5b-9 in the kidney is essential. Because immunohistochemical staining of C5b-9 has not been routinely conducted and never been compared across studies, we provide a review of studies on deposition of C5b-9 in healthy and diseased human kidneys. We describe techniques to stain deposits and compare the occurrence of deposits in healthy kidneys and in a wide spectrum of kidney diseases, including hypertensive nephropathy, diabetic nephropathy, membranous nephropathy, IgA nephropathy, lupus nephritis, C3 glomerulopathy, and thrombotic microangiopathies such as the atypical hemolytic uremic syndrome, vasculitis, interstitial nephritis, acute tubular necrosis, kidney tumors, and rejection of kidney transplants. We summarize how these deposits are related with other histological lesions and clinical characteristics. We evaluate the prognostic relevance of these deposits in the light of possible treatment with complement inhibitors.

The pathogenesis of human membranous nephropathy: we are (almost) there

Primary membranous nephropathy is an autoimmune disease usually associated with antibody to phospholipase A2 receptor (anti-PLA2R). The study by Meyer-Schwesinger et al. describes the first mouse model induced using a PLA2R system to study the pathogenicity of anti-PLA2R. Hyperimmune rabbit anti-PLA2R IgG can induce a primary membranous nephropathy-like glomerulopathy with proteinuria in mice. However, to conclusively establish the pathogenicity of human anti-PLA2R will require additional studies using PLA2R and anti-PLA2R of human origin.

Urinary Clusterin Is Upregulated in Nephropathia Epidemica

Kidney insufficiency is a hallmark of nephropathia epidemica (NE). Little is known about the mechanisms of the NE kidney pathology, with current knowledge mainly based on findings in postmortem tissue. We have analyzed kidney damage biomarkers in urine collected from early- and late-phase NE using Bio-Plex kidney toxicity panels 1 and 2. To determine the disease specificity, kidney damage biomarkers were also analyzed in urine samples from patients diagnosed with gout, type 2 diabetes, systemic lupus erythematosus, and chronic kidney insufficiency. Analysis of 12 biomarkers suggests damage to the kidney proximal tubule at the onset of NE. Also, upregulation of biomarkers of inflammation and leukocyte chemotaxis were detected in NE urine. Furthermore, increased clusterin levels were found in early- and late-phase NE urine. Comparative analysis revealed that clusterin is a biomarker, upregulated in NE urine.

Primary Membranous Nephropathy

Membranous nephropathy (MN) is a unique glomerular lesion that is the most common cause of idiopathic nephrotic syndrome in nondiabetic white adults. About 80% of cases are renal limited (primary MN, PMN) and 20% are associated with other systemic diseases or exposures (secondary MN). This review focuses only on PMN. Most cases of PMN have circulating IgG4 autoantibody to the podocyte membrane antigen PLA2R (70%), biopsy evidence PLA2R staining indicating recent immunologic disease activity despite negative serum antibody levels (15%), or serum anti-THSD7A (3%-5%). The remaining 10% without demonstrable anti-PLA2R/THSd7A antibody or antigen likely have PMN probably secondary to a different, still unidentified, anti-podocyte antibody. Considerable clinical and experimental data now suggests these antibodies are pathogenic. Clinically, 80% of patients with PMN present with nephrotic syndrome and 20% with non-nephrotic proteinuria. Untreated, about one third undergo spontaneous remission, especially those with absent or low anti-PLA2R levels, one-third progress to ESRD over 10 years, and the remainder develop nonprogressive CKD. Proteinuria can persist for months after circulating anti-PLA2R/THSD7A antibody is no longer detectable (immunologic remission). All patients with PMN should be treated with supportive care from the time of diagnosis to minimize protein excretion. Patients with elevated anti-PLA2R/THSD7A levels and proteinuria >3.5 g/d at diagnosis, and those who fail to reduce proteinuria to <3.5 g after 6 months of supportive care or have complications of nephrotic syndrome, should be considered for immunosuppressive therapy. Accepted regimens include steroids/cyclophosphamide, calcineurin inhibitors, and B cell depletion. With proper management, only 10% or less will develop ESRD over the subsequent 10 years.

CD47 deficiency ameliorates autoimmune nephritis in Fas(lpr) mice by suppressing IgG autoantibody production

CD47, a self-recognition marker, plays an important role in both innate and adaptive immune responses. To explore the potential role of CD47 in activation of autoreactive T and B cells and the production of autoantibodies in autoimmune disease, especially systemic lupus erythematosus (SLE), we have generated CD47 knockout Fas(lpr) (CD47(-/-) -Fas(lpr) ) mice and examined histopathological changes in the kidneys, cumulative survival rates, proteinuria, extent of splenomegaly and autoantibodies, serum chemistry and immunological parameters. In comparison with Fas(lpr) mice, CD47(-/-) -Fas(lpr) mice exhibit a prolonged lifespan and delayed autoimmune nephritis, including glomerular cell proliferation, basement membrane thickening, acute tubular atrophy and vacuolization. CD47(-/-) -Fas(lpr) mice have lower levels of proteinuria, associated with reduced deposition of complement C3 and C1q, and IgG but not IgM in the glomeruli, compared to age-matched Fas(lpr) mice. Serum levels of antinuclear antibodies and anti-double-stranded DNA antibodies are significantly lower in CD47(-/-) -Fas(lpr) than in Fas(lpr) mice. CD47(-/-) -Fas(lpr) mice also display less pronounced splenomegaly than Fas(lpr) mice. The mechanistic studies further suggest that CD47 deficiency impairs the antigenic challenge-induced production of IgG but not IgM, and that this effect is associated with reduction of T follicular cells and impairment of germinal centre development in lymphoid tissues. In conclusion, our results demonstrate that CD47 deficiency ameliorates lupus nephritis in Fas(lpr) mice via suppression of IgG autoantibody production.

Extracellular chaperones

The maintenance of the levels and correct folding state of proteins (proteostasis) is a fundamental prerequisite for life. Life has evolved complex mechanisms to maintain proteostasis and many of these that operate inside cells are now well understood. The same cannot yet be said of corresponding processes in extracellular fluids of the human body, where inappropriate protein aggregation is known to underpin many serious diseases such as Alzheimer's disease, type II diabetes and prion diseases. Recent research has uncovered a growing family of abundant extracellular chaperones in body fluids which appear to selectively bind to exposed regions of hydrophobicity on misfolded proteins to inhibit their toxicity and prevent them from aggregating to form insoluble deposits. These extracellular chaperones are also implicated in clearing the soluble, stabilized misfolded proteins from body fluids via receptor-mediated endocytosis for subsequent lysosomal degradation. Recent work also raises the possibility that extracellular chaperones may play roles in modulating the immune response. Future work will better define the in vivo functions of extracellular chaperones in proteostasis and immunology and pave the way for the development of new treatments for serious diseases.

Extracellular Chaperones

The maintenance of the levels and correct folding state of proteins (proteostasis) is a fundamental prerequisite for life. Life has evolved complex mechanisms to maintain proteostasis and many of these that operate inside cells are now well understood. The same cannot yet be said of corresponding processes in extracellular fluids of the human body, where inappropriate protein aggregation is known to underpin many serious diseases such as Alzheimer's disease, type II diabetes and prion diseases. Recent research has uncovered a growing family of abundant extracellular chaperones in body fluids which appear to selectively bind to exposed regions of hydrophobicity on misfolded proteins to inhibit their toxicity and prevent them from aggregating to form insoluble deposits. These extracellular chaperones are also implicated in clearing the soluble, stabilized misfolded proteins from body fluids via receptor-mediated endocytosis for subsequent lysosomal degradation. Recent work also raises the possibility that extracellular chaperones may play roles in modulating the immune response. Future work will better define the in vivo functions of extracellular chaperones in proteostasis and immunology and pave the way for the development of new treatments for serious diseases.

Loss of clusterin expression worsens renal ischemia-reperfusion injury

Prevention of ischemia-reperfusion injury (IRI) is a challenge in clinical care of the patients with kidney transplants or acute kidney injury, and understanding of the intrinsic mechanisms of resistance to injury in the kidney will lead to a novel therapy. Clusterin, a secreted glycoprotein, is an antiapoptotic protein in cancer cells. Our study is to investigate the role of clusterin in renal IRI. Renal IRI in mice was induced by clamping renal vein and artery for 45 or 50 min at 32°C. Apoptosis of renal tubular epithelial cells (TECs) was determined by FACS analysis. Clusterin expression was examined by Western blot or immunohistochemistry. Here, we showed that clusterin protein was induced in TECs following IRI, and more tubules expressed clusterin in the kidneys following ischemia at higher temperatures. In human proximal TEC HKC-8 cultures, clusterin was upregulated by removal of serum and growth factors in medium and was downregulated by TNF-α-IFN-γ mixture. The levels of clusterin were positively correlated with cell survival in these conditions. Knockdown or knockout of clusterin expression enhanced the sensitivity of TECs to apoptosis. In experimental models of renal IRI, deficiency in clusterin expression worsened the injury, as indicated by a significant increase in renal tissue damage with higher levels of serum creatinine and blood urea nitrogen and by a poorer recovery from the injury in clusterin-deficient mice compared with wild-type mice. Our data indicate that the reduction of inducible expression of clusterin results in an increase in TEC apoptosis in the cultures and renders mice susceptibility to IRI, implying a protective role of clusterin in kidney injury.

Identification of human plasma proteins as major clients for the extracellular chaperone clusterin

Clusterin (CLU) is an extracellular chaperone that is likely to play an important role in protein folding quality control. This study identified three deposition disease-associated proteins as major plasma clients for clusterin by studying CLU-client complexes formed in response to physiologically relevant stress (shear stress, approximately 36 dynes/cm(2) at 37 degrees C). Analysis of plasma samples by size exclusion chromatography indicated that (i) relative to control plasma, stressed plasma contained proportionally more soluble protein species of high molecular weight, and (ii) high molecular weight species were far more abundant when proteins purified by anti-CLU immunoaffinity chromatography from stressed plasma were compared with those purified from control plasma. SDS-PAGE and Western blot analyses indicated that a variety of proteins co-purified with CLU from both stressed and control plasma; however, several proteins were uniquely present or much more abundant when plasma was stressed. These proteins were identified by mass spectrometry as ceruloplasmin, fibrinogen, and albumin. Immunodot blot analysis of size exclusion chromatography fractionated plasma suggested that CLU-client complexes generated in situ are very large and may reach >or=4 x 10(7) Da. Lastly, sandwich enzyme-linked immunosorbent assay detected complexes containing CLU and ceruloplasmin, fibrinogen, or albumin in stressed but not control plasma. We have previously proposed that CLU-client complexes serve as vehicles to dispose of damaged misfolded extracellular proteins in vivo via receptor-mediated endocytosis. A better understanding of these mechanisms is likely to ultimately lead to the identification of new therapies for extracellular protein deposition disorders.

Chapter 6: The chaperone action of Clusterin and its putative role in quality control of extracellular protein folding

The function(s) of clusterin may depend upon its topological location. A variety of intracellular "isoforms" of clusterin have been reported but further work is required to better define their identity. The secreted form of clusterin has a potent ability to inhibit both amorphous and amyloid protein aggregation. In the case of amorphous protein aggregation, clusterin forms stable, soluble high-molecular-weight complexes with misfolded client proteins. Clusterin expression is increased during many types of physiological and pathological stresses and is thought to function as an extracellular chaperone (EC). The pathology of a variety of serious human diseases is thought to arise as a consequence of the inappropriate aggregation of specific extracellular proteins (e.g., Abeta peptide in Alzheimer's disease and beta(2)-microglobulin in dialysis-related amyloidosis). We have proposed that together with other abundant ECs (e.g., haptoglobin and alpha(2)-macroglobulin), clusterin forms part of a previously unknown quality-control (QC) system for protein folding that mediates the recognition and disposal of extracellular misfolded proteins via receptor-mediated endocytosis and lysosomal degradation. Characterizing the mechanisms of this extracellular QC system will thus have major implications for our understanding of diseases of this type and may eventually lead to the development of new therapies.

Structural characterization of clusterin-chaperone client protein complexes

Clusterin (CLU) is a potent extracellular chaperone that inhibits protein aggregation and precipitation otherwise caused by physical or chemical stresses (e.g. heat, reduction). This action involves CLU forming soluble high molecular weight (HMW) complexes with the client protein. Other than their unquantified large size, the physical characteristics of these complexes were previously unknown. In this study, HMW CLU-citrate synthase (CS), HMW CLU-fibrinogen (FGN), and HMW CLU-glutathione S-transferase (GST) complexes were generated in vitro, and their structures studied using size exclusion chromatography (SEC), ELISA, SDS-PAGE, dynamic light scattering (DLS), bisANS fluorescence, and circular dichroism spectrophotometry (CD). Densitometry of Coomassie Blue-stained SDS-PAGE gels indicated that all three HMW CLU-client protein complexes had an approximate mass ratio of 1:2 (CLU:client protein). SEC indicated that all three clients formed complexes with CLU>or=4x10(7) Da; however, DLS estimated HMW CLU-FGN to have a diameter of 108.57+/-18.09 nm, while HMW CLU-CS and HMW CLU-GST were smaller with estimated diameters of 51.06+/-6.87 nm and 52.61+/-7.71 nm, respectively. Measurements of bisANS fluorescence suggest that the chaperone action of CLU involves preventing the exposure to aqueous solvent of hydrophobic regions that are normally exposed by the client protein during heat-induced unfolding. CD analysis indicated that, depending on the individual client protein, CLU may interact with a variety of intermediates on protein unfolding pathways with different amounts of native secondary structure. In vivo, soluble complexes like those studied here are likely to serve as vehicles to dispose of otherwise dangerous aggregation-prone misfolded extracellular proteins.

Clusterin expression during fetal and postnatal CNS development in mouse

Clusterin (or apolipoprotein J) is a widely distributed multifunctional glycoprotein involved in CNS plasticity and post-traumatic remodeling. Using biochemical and morphological approaches, we investigated the clusterin ontogeny in the CNS of wild-type (WT) mice and explored developmental consequences of clusterin gene knock-out in clusterin null (Clu-/-) mice. A punctiform expression of clusterin mRNA was detected through the hypothalamic region, neocortex and hippocampus at embryonic stages E14/E15. From embryonic stage E16 to the first week of the postnatal life, the vast majority of CNS neurons expressed low levels of clusterin mRNA. In contrast, a very strong hybridizing signal mainly localized in pontobulbar and spinal cord motor nuclei was observed from the end of the first postnatal week to adulthood. Astrocytes expressing clusterin mRNA were often detected through the hippocampus and neocortex in neonatal mice. Real-time polymerase chain amplification and clusterin-immunoreactivity dot-blot analyses indicated that clusterin levels paralleled mRNA expression. Comparative analyses between WT and Clu-/- mice during postnatal development showed no significant differences in brain weight, neuronal, synaptic and astrocyte markers as well myelin basic protein expression. However, quantitative estimation of large motor neuron populations in the facial nucleus revealed a significant deficit in motor cells (-16%) in Clu-/- compared with WT mice. Our data suggest that clusterin expression is already present in fetal life mainly in subcortical structures. Although the lack of this protein does not significantly alter basic aspects of the CNS development, it may have a negative impact on neuronal development in certain motor nuclei.

Presence of plasma complement regulatory proteins clusterin (Apo J) and vitronectin (S40) on circulating immune complexes (CIC)

The complement regulatory (CR) proteins clusterin and vitronectin bind to the membrane attack complex (MAC) and thus prevent cytolysis. In this report, we demonstrate the presence of both of these CR proteins on MAC bound to circulating immune complexes (CIC). We measured the amount of clusterin and vitronectin on MAC in plasma, also referred to as soluble MAC (SMAC), as well as on MAC bound to CIC (MAC-CIC), using antibody directed to polymerized C9 in systemic lupus erythematosus (SLE) patients. We observed a strong correlation among the quantities of SMAC and MAC-CIC. The amount of both clusterin and vitronectin associated with MAC-CIC was two- to threefold higher in comparison to the SMAC. Patients with high levels of clusterin and vitronectin demonstrated renal involvement. We hypothesize that these complement regulatory proteins besides regulating the insertion of MAC play other critical roles, in disease pathogenesis.

Complement and systemic lupus erythematosus

Complement is implicated in the pathogenesis of systemic lupus erythematosus (SLE) in several ways and may act as both friend and foe. Homozygous deficiency of any of the proteins of the classical pathway is causally associated with susceptibility to the development of SLE, especially deficiency of the earliest proteins of the activation pathway. However, complement is also implicated in the effector inflammatory phase of the autoimmune response that characterizes the disease. Complement proteins are deposited in inflamed tissues and, in experimental models, inhibition of C5 ameliorates disease in a murine model. As a further twist to the associations between the complement system and SLE, autoantibodies to some complement proteins, especially to C1q, develop as part of the autoantibody response. The presence of anti-C1q autoantibodies is associated with severe illness, including glomerulonephritis. In this chapter the role of the complement system in SLE is reviewed and hypotheses are advanced to explain the complex relationships between complement and lupus.

Clusterin is up-regulated in glomerular mesangial cells in complement-mediated injury

BACKGROUND

Clusterin is a soluble complement regulatory protein that binds to C5b-7 and inhibits generation of membrane attack complex, C5b-9. Glomerular deposition of clusterin has been observed in human and experimental membranous nephropathy in association with C5b-9 and immune deposits. However, it is controversial as to whether clusterin observed in glomeruli is synthesized by the resident glomerular cells or is derived from the circulation. We examined whether clusterin is expressed by resident glomerular cells exposed to complement-mediated injury.

METHODS

In vitro, cultured mesangial cells were exposed to antithymocyte serum immunoglobulin G and 5% normal rat serum as a complement source. In vivo, we induced anti-Thy1 nephritis in rats and examined the kidneys on days 8 and 29.

RESULTS

We observed increased expression of clusterin in cultured rat glomerular mesangial cells stimulated by sublytic complement attack. We also demonstrated that in comparison with control rats, both a marked increase in clusterin mRNA in the glomeruli and marked deposition of clusterin protein in the mesangial area occurred in the OX-7-treated rats on day 8 in association with C5b-9 deposition and on day 29.

CONCLUSION

Clusterin was induced in glomerular mesangial cells during the course of immune-mediated injuries. This up-regulation of clusterin may play a critical role in protecting mesangial cells from complement attack.

Spatial arrangement of subepithelial deposits in lupus and nonlupus membranous nephropathy

Subepithelial deposits are a common feature of idiopathic membranous glomerulonephritis (MGN) and lupus membranous glomerulopathy (LMGN). We investigated the spatial arrangement of immunoglobulin G (IgG) and C3c fraction of complement (C3c) in the immune deposits of MGN and LMGN with confocal laser scanning microscopy to correlate specific patterns of IgG-C3 interactions with different diseases. Ten patients with MGN and 8 patients with LMGN (World Health Organization class VB) were selected. A determination of the spatial arrangement of the two fluorochromes and the glomerular area occupied by each fluorochrome was performed for each case. Our results showed MGN specimens have an orderly distribution of IgG and C3c, with each deposit showing an outer ring of sole IgG. IgG was always more abundant than C3c (1,619 +/- 271 v 790 +/- 105 micrometer(2), P = 0.002). In LMGN, IgG and C3c were haphazardly arranged, with deposits made of C3c only and an outer ring of IgG only rarely present. Also, the relative amounts of the two antigens were variable, and two groups could be identified (group 1: IgG, 5,515 +/- 1,179 micrometer(2) v C3c, 4,810 +/- 1,174 micrometer(2); P = 0.02; group 2: IgG, 3,358 +/- 658 micrometer(2) v C3c, 4,047 +/- 740 micrometer(2); P = 0.03). Our data show that diffuse IgG capping of the subepithelial immune deposits is diagnostic of MGN. The absence of an orderly three-dimensional arrangement in LMGN deposits (ie, outer ring of IgG) is likely to render active complement components more readily available to inflammatory activities.

A reexamination of the role of clusterin as a complement regulator

Clusterin is a highly conserved glycoprotein which has been proposed to protect host cells against complement-mediated cytolysis. We tested the hypothesis that clusterin is a complement regulator using erythrocytes and cells which had been stably transfected with a membrane-anchored form of clusterin as targets for complement-mediated cytolysis. Clusterin gave dose-dependent protection of antibody-coated sheep erythrocytes against complement-mediated lysis by diluted normal human serum. There was a linear relationship between the concentration of clusterin giving 50% protection and the concentration of serum; extrapolation of this to the case of undiluted human serum showed that a clusterin concentration at least two orders of magnitude greater than its physiological plasma concentration would be needed to confer protection against complement-mediated cytolysis under physiological conditions. Physiological concentrations of clusterin did not protect rabbit erythrocytes against alternative complement pathway-mediated lysis using dilute human serum. Exogenous clusterin had no effect on lysis of human erythrocytes triggered by the addition of inulin to autologous human serum. Induction of cell-surface clusterin expression by L929 (murine fibroblast) cells which had been stably transfected with cDNA for human clusterin linked to DNA coding for the 44 C-terminal amino acid residues of CD55 did not protect the cells against complement-mediated lysis by either normal or clusterin-depleted human serum. These data suggest that clusterin may not be a physiologically relevant regulator of complement activation.

The concept of glomerular self-defense

The balance between local offense factors and defense machinery determines the fate of tissue injury: progression or resolution. In glomerular research, the most interest has been on the offensive side, for example, the roles of leukocytes, platelets, complement, cytokines, eicosanoids, and oxygen radical intermediates. There has been little focus on the defensive side, which is responsible for the attenuation and resolution of disease. The aim of this review is to address possible mechanisms of local defense that may be exerted during glomerular injury. Cytokine inhibitors, proteinase inhibitors, complement regulatory proteins, anti-inflammatory cytokines, anti-inflammatory eicosanoids, antithrombotic molecules, and extracellular matrix proteins can participate in the extracellular and/or cell surface defense. Heat shock proteins, antioxidants, protein phosphatases, and cyclin kinase inhibitors may contribute to the intracellular defense. This article outlines how the glomerulus, when faced with injurious cells or exposed to pathogenic mediators, defends itself via the intrinsic machinery that is brought into play in resident glomerular cells.

Complement regulatory proteins in glomerular diseases

Complement activation plays a critical role in the pathogenesis of many forms of glomerulonephritis. Complement activation leads to tissue injury through various mechanisms including the generation of chemotactic factors and activation of the resident glomerular cells following C5b-9 insertion. Recent advances have disclosed the mechanisms of regulation of complement activation by discovery of a number of complement regulatory proteins. Decay accelerating factor (DAF), membrane cofactor protein (MCP), and complement receptor type 1 (CR1) act by inactivating C3/C5 convertase. They belong to the gene superfamily known as the regulators of complement activation (RCA), and share a common structural motif called a short consensus repeat (SCR). In contrast, CD59 works by inhibiting formation of C5b-9. The glomerulus is particularly well endowed with these membrane-bound complement regulatory proteins. DAF, MCP, and CD59 are ubiquitously expressed by all three resident glomerular cells, while CR1 is localized exclusively in podocytes. Expression of complement regulatory proteins can be changed by many factors including complement attack itself, and their expression levels are affected in various glomerular disorders. Studies utilizing cultured glomerular cells and animal models of glomerular diseases suggest important protective roles of complement regulatory proteins against immune-mediated renal injury. Recent progress in molecular biological techniques has made new therapeutic strategy feasible. Systemic administration of soluble recombinant complement regulatory proteins and local overexpression of complement regulatory proteins are promising therapeutic approaches.

Tubular up-regulation of clusterin mRNA in murine lupus-like nephritis

Clusterin, a widely distributed glycoprotein, is detected in most tissues and in numerous physiological fluids. In the kidney, this protein is constitutively expressed in tubular epithelial cells, and its expression is enhanced following tubular injuries. In addition, clusterin has been detected in glomerular immune deposits of glomerulonephritis. The present study was designed to define the sites of clusterin mRNA accumulation in murine lupus-like nephritis in comparison with murine tubulopathies. In lupus-like nephritis, a significant increase of clusterin mRNA abundance was demonstrated. This up-regulation was localized exclusively in tubular epithelial cells exhibiting tubulointerstitial alterations, whereas no clusterin mRNA was detectable in diseased glomeruli, excluding an active synthesis of clusterin by glomerular cells. A similar tubular increase of clusterin mRNA abundance was observed in myeloma-like cast nephropathy induced by IgG3 monoclonal cryoglobulins and even in the absence of any detectable histological alterations in a model of septic shock induced by the injection of bacterial lipopolysaccharides. Our results suggest that tubular epithelial cells are the only sites of clusterin mRNA accumulation during the course of lupus-like nephritis and that the tubular up-regulation of clusterin gene expression may reflect the cellular response to various types of tubular injuries.

Biochemical analysis of the interaction of fibronectin with IgG and localization of the respective binding sites

Fibronectin (Fn), a mosaic protein composed of multiple copies of three different module types (Fl, F2 and F3), has been found associated with circulating immune complexes (ICs) and immunoglobulin (Ig) aggregates in a variety of IC diseases and myeloproliferative disorders. We have previously shown that a proteolytic fragment of Mr = 25,900 Da, from the NH2-terminal domain of Fn, composed of five type 1 modules (1Fl -5Fl) binds to the major Ig classes under physiologic conditions, suggesting that the presence of Fn in ICs and cryoglobulins results from a physicochemical binding interaction between these two molecules. Using an ELISA, we now show that the interaction between Fn and IgG is: (1) not influenced by any other constituent of plasma; (2) unaffected by temperature; and (3) has an estimated Kd of 3.77 x 10(-9) M. In addition, we have further delineated the respective sites involved in the interaction between Fn and IgG. Recombinant type l module pairs (1Fl.2Fl and 4Fl.5Fl) from the NH2-terminus of Fn, expressed in yeast, were employed in an ELISA and affinity chromatography and compared with the 25.9 kDa (1Fl - 5Fl) fragment and intact Fn for binding to IgG. The 4Fl.5Fl and the 25.9 kDa fragment bound to immobilized IgG and inhibited Fn binding to IgG to nearly the same extent as the intact molecule (IC50: Fn = 6.77 x 1O(-9) M; 25.9 kDa fragment = 5 x 10(-9) M; 4Fl.5Fl = 7.6 x 10(-9) M). Thus, the binding site for IgG on the Fn molecule is localized to and completely conferred by the 4Fl.5Fl module pair (residues 151-244). Similar experiments using papain-generated Fab and Fc fragments of IgG localized the Fn binding site on IgG to the Fe region of the IgG molecule. Fn bound to the Fc fragment with a nearly identical Kd of 3.69 x 10(-9) M, as to intact IgG (3.77 x 10(-9) M). These studies support the hypothesis that the interaction between Fn and Ig may contribute to the pathophysiology of immune complex related disorders.

Protein SIC, a novel extracellular protein of Streptococcus pyogenes interfering with complement function

The human pathogen Streptococcus pyogenes possesses a chromosomal region, the mga regulon, that contains co-regulated genes important to the virulence of these bacteria. A novel gene located in the mga regulon of a S. pyogenes strain of serotype M1 was cloned and sequenced. It translates into a protein of 305 amino acid residues, including a signal sequence of 32 amino acids and a central region consisting of three tandem repeats. The sequence represents a novel structure with no significant homology to any previously published sequence. The protein was purified from the streptococcal culture media where it is present in substantial amounts. Affinity chromatography of human plasma on Sepharose coupled with the protein specifically absorbed two plasma proteins which were identified as clusterin and histidine-rich glycoprotein (HRG). The interactions between the streptococcal protein and the plasma proteins were further characterized using purified clusterin and HRG. Inhibition experiments indicated that they have affinity for overlapping or closely located sites in the streptococcal protein. Both clusterin and HRG are regulators of the membrane attack complex (C5b-C9) of complement. When the streptococcal protein was added to serum, complement-mediated lysis of sensitized sheep erythrocytes and guinea pig erythrocytes was inhibited. In addition, the streptococcal protein was incorporated into C5b-C9 in serum, indicating the location of its action. The name, protein SIC, streptococcal inhibitor of complement-mediated lysis, is therefore suggested for this novel protein. The occurrence of protein SIC and its gene was investigated in a collection of S. pyogenes strains comprising 55 different M serotypes. Only M1 and M57 strains were positive in this screening, indicating that protein SIC could be a virulence determinant. Thus, during recent years, the M1 serotype has been connected with a world-wide increase of severe and toxic S. pyogenes infections.

