Severe proteinuria, sustained for 6 months, induces tubular epithelial cell injury and cell infiltration in rats but not progressive interstitial fibrosis

Inflammatory processes in renal fibrosis

Many types of kidney injury induce inflammation as a protective response. However, unresolved inflammation promotes progressive renal fibrosis, which can culminate in end-stage renal disease. Kidney inflammation involves cells of the immune system as well as activation of intrinsic renal cells, with the consequent production and release of profibrotic cytokines and growth factors that drive the fibrotic process. In glomerular diseases, the development of glomerular inflammation precedes interstitial fibrosis; although the mechanisms linking these events are poorly understood, an important role for tubular epithelial cells in mediating this link is gaining support. Data have implicated macrophages in promoting both glomerular and interstitial fibrosis, whereas limited evidence suggests that CD4(+) T cells and mast cells are involved in interstitial fibrosis. However, macrophages can also promote renal repair when the cause of renal injury can be resolved, highlighting their plasticity. Understanding the mechanisms by which inflammation drives renal fibrosis is necessary to facilitate the development of therapeutics to halt the progression of chronic kidney disease.

The podocyte's response to stress: the enigma of foot process effacement

Progressive loss of podocytes is the most frequent cause accounting for end-stage renal failure. Podocytes are complex, terminally differentiated cells incapable of replicating. Thus lost podocytes cannot be replaced by proliferation of neighboring undamaged cells. Moreover, podocytes occupy a unique position as epithelial cells, adhering to the glomerular basement membrane (GBM) only by their processes, whereas their cell bodies float within the filtrate in Bowman's space. This exposes podocytes to the danger of being lost by detachment as viable cells from the GBM. Indeed, podocytes are continually excreted as viable cells in the urine, and the rate of excretion dramatically increases in glomerular diseases. Given this situation, it is likely that evolution has developed particular mechanisms whereby podocytes resist cell detachment. Podocytes respond to stress and injury by undergoing tremendous changes in shape. Foot process effacement is the most prominent and, yet in some ways, the most enigmatic of those changes. This review summarizes the various structural responses of podocytes to injury, focusing on foot process effacement and detachment. We raise the hypothesis that foot process effacement represents a protective response of podocytes to escape detachment from the GBM.

Urinary OPN excretion in children with glomerular proteinuria

PURPOSE

The aim of the study was to investigate urinary levels and clinical significance of osteopontin (uOPN) in children with different glomerular diseases according to histological diagnosis and degree of proteinuria.

MATERIALS AND METHODS

The examinations were conducted in 3 groups of children: I - 20 children with minimal change disease (MCD) examined twice: A - in relapse; B - in remission, group II - 17 children with focal segmental glomerulosclerosis (FSGS), III - 12 children with IgA nephropathy (IgAN). The control group (C) contained 20 healthy children. OPN was measured in the urine using ELISA commercial available kit (R&D Quantikine) and was expressed in ng/ mg cr.

RESULTS

The median uOPN/ cr. in MCD children in relapse (IA) was median 134.98 ng/ mg cr. and was higher when compared to controls (p< 0.01). In exam IB, when proteinuria subsided, OPN/ cr. increased to median 172.96 ng/ mg cr. and was higher in comparison to healthy subjects (p< 0.01) and MCD children in relapse (p<0.05). Children from group II revealed higher uOPN/ cr. levels when compared to groups I, III and C (p< 0.01). UOPN/ cr. positively correlated with protein/ creatinine ratio in all examined groups of children (p< 0.01).

CONCLUSION

We found significantly higher uOPN/ cr. in all the groups of children with glomerulonephritis. The highest uOPN/ cr. levels were found in patients with FSGS and correlated significantly with both interstitial changes and mesangial expansion found in kidney biopsy.

Cell biology and pathology of podocytes

As an integral member of the filtration barrier in the kidney glomerulus, the podocyte is in a unique geographical position: It is exposed to chemical signals from the urinary space (Bowman's capsule), it receives and transmits chemical and mechanical signals to/from the glomerular basement membrane upon which it elaborates, and it receives chemical and mechanical signals from the vascular space with which it also communicates. As with every cell, the ability of the podocyte to receive signals from the surrounding environment and to translate them to the intracellular milieu is dependent largely on molecules residing on the cell membrane. These molecules are the first-line soldiers in the ongoing battle to sense the environment, to respond to friendly signals, and to defend against injurious foes. In this review, we take a membrane biologist's view of the podocyte, examining the many membrane receptors, channels, and other signaling molecules that have been implicated in podocyte biology. Although we attempt to be comprehensive, our goal is not to capture every membrane-mediated pathway but rather to emphasize that this approach may be fruitful in understanding the podocyte and its unique properties.

Proteinuria: an enzymatic disease of the podocyte?

Proteinuria is a major health-care problem that affects several hundred million people worldwide. Proteinuria is a cardinal sign and a prognostic marker of kidney disease, and also an independent risk factor for cardiovascular morbidity and mortality. Microalbuminuria is the earliest cue of renal complications of diabetes, obesity, and the metabolic syndrome. It can often progress to overt proteinuria that in 10-50% of patients is associated with the development of chronic kidney disease, ultimately requiring dialysis or transplantation. Therefore, reduction or prevention of proteinuria is highly desirable. Here we review recent novel insights into the pathogenesis and treatment of proteinuria, with a special emphasis on the emerging concept that proteinuria can result from enzymatic cleavage of essential regulators of podocyte actin dynamics by cytosolic cathepsin L (CatL), resulting in a motile podocyte phenotype. Finally, we describe signaling pathways controlling the podocyte actin cytoskeleton and motility and how these pathways can be manipulated for therapeutic benefit.

Podocyte foot process effacement as a diagnostic tool in focal segmental glomerulosclerosis

Podocyte foot process effacement is characteristic of proteinuric renal diseases. In minimal change nephrotic syndrome (MCNS) foot processes are diffusely effaced whereas the extent of effacement varies in focal segmental glomerulosclerosis (FSGS). Here we measured foot process effacement in FSGS and compared it to that in MCNS and in normal kidneys. A clinical diagnosