Glomerular basement membrane degradation by endogenous cysteine proteinases in isolated rat glomeruli

Behind every smile there's teeth: Cathepsin B's function in health and disease with a kidney view

Cathepsin B (CatB) is a very abundant lysosomal protease with endo- and carboxydipeptidase activities and even ligase features. In this review, we will provide a general characterization of CatB and describe structure, structure-derived properties and location-dependent proteolytic actions. We depict CatB action within lysosome and its important roles in lysosomal biogenesis, lysosomal homeostasis and autophagy rendering this protease a key player in orchestrating lysosomal functions. Lysosomal leakage and subsequent escape of CatB into the cytosol lead to harmful actions, e.g. the role in activating the NLPR3 inflammasome, affecting immune responses and cell death. The second focus of this review addresses CatB functions in the kidney, i.e. the glomerulus, the proximal tubule and collecting duct with strong emphasis of its role in pathology of the respective segment. Finally, observations regarding CatB functions that need to be considered in cell culture will be discussed. In conclusion, CatB a physiologically important molecule may, upon aberrant expression in different cellular context, become a harmful player effectively showing its teeth behind its smile.

Proteomic Analysis Identifies Distinct Glomerular Extracellular Matrix in Collapsing Focal Segmental Glomerulosclerosis

BACKGROUND

The mechanisms leading to extracellular matrix (ECM) replacement of areas of glomerular capillaries in histologic variants of FSGS are unknown. This study used proteomics to test the hypothesis that glomerular ECM composition in collapsing FSGS (cFSGS) differs from that of other variants.

METHODS

ECM proteins in glomeruli from biopsy specimens of patients with FSGS not otherwise specified (FSGS-NOS) or cFSGS and from normal controls were distinguished and quantified using mass spectrometry, verified and localized using immunohistochemistry (IHC) and confocal microscopy, and assessed for gene expression. The analysis also quantified urinary excretion of ECM proteins and peptides.

RESULTS

Of 58 ECM proteins that differed in abundance between cFSGS and FSGS-NOS, 41 were more abundant in cFSGS and 17 in FSGS-NOS. IHC showed that glomerular tuft staining for cathepsin B, cathepsin C, and annexin A3 in cFSGS was significantly greater than in other FSGS variants, in minimal change disease, or in membranous nephropathy. Annexin A3 colocalized with cathepsin B and C, claudin-1, phosphorylated ERK1/2, and CD44, but not with synaptopodin, in parietal epithelial cells (PECs) infiltrating cFSGS glomeruli. Transcripts for cathepsins B and C were increased in FSGS glomeruli compared with normal controls, and urinary excretion of both cathepsins was significantly greater in cFSGS compared with FSGS-NOS. Urinary excretion of ECM-derived peptides was enhanced in cFSGS, although in silico analysis did not identify enhanced excretion of peptides derived from cathepsin B or C.

CONCLUSIONS

ECM differences suggest that glomerular sclerosis in cFSGS differs from that in other FSGS variants. Infiltration of activated PECs may disrupt ECM remodeling in cFSGS. These cells and their cathepsins may be therapeutic targets.

Role of cathepsin L in idiopathic nephrotic syndrome in children

Nephrotic syndrome (NS) is one of the most common glomerular diseases in children. Glomerular podocyte dysfunction can result in proteinuria, the presence of a large amount of protein in the urine. Podocytes are unique epithelial cells that divide into 3 separate structural and functional segments: a cell body, major processes, and foot processes. Since synaptopodin, dynamin, and actin are crucial components of the podocyte cytoskeleton, degradation of these proteins is associated with cytoskeleton instability, resulting in the development of proteinuria. Cathepsin L (CatL), a cysteine proteinase, plays a crucial role in various renal diseases. CatL expression is elevated in rats with puromycin aminonucleoside-induced nephropathy, which is used as a model of minimal change NS. In CatL-deficient mice, which do not develop proteinuria, dynamin is retained through the escape of CatL-mediated decomposition, resulting in no changes in the filtration barrier of podocytes. However, there is limited information on the roles of CatL in NS. Based on these data, CatL might play an important role in the development of proteinuria. Furthermore, identifying the functions of CatL may contribute to a better understanding of the pathogenesis of childhood-onset NS. We hypothesize that high levels of CatL can lead to cytoskeletal instability of podocytes, resulting in proteinuria in childhood-onset NS.

Expression of Cathepsin L and Its Intrinsic Inhibitors in Glomeruli of Rats With Puromycin Aminonucleoside Nephrosis

Cathepsin L, a lysosomal cysteine proteinase, may have a key role in various biological and disease processes by intracellular and extracellular degradation of proteins. We examined the levels of cathepsin L and its intrinsic inhibitors in glomeruli of rats with puromycin aminonucleoside (PAN) nephrosis. In contrast to the weak levels of cathepsin L in normal glomeruli, on days 4 and 8, strong immunostaining was detected in almost all podocytes when proteinuria and pathological changes of the podocytes developed. Cathepsin L was reduced after day 28, but remained in a focal and segmental manner. Cystatin β, an intracellular inhibitor, was not detected in podocytes. However, cystatin C, an extracellular inhibitor, was detected in podocytes after day 4, coincident with cathepsin L. Cystatin C levels were gradually reduced but sustained in many podocytes on day 28, while cystatin C was not detected in podocytes sustained cathepsin L. These results demonstrated that cathepsin L levels are not always accompanied by the levels of its inhibitors in podocytes of PAN nephrosis, suggesting a potential role of cathepsin L in podocyte injury, which is a critical process for the development and progression of tuft adhesion and sclerosis.

Cyclosporine A induced histological changes of Cathepsin L and CD2AP expression in renal glomeruli and tubules

BACKGROUND

Cyclosporine A (CsA) is used globally as an immunosuppressant for the treatment of immune-mediated nephrotic syndrome (NS). However, its long-term use causes nephrotoxicity characterized by tubulointerstitial injury and glomerulosclerosis. The present study aimed to investigate the associations between histomorphological findings and immunohistological expression of Cathepsin L (CatL) and CD2-associated protein (CD2AP) in patients with NS mediated with CsA.

METHODS

A total of 18 patients with child-onset NS were divided into two groups after treatment with CsA for 2 years (group A; n = 10) and more than 4 years (group B; n = 8), respectively. Analyses of relationships between tubulointerstitial disorders and expression of CatL and CD2AP proteins were performed using immunohistochemistry of paired renal specimens.

RESULTS

Glomeruli with arteriole hyalinization were significantly increased in both groups depending on dosage periods, although degrees of tubule and interstitial injury did not differ between groups. CD2AP expression was significantly greater in podocytes (P = 0.046) and was significantly less in proximal tubule cells (P = 0.014) in patients of group B compared with those of group A. Moreover, CD2AP expression was significantly increased in lateral tubule cells in both groups (group A, P = 0.02; group B, P = 0.001), and CatL expression in glomeruli and tubule cells did not change with the duration of CsA treatment in either patient group.

CONCLUSIONS

CD2AP expression in renal tubules may histologically associate with tissue hypoxia and reflected recovery from CsA-mediated renal injury in patients, even with mild histological features of tubulointerstitial disorder.

Enzymatic disease of the podocyte

Proteinuria is an early sign of kidney disease and has gained increasing attention over the past decade because of its close association with cardio-vascular and renal morbidity and mortality. Podocytes have emerged as the cell type that is critical in maintaining proper functioning of the kidney filter. A few genes have been identified that explain genetic glomerular failure and recent insights shed light on the pathogenesis of acquired proteinuric diseases. This review highlights the unique role of the cysteine protease cathepsin L as a regulatory rather than a digestive protease and its action on podocyte structure and function. We provide arguments why many glomerular diseases can be regarded as podocyte enzymatic disorders.

Identification of proteins that interact with podocin using the yeast 2-hybrid system

PURPOSE

As a membrane protein at the insertion site of the slit diaphragm (SD) complex in podocyte foot processes, podocin has been reported to act as a scaffolding protein required to maintain or regulate the structural integrity of the SD. In order to identify proteins that associate or interact with podocin, we screened a mouse kidney complementary DNA (cDNA) library using a yeast 2-hybrid system.

MATERIALS AND METHODS

1) The full-length cDNA of podocin from the mouse kidney was amplified by Polymerase Chain Reaction (PCR), 2) The PCR product was cloned into a pGBKT7 vector, pGBKT7-podocin, 3) After the pGBKT7-podocin was transformed into AH109, the AH109/pGBKT7-podocin product was obtained, 4) The mouse kidney cDNA library was transformed into the AH109/pGBKT7-podocin and screened by selection steps, 5) Next, twelve clones were cultured and isolated, 6) The yeast-purified plasmids were transformed into Escherichia coli (E. coli) by heat shock, and 7) To identify the activation domain (AD)/library inserts, we digested them with Him III, and the fragments were then sequenced.

RESULTS

12 positive clones that interacted with podocin were obtained by screening a mouse kidney cDNA library using pGBKT7-podocin. Among them, only 4 clones were found to function at the podocyte where podocin is present.

CONCLUSION

Additional studies are needed to clarify the role and interaction with podocin and candidates.

Selective modulation of the secretion of proteinases and their inhibitors by growth factors in cultured differentiated podocytes

Selective modulation of the secretion of proteinases and their inhibitors by growth factors in cultured differentiated podocytes.

BACKGROUND

Podocyte damage is considered to be an important factor in the development of glomerulosclerosis. Morphological studies on experimental models of progressive glomerular disease have identified the detachment of podocytes from the glomerular basement membrane (GBM) as a critical step in the development and progression of glomerulosclerosis. Degradation of the GBM by proteinases also might be a potential mechanism of the detachment because the process impairs the connection between podocytes and the GBM. The present study examined the effects of basic fibroblast growth factor (bFGF), transforming growth factor-beta1 (TGF-beta1) and platelet-derived growth factor (PDGF) on the secretion of proteinases [cathepsin L and matrix metalloproteinases (MMPs)] and their inhibitors [cystatin C and tissue inhibitor of metalloproteinase-2 (TIMP-2)] from differentiated podocytes in culture.

METHODS

Expression of mRNAs for receptors of growth factors (bFGF, PDGF, TGF-beta1), the proteinases and their inhibitors in differentiated podocytes were shown by RT-PCR. The secretion of cathepsin L, cystatin C and TIMP-2 from differentiated podocytes were shown by immunoblot analysis. The activities of MMPs-2 and -9 from differentiated podocytes were shown by gelatin zymography.

RESULTS

Expression of mRNAs for receptors of the growth factors, the proteinases and their inhibitors were confirmed. bFGF increased the secretion of cathepsin L (5.04-fold at 20 ng/mL), but did not alter the secretion of its extracellular inhibitor, cystatin C. In contrast, TGF-beta1 increased the activities of MMPs-2 and -9 (3.23-fold at 10 ng/mL and 25.3-fold at 10 ng/mL, respectively) from differentiated podocytes, but did not enhance the secretion of its inhibitor, TIMP-2. In addition, bFGF enhanced the secretion of TIMP-2 (2.75-fold at 20 ng/mL) and TGF-beta1 enhanced the secretion of cystatin C (2.32-fold at 20 ng/mL). These results demonstrate the imbalance of the secretion of proteinases and their inhibitors after incubation of such growth factors. Of particular interest was the observation of differences in regulation of proteinases and their extracellular inhibitors in response to bFGF and TGF-beta1. PDGF only slightly increased the secretion of cathepsin L (2.54-fold at 20 ng/mL) but exerted no effect on the secretion of cystatin C, MMPs, and TIMP-2 from differentiated podocytes.

CONCLUSION

These results indicate, to our knowledge for the first time, that in differentiated podocytes, both cathepsin L and its inhibitor are independently regulated by different growth factors. It appears that increases in proteolytic activities may induce degradation of the glomerular basement membrane (GBM), which plays an important role in the progression of glomerulosclerosis.

Interleukin-4 and -13 promote basolateral secretion of H(+) and cathepsin L by glomerular epithelial cells

Minimal change nephrosis (MCN) is characterized by massive proteinuria and ultrastructural alterations of glomerular visceral epithelial cells (GVEC). MCN has been associated with elevated production of interleukin (IL)-13 by circulating T lymphocytes and with T helper 2 lymphocyte-dependent conditions. We recently showed that GVEC express IL-4 and IL-13 receptors and that IL-4 and IL-13 increase transcellular ion transport over GVEC monolayers. We therefore hypothesized that IL-13 may directly injure GVEC. Here we demonstrate that IL-4 and IL-13 induce bafilomycin A1-sensitive basolateral proton secretion by cultured GVEC, indicating involvement of vacuolar H(+)-ATPase. The effects of IL-4 and IL-13 were accompanied by redistribution of the small GTPases Rab5b and Rab7, as shown by confocal immunofluorescence studies. Furthermore, Western blot analysis and assays for cysteine proteinase activity revealed basolateral secretion of the lysosomal proteinase procathepsin L by cultured GVEC, stimulated by IL-4 and IL-13. We speculate that IL-4 and IL-13 influence intracellular trafficking of proteins and promote proteolysis at the basolateral surface of GVEC, which may play a pathogenic role in altered glomerular permeability.

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.

Isoform switching of type IV collagen is developmentally arrested in X-linked Alport syndrome leading to increased susceptibility of renal basement membranes to endoproteolysis

Normal glomerular capillaries filter plasma through a basement membrane (GBM) rich in alpha3(IV), alpha4(IV), and alpha5(IV) chains of type IV collagen. We now show that these latter isoforms are absent biochemically from the glomeruli in patients with X-linked Alport syndrome (XAS). Their GBM instead retain a fetal distribution of alpha1(IV) and alpha2(IV) isoforms because they fail to developmentally switch their alpha-chain use. The anomalous persistence of these fetal isoforms of type IV collagen in the GBM in XAS also confers an unexpected increase in susceptibility to proteolytic attack by collagenases and cathepsins. The incorporation of cysteine-rich alpha3(IV), alpha4(IV), and alpha5(IV) chains into specialized basement membranes like the GBM may have normally evolved to protectively enhance their resistance to proteolytic degradation at the site of glomerular filtration. The relative absence of these potentially protective collagen IV isoforms in GBM from XAS may explain the progressive basement membrane splitting and increased damage as these kidneys deteriorate.

Presence of cerium-cytochemical reactions of glomerular phosphatases of normal gerbil Meriones crassus: an ultrastructural localization study

Phosphatase cytochemical activity in the normal glomerulus of the desert gerbil Meriones crassus was demonstrated using cerium ions as capturing agents. Three major enzymes have been recognized: sodium-potassium adenosine triphosphatase (Na(+)-K(+)-ATPase), alkaline phosphatase (ALPase) and acid phosphatase (ACPase). However, cytochemical staining for these markers to map their localizations and distributions reveal a high positivity of Na(+)-K(+)-ATPase. This appeared as uniform dense precipitates surrounding the glomerular basement membrane (GBM) and the plasma membranes of the epithelial and endothelial cells of the glomerular layers. Negligible ALKase reaction product being over the glomerular epithelia including the GBM. In contrast, the cytochemical profiles of ACPase was unusual, with dense reaction products extensively covering the endoplasmic reticulum at the region of Golgi apparatus products lysosomes (GERL) complex, including its cisternal and tubular elements and the lysosomal-vacuolar apparatus of the glomerular epithelial cells. All other subcellular organelles showed no activity. For Na(+)-K(+)-ATPase, the reaction product was successive when acetate buffer (as decalcifying agent, pH 5.0) was used. This reaction was still seen when a medium containing levamisole was used. Cytochemical controls for all enzymes were incubated in substrate-free media including those using levamisole as an inhibitor of ALPase. The data presented, which is reported for the first time, is not an attempt to determine the contribution of the selected phosphatases in the glomerular physiology and pathology. Such findings may, nevertheless, have functional implications in the fact that these markers may be involved in the ultrafiltration and other metabolic activities of the glomerulus at the molecular and/or cellular level. In addition to earlier morphological and recent histochemical work, the present study updates and recognizes information to be used as a baseline to which the gerbil model can now be employed to investigate the behavioural adaptations of the desert rodents.

Alterations of cathepsins B, H and L in proximal tubules from polycystic kidneys of the Han:SPRD rat

Abnormalities of tubular matrix metalloproteinases have been shown recently to occur early in the course of polycystic kidney disease (PKD). The present study was conducted to determine whether lysosomal cysteine proteinases were altered in proximal tubules from 2-month-old, heterozygous Han:SPRD rats. The activities of cathepsins B (-45%), H (-39%) and L (-37%) were significantly lower in proximal tubules from PKD rats as compared to healthy offspring. Enzyme proteins were also decreased (cath. B, 2.4 +/- 0.7-fold; cath. H, 1.9 +/- 0.6-fold; N = 4, P < 0.05), while mRNA levels for cathepsins B, H and L were not different. Tubular cystatin C, a major inhibitor of cathepsins, was normal with regard to protein and mRNA levels in PKD animals. The decrease in cathepsins in PKD was specific for tubules, as enzyme activities in glomeruli and liver tissue were unchanged and limited to the lysosomal compartment, since marker enzymes for cytoplasm, endoplasmatic reticulum and mitochondria were all normal. Intralysosomally, soluble enzymes like cathepsins and beta-NAG were decreased, while membrane-bound acid phosphatase was unchanged. The presence of cathepsins could be demonstrated in cyst fluid from homozygous PKD rats and urinary excretion of cathepsins was enhanced in heterozygous animals. Taken together, these findings indicate that the reduction in tubular cathepsins B, H and L was neither due to decreased gene expression nor to upregulation of specific inhibitors, but was likely due to enhanced apical secretion of these enzymes.

The aging rat kidney displays low glomerular and tubular proteinase activities

The present study was conducted to investigate the relationship of age-related changes in renal function and structure with changes in glomerular and tubular proteinase activities in young (3 weeks), mature (3 months), and older (18 months) male Wistar rats. Glomerular filtration rate, expressed per 100 g body weight, remained unchanged during adolescence, but declined significantly (-44%) in aging animals. In parallel, albuminuria, which was barely detectable in young and mature rats, increased almost 10-fold in the aging animals. In comparison to young animals, the kidney weight in aging rats was 10-fold higher, signifying considerable tubular hypertrophy. The glomerular protein to DNA ratio increased by almost 70%, suggesting deposition of mesangial matrix within the glomerulus. These structural changes were associated with significant reductions in glomerular cysteine and metalloproteinase activities in the adolescent and older animals. Similarly, lower activities of both types of proteinases were observed in isolated proximal tubules. This behavior of proteolytic enzyme activities in the aging rat kidney corresponds well to the 10-fold increase in kidney weight (proximal tubular hypertrophy) and to the enhanced deposition of glomerular matrix. This study suggests a causal involvement of renal cysteine proteinases and metalloproteinases in the protein accumulation of the aging rat kidney.

Ultrastructure of developing kidney glomerular basement membranes: temporal changes in binding of anti-laminin IgG and cationized ferritin

In vivo labeling of infant rat and mouse glomerular basement membranes (GBMs) with polyclonal anti-laminin IgGs results in binding across the full widths of GBMs at all stages of development. These stages include the pre-fusion, double basement membranes found beneath endothelial cells and podocytes in early glomeruli, and the subepithelial matrix outpockets where newly synthesized GBM is spliced into fused basement membrane during glomerular maturation. Identical binding results are obtained either with peroxidase or post-embedding immunogold techniques. Although injected cationized ferritin also binds abundantly to all developing GBMs, it quickly disappears and, 24 hours after injection, is generally absent from GBMs but remains within mesangial matrices. Injection of newborn mice with monoclonal anti-laminin IgGs results in dense labeling of pre-fusion GBMs but post-fusion GBMs and subepithelial outpockets are weak-negative. Although masking can not be excluded, these results indicate that laminin epitopes are removed during GBM fusion and splicing, either by isoform substitution or proteolytic processing. The loss of bound cationized ferritin is believed to occur mainly through rapid turnover of GBM proteoglycans.

Glomerular metalloprotease activity in streptozotocin- treated rats and in spontaneously diabetic rats (BB/DP)

Differences in the type and extent of diabetic glomerular injury occur in rats made diabetic with streptozotocin (STZ) and in a spontaneously diabetic rat model of the BB/Wor strain. The diabetes prone (BB/DP) rat develops no mesangial matrix expansion with diabetes, whereas the STZ-treated Sprague Dawley rat does. The present study was performed to determine if glomerular metalloprotease activities were decreased in these two rat models of diabetes. Rats were studied at 8 weeks following the onset of hyperglycemia. All diabetic rats were maintained moderately hyperglycemic (plasma glucose = 350 mg/dL) with daily injection of insulin. Glomerular preparations were made and metalloprotease activities were assayed by hydrolysis of gelatin. Metalloprotease activities were decreased by 40-50% in STZ-treated Sprague Dawley rats, but not in BB/DP rats or diabetes resistant rats (BD/DR--the control for the BB/DP) made diabetic by STZ-injection. These data suggest that the decrease in glomerular metalloprotease activity may contribute to mesangial matrix expansion in the diabetic Sprague Dawley rats.

Regulation of proteolytic activity in tissues

Degradation of tissue proteins is controlled by multiple means. These include regulation of the synthesis of proteinases, activation of the zymogen forms, the activity of the mature proteinase, and the degradation of these enzymes and the substrates. Mature proteinases can be controlled by pH, calcium ions, ATP, lipids and the formation of complexes with other proteinases, proteoglycans, and inhibitors.

Characterization of neutral endopeptidase 24.11 in dog glomeruli

Neutral endopeptidase (NEP; also known as neprilysin and enkephalinase; EC 3.4.24.11) is a cell-surface metallopeptidase that is present in many mammalian tissues. It is particularly abundant on the brush-border membranes of the kidney proximal tubule. In this paper, the presence of NEP in purified glomeruli from dog kidney was assessed by measuring phosphoramidon- and thiorphan-sensitive [D-Ala2,Leu5]enkephalin-degrading activity. Using this assay, the Km and kcat. of the glomerular enzyme were found to be identical to those of the tubular enzyme. By Western blotting the apparent M(r) of the glomerular enzyme was found to be 104,000, compared with 94,000 for the tubular enzyme. This might be due to a different glycosylation pattern, since endoglycosidase F treatment of NEP obtained from both tissues yielded deglycosylated enzymes with similar electrophoretic mobilities. The glomerular enzyme also appears to be membrane-bound, since it was retained in the detergent-rich phase after phase separation with Triton X-114. Autoradiography experiments performed with RB104, a new highly selective and potent NEP inhibitor, showed that NEP was expressed in both glomeruli and proximal tubules. The presence in glomeruli of NEP and some other brush-border peptidases (dipeptidyl-dipeptidase IV, aminopeptidase N and angiotensin I-converting enzyme) suggests that cell-surface peptidases might play an important role as regulators of plasma-derived peptides in this part of the nephron.

Glomerular thromboxane A2/prostaglandin H2 receptors: characterization and effect of adriamycin-induced nephrotic syndrome

We have characterized the thromboxane (TX) A2/prostaglandin (PG) H2 receptor in glomeruli isolated from the rat using the agonist radioligand [125I]-BOP. Binding of [125I]-BOP was highly specific, stereoselective, and to a single class of high affinity binding sites (Kd = 1.16 +/- 0.22 nM and Bmax = 348 +/- 32 fmol/mg protein; n = 6). Binding of [125I]-BOP was competed for by the agonist ONO11113 (Kd = 50.8 +/- 8.0 nM; n = 4) and the antagonists SQ29548 (Kd = 15.8 +/- 1.0 nM; n = 3), L657925 (Kd = 12.1 +/- 2.2 nM; n = 3) and L657926 (Kd = 1642 +/- 135 nM; n = 3). I-BOP also produced a TXA2/PGH2 receptor-mediated rise in [Ca2+]i in isolated glomeruli In adriamycin-induced nephrotic syndrome in the rat, the development of proteinuria is reported to be dependent on increased renal TXA2 production. We therefore examined whether or not changes in glomerular TXA2/PGH2 receptors occur between control and nephrotic rats. No changes in expression or affinity of either glomerular or platelet TXA2/PGH2 receptors were observed. Kd and Bmax values for isolated glomeruli were 1.45 +/- 0.24 nM and 406 +/- 72 fmol/mg for controls and 1.22 +/- 0.25 nM and 321 +/- 62 fmol/mg for nephrotic rats (n = 6).