Clusterin promotes the aggregation and adhesion of renal porcine epithelial cells

The function of clusterin, a heterodimeric glycoprotein markedly induced in renal and other organ injuries, is unclear. Since renal injury is accompanied by alterations in cell attachment, it is possible that clusterin functions to promote cell-cell and cell-substratum interactions. In this study, a single cell suspension of renal epithelial (LLC-PK1) cells was treated with purified human clusterin, resulting in time- and dose-dependent cell aggregation. Electron microscopy of the cell aggregates demonstrated cell junction and lumen formation. To determine the effect of clusterin on cell adhesion, tissue culture plates were coated with clusterin, fibronectin, PBS, or albumin. Clusterin and fibronectin promoted cell adhesion to the same extent. The adhesion to clusterin was dose dependent and specific, as a monoclonal antibody against clusterin inhibited cell adhesion to clusterin but not fibronectin. Perterbations of the cytoskeleton may underlie the alterations in cell attachment which occur in renal injury. Induction of clusterin mRNA was seen after disruption of both microtubules and microfilaments and after inhibition of cell-substratum interactions. In conclusion, clusterin is a potent renal epithelial cell aggregation and adhesion molecule. We speculate that clusterin functions to promote cell-cell and cell-substratum interactions which are perturbed in the setting of renal injury, thereby preserving the integrity of the renal epithelial barrier.

Spatial arrangement of IgA and C3 as a prognostic indicator of IgA nephropathy

The histological picture of primary glomerulonephritis with glomerular IgA deposition (IgA nephropathy and Henoch-Schönlein disease) can vary from minimal mesangial involvement to severe endocapillary and/or extracapillary proliferation. Local activation of the complement cascade by glomerular IgA deposits and release of anaphylactoid factor are considered to be major triggers of inflammation, but clear-cut correlations between the severity of the histological findings and the intensity of glomerular deposition of immunoglobulin and complement fractions are still lacking. The purpose of this study was to investigate the spatial distribution of IgA and complement in mesangial deposits with confocal laser scanning microscopy (CLSM) and to correlate specific patterns of IgA-complement interaction with glomerular damage. Two groups of patients have been studied, one with mild to moderate diffuse mesangial proliferation and the other with diffuse proliferative endocapillary and/or extracapillary patterns. In milder forms of the disease, the majority of the immune deposits are composed of both IgA and C3, coated by an outer layer of IgA alone. Large C3 deposits, or deposits composed of IgA and C3 without an outer coat of IgA, were associated with more severe histological lesions. The results suggest that free access of active complement components to cell and/or mesangial matrix receptors could trigger a cytolytic reaction and that immunoglobulins seem to act as a protective layer on C3 components.

Clusterin: physiologic and pathophysiologic considerations

Clusterin is a heterodimeric glycoprotein produced by a wide array of tissues and found in most biologic fluids. A number of physiologic functions have been proposed for clusterin based on its distribution and in vitro properties. These include complement regulation, lipid transport, sperm maturation, initiation of apoptosis, endocrine secretion, membrane protection, and promotion of cell interactions. A prominent and defining feature of clusterin is its induction in such disease states as glomerulonephritis, polycystic kidney disease, renal tubular injury, neurodegenerative conditions including Alzheimer's disease, atherosclerosis, and myocardial infarction. The expression of clusterin in these states is puzzling, from the specific molecular species and cellular pathways eliciting such expression, to the roles subserved by clusterin once induced. This review will discuss these physiologic and pathophysiologic aspects of clusterin and speculate on its role in disease.

The kinetics and distribution of C9 and SC5b-9 in vivo: effects of complement activation

Many diseases associated with complement activation are characterized by tissue deposition of components of the terminal complement complex (TCC). The ninth component of complement (C9) plays an important role in the cytolytic effects, and may contribute to the non-lethal cell-regulating functions of the TCC. In this study we examined the behaviour of radiolabelled human C9 and its soluble complexed form SC5b-9 in vivo in order to determine the effects of complement activation on its turnover, distribution and molecular size. In normal rabbits the metabolic parameters of 125I-C9 (median and range) were: plasma half-life (t1/2) 25.9 (20.6-29.5) h, fractional catabolic rate (FCR) 5.7 (5.3-7.0)%/h, and extravascular/intravascular ratio (EV/IV) 0.7 (0.6-1.1). The distribution of radiolabelled C9 amongst body tissues was similar to that observed for rabbit serum albumin (RSA). Activation of the complement cascade with i.v. injection of cobra venom factor (CVF) resulted in rapid disappearance of C9 from the plasma and accumulation of protein-bound radiolabeled in the spleen (exceeding the plasma concentration) and the liver. RSA metabolism and distribution were unaffected by CVF. Fine performance liquid chromatography (FPLC) gel filtration of plasma samples suggested that monomeric C9 was the only major radiolabelled protein present during normal turnovers, whereas CVF administration was accompanied by the prompt appearance of a high mol. wt species consistent in size with SC5b-9. When injected directly, 125I-SC5b-9 disappeared rapidly from the plasma, falling by 50% in 0.7 (0.6-0.8) h, and less than 15% remaining after 4 h with accumulation of protein-bound label in the spleen and liver. These results demonstrate the complexity of C9 metabolism during complement activation.

Clusterin expression and apoptosis in tissue remodeling associated with renal regeneration

To analyze the role of clusterin in renal diseases involving a regenerative process, we have used a novel rodent model to compare temporal and spatial expression of clusterin mRNA. Thus, renal artery stenosis was used to induce unilateral non-infarctive renal atrophy. After several weeks, when cellular pathology of atrophic kidneys involved minimal apoptosis or inflammatory response and mitosis was at normal levels, regeneration of atrophic kidneys was stimulated by removal of the contralateral healthy kidneys. The regrowth response was very rapid and involved renal hyperplasia rather than hypertrophy. Regenerating kidneys were studied 0, 4, 8, 24 hours and 2, 3, 5, 7, and 14 days after contralateral nephrectomy. Several parameters were compared: level and localization of clusterin mRNA; cell proliferation; cell dedifferentiation and redifferentiation and apoptosis. During the acute regenerative phase (first 24 hr) clusterin expression was markedly increased, decreasing to untraceable levels by five days of regeneration. Clusterin mRNA was localized in dilated or collapsed atrophic tubules that had lost identifying surface structures of normal tubular epithelium (termed dedifferentiated). Clusterin was also localized in the periphery of some blood vessel walls. Cell proliferation peaked at three to five days of regeneration, and was also localized in dedifferentiated tubules. Despite the regenerative stimulus, an unexpected result was a transient but marked increase in apoptotic cell death in atrophic tubules in the first 24 hours of regeneration. Our results provide evidence of a temporal association between increased clusterin expression and apoptosis, but in situ localization showed clusterin mRNA over apparently viable, as well as apoptotic, cells in the epithelium of tubules showing clusterin expression. Clusterin mRNA was rarely identified over epithelial cells in foci of non-atrophic (non-dedifferentiated) nephrons that responded to the regenerative stimulus by cellular hypertrophy. The dramatic response after initiation of regeneration, especially the initiation of apoptosis in the tubular epithelium, may have applications for the study of genetic changes leading to renal oncogenesis.

Potential role of apolipoprotein-E in fibrillogenesis

Immunohistochemical and biochemical studies have demonstrated several different proteins in amyloid deposits that are not intrinsic components of the fibril itself but may play a role in their deposition and fibril formation. We compared the distribution of several amyloid-associated proteins, ie, amyloid P component, apolipoprotein-E, apolipoprotein-J, and vitronectin, in the deposits of several different amyloids, in particular light chain amyloid, with those in the deposits of nonamyloid monoclonal immunoglobulin, which may be considered a form of preamyloid disease. Although 100% of amyloid specimens (7 amyloid A, 15 immunoglobulin light chain, and 1 transthyretin) had amyloid P component and 100% had apolipoprotein-E (2 amyloid A, 10 immunoglobulin light chain, and 1 transthyretin) co-localized with the primary amyloid protein, none of the monoclonal nonamyloid cases (14 light chain deposition disease and 6 light and heavy chain deposition disease) had amyloid P component and only 1 of 11 had apolipoprotein-E. On the other hand, staining for apolipoprotein-J and vitronectin was positive in 100% of cases of amyloid and nonamyloid monoclonal deposits. The association between the presence of apolipoprotein-E and amyloid P component in the fibrillar form of monoclonal light chain deposits and their absence in the nonfibrillar form of deposits suggest a role for these proteins in the process of fibrillogenesis. This lends support for the previously proposed concept that apolipoprotein-E functions as a pathological chaperone by altering the conformation of amyloidogenic proteins.

Expression and function of multiple regulators of complement activation in autoimmune thyroid disease

Membrane attack complexes of complement occur around thyroid follicles in Graves' disease and Hashimoto's thyroiditis. The lytic potential of such complexes is controlled by membrane-bound and fluid phase regulators and we have investigated the role of these in autoimmune thyroid disease. By immunohistochemical staining, clusterin and S-protein were found in all nine thyroid specimens from patients with Graves' disease and S-protein was found in one of two Hashimoto glands. CD46, CD55 and CD59 were found on thyroid cells in all specimens. CD46 and CD55 expression occurred on thyroid cells cultured in vitro and was increased significantly by culture with interleukin-1 (IL-1) and interferon-gamma (IFN-gamma), which are known to be released by the lymphocytic infiltrate in these conditions. Blocking CD55 had a weak and inconsistent effect on complement-mediated thyroid cell killing in vitro but, in four of five experiments, blocking CD46 enhanced killing. However, the effect of blocking CD59 was greater in all cases than blocking CD46 or CD55. Expression of these fluid phase and membrane-bound proteins may be important in determining the severity of thyroid damage produced by complement fixation in Graves' disease and Hashimoto's thyroiditis.

Clusterin depletion enhances immune glomerular injury in the isolated perfused kidney

Clusterin is a normal plasma protein, shown to be an inhibitor of reactive complement hemolysis and a component of the fluid phase SC5b-9 terminal complement complexes. It is a component of glomerular immune deposits in human and experimental glomerulonephritis. Using the complement-dependent isolated perfused rat kidney model of autologous phase passive Heymann nephritis, we have studied the effect of clusterin depletion of perfused plasma on the development of glomerular injury. Kidneys with planted glomerular sheep anti-rat Fx1A antibody were perfused with human plasma either depleted of clusterin to < or = 30%, or control plasma depleted of plasma fibronectin. Glomerular injury was then initiated by the addition of guinea pig anti-sheep immunoglobulins to the perfusate. Kidneys perfused with clusterin depleted plasma developed significantly greater proteinuria at all time points when compared to control kidneys. Glomerular antibody binding and C3 deposition were similar in the two groups, but terminal complement components were deposited in larger amounts in the clusterin depleted group. These data support a possible role for clusterin in vivo in the protection of complement-induced glomerular injury.