Proteolytic machinery of glomerular epithelial cells against IgG

Accumulation of immune complexes in the subepithelial region of the glomerular basement membrane results in the lesion of membranous nephropathy. The inefficient handling of immune complexes by the glomerular epithelial cell was investigated by studying the mechanism of IgG proteolysis by the intracellular proteases of cultured epithelial cells. Radiolabelled IgG was incubated with extracts of cells and the digestion of IgG was monitored by SDS-PAGE analysis. Prolonged incubation of IgG with the cell extracts resulted in only partial degradation of the IgG. The enzyme responsible for the breakdown was determined to be the lysosomal cathepsin D based on the pH optimum and the presence of aspartate in the active site of the enzyme. SDS-PAGE analysis of the digestion fragments revealed that a large proportion of the incompletely degraded IgG was the (Fab)2 fragment, which was resistant to further proteolysis. This could be one of the possible explanations for the slow removal of IgG from the subepithelial location of the basement membrane.

Cystatin C secretion by rat glomerular mesangial cells: autocrine loop for in vitro growth-promoting activity

Cystatin C, the major inhibitor of the cysteine proteinases found in human and rat body fluids, is particularly abundant in seminal plasma and cerebrospinal fluid. In a precedent report, we have evidenced noteworthy levels of cystatin C in rat kidney cortex. In the present study, we show that rat mesangial glomerular cells produce cystatin C. Immunoprecipitation of extracts of metabolically labeled cells and culture media showed that the synthesized cystatin C is a 15.5 +/- 0.5 kDa protein. The protein was released into the culture supernatant (1.6 +/- 0.26 micrograms/10(6) cells/24 h). Urinary rat cystatin C and PPPR synthetic peptide (5-8 N-terminal sequence of rat cystatin C) increased mesangial cell proliferation. Affinity chromatography on Ultrogel-avidin-biotin-PPPR of extracts of metabolically labeled cells indicate the existence of a PPPR binding protein of 46 kDa. The results described in this work suggest, for glomerular rat mesangial cells in vitro, an autocrine regulation of proliferation by cystatin C.

Evidence suggesting a role for cathepsin L in an experimental model of glomerulonephritis

We have utilized specific, irreversible inhibitors of cysteine proteinases to examine the role of renal cathepsin B and cathepsin L in the proteinuria which occurs in an experimental model of human glomerular disease. Administration of trans-epoxysuccinyl-L-leucylamido-(3-methyl)butane (Ep475) a specific, irreversible inhibitor of cysteine proteinases, including cathepsins B and L, significantly reduced proteinuria in rats with experimentally induced, neutrophil-independent, anti-GBM antibody disease (controls: 10 +/- 1 mg/24 h, N = 8; anti-GBM antibody disease: 203 +/- 30 mg/24 h, N = 8; anti-GBM antibody disease + Ep475: 112 +/- 13 mg/24 h, mean +/- SEM, N = 6, P less than 0.05). There was a marked reduction in the activity of both cathepsin B and cathepsin L in renal cortices obtained from Ep475-treated rats compared to either saline-treated controls or rats treated with anti-GBM IgG only. Administration of Z-Phe-Tyr(O-t-butyl)CHN2, a specific, irreversible cysteine proteinase inhibitor with a high degree of selectivity toward cathepsin L, also caused a reduction in anti-GBM antibody-induced proteinuria (90 +/- 18 mg/24 h, N = 6, P less than 0.05). This reduction in proteinuria was accompanied by a marked decrease (-84%) in the specific activity of renal cortical cathepsin L in Z-Phe-Tyr(O-t-butyl)CHN2-treated rats. However, cathepsin B activity was unchanged. There was no significant change in the renal anti-GBM antibody uptake, plasma urea nitrogen, or plasma creatinine values in the Z-Phe-Tyr(O-t-butyl)CHN2-treated rats compared to rats treated with anti-GBM IgG only or saline-treated controls. These data document the ability of cysteine proteinase inhibitors to decrease the proteinuria which occurs in a neutrophil-independent model of human anti-GBM antibody disease and suggest an important role for cathepsin L in the pathophysiology of the proteinuria which occurs in this model.