Expression of clusterin in human renal diseases

Clusterin, a glycoprotein with potent cohesive properties, is induced in a wide variety of acute and chronic experimental renal diseases. The purpose of this study was to examine clusterin expression in human renal diseases. Clusterin immunostaining was examined in nephrectomy specimens from patients with autosomal-dominant polycystic kidney disease (N = 5), autosomal-recessive polycystic kidney disease (N = 3), multilocular cyst of the kidney (N = 2), renal hypoplasia/dysplasia (N = 7), Wilms' tumor (nephroblastoma) (N = 6), renal cell carcinoma (N = 9), and acute and/or chronic renal transplant rejection (N = 15). No clusterin staining was detected in normal renal tissue distant from renal cell carcinomas. Increased expression of clusterin was found in epithelial cells lining cysts in all of the cystic disorders studied. Clusterin expression was found in some immature tubules in hypoplastic/dysplastic kidneys and in tubules of rejected renal allografts, but was not a prominent finding in renal neoplasms, although some renal cell carcinomas expressed clusterin in a focal manner. Common features of clusterin induction included exclusively epithelial production of clusterin in cysts, immature nephrons, and injured tubules, heterogeneity of clusterin expression, with only some tubules and/or cysts in a given area staining for clusterin, and uniform clusterin staining of epithelial cells in a given tubule or cyst in most cases. Based on its cohesive properties, we speculate clusterin functions to maintain cell-cell and cell-substratum interactions which become perturbed in the setting of renal injury and cystic diseases.

Co-deposition of clusterin with the complement membrane attack complex in myocardial infarction

Clusterin is a multi-functional plasma glycoprotein that has been shown to inhibit formation of the complement membrane attack complex (MAC) by preventing the association of terminal complement complexes with target cell membranes. Recent studies have suggested that complement activation is involved in the development of tissue injury of myocardial infarction. In this study we observed that clusterin is selectively deposited in the infarcted areas of human myocardium. Clusterin deposits were observed in the heart tissue of 10 patients whose infarcted lesions were 8 hr to 14 days old, but not in patients who died from other causes. Clusterin co-localized with the MAC on the surface of damaged cardiomyocytes. In normal myocardium only endothelial lining of blood vessels occasionally stained positive for clusterin. The 80,000 MW clusterin was also detected by Western blot analysis in extracts of myocardial infarction lesions, but only faintly in extracts of normal heart. As clusterin has apparently failed in protecting myocardium against complement-mediated cell injury its main role might be to participate in the clearance of damaged and necrotic tissue together with the MAC.

Murine clusterin: molecular cloning and mRNA localization of a gene associated with epithelial differentiation processes during embryogenesis

Clusterin is a broadly distributed glycoprotein constitutively expressed by various tissues and cell types, that has been shown to be involved in cell-cell adhesion and expressed during cellular differentiation in vitro. To assess the suggested participation of clusterin in these processes in vivo, we have cloned the cDNA encoding murine clusterin and studied the cellular distribution of clusterin mRNA during murine embryogenesis. Sequence analysis of the cDNA encoding murine clusterin revealed 92 and 75% sequence identity with the rat and human cDNAs, respectively, and conservation of the predicted structural features which include alpha-helical regions and heparin-binding domains. From 12.5 d of development onwards, the clusterin gene is widely expressed in developing epithelia, and selectively localized within the differentiating cell layers of tissues such as the developing skin, tooth, and duodenum where proliferating and differentiating compartments are readily distinguished. In addition, transient and localized clusterin gene expression was detected in certain morphogenetically active epithelia. In the lung, abundant gene transcripts were detected in cuboidal epithelial cells of the terminal lung buds during branching morphogenesis, and in the kidney, clusterin gene expression in the epithelial cells of comma and S-shaped bodies coincided with the process of polarization. Our results demonstrate the in vivo expression of the clusterin gene by differentiating epithelial cells during murine embryogenesis, and provide novel evidence suggesting that clusterin may be involved in the differentiation and morphogenesis of certain epithelia.

V-src-induced-transcription of the avian clusterin gene

We have isolated the avian gene T64 corresponding to the mammalian clusterin, on the basis of high accumulation of its template mRNA in cells infected with oncogenic retroviruses. Since the clusterin was shown to have a protective effect against the immune system, its induction by oncogenic viruses is of major biological importance. The unique, short 5 kb-long T64 genomic locus is inactive in normal quail embryo fibroblasts in primary culture whereas it shows a high transcriptional activity after transformation by the Rous sarcoma virus. The 963 bp-long 5' flanking region is sufficient to drive the transcription of the chloramphenicol acetyltransferase reporter gene in a thermodependent manner when a thermosensitive version of pp60v-src is used. Deletion and point mutation analyses of the promoter show that the v-src response requires at least two separate elements: PUR and AP-1, located respectively at positions -167 to -152 and -25 to -19 relative to the single transcription initiation site. In addition, the binding of specific nuclear factors to these responsive elements correlates with the T64 promoter activation.

Distinct sites of production and deposition of the putative cell death marker clusterin in the human thymus

Clusterin is a multifunctional protein endowed with cell-aggregating, complement-inhibitory, and lipid-binding properties. Since several studies have demonstrated highly increased clusterin gene expression in epithelial and nervous tissues regressing as a consequence of tissue involution and apoptotic cell death, clusterin is also considered as a specific marker of dying cells. To determine whether clusterin expression is also upregulated during thymocyte death occurring during the negative selection process we analyzed the cellular distribution of clusterin mRNA and protein by in situ hybridization and immunocytochemistry in the human thymus. We observed that the expression of clusterin mRNA was confined to cells present in the thymic medulla, concentrated mainly around Hassal's bodies. Immunostaining of adjacent sections with antikeratin Ab revealed that cells containing clusterin mRNA were predominantly epithelial. By contrast no clusterin mRNA was found in thymocytes by in situ hybridization and Northern blot analysis of total RNA from purified thymocyte populations. Clusterin protein colocalized with the membrane attack complex of complement and vitronectin in the center of the largest Hassal's bodies, but was not detectable by immunocytochemistry in or at the surface of epithelial cells. Our results demonstrate that clusterin gene expression does not take place in apoptotic thymocytes, and therefore that clusterin synthesis by the dying cell is probably not a prerequisite to its death. However, synthesis of clusterin by medullary epithelial cells may be related to their terminal differentiation, and, furthermore, its presence in Hassal's bodies raises the possibility that the secreted protein is involved in the disposal of cell debris resulting from thymocyte apoptosis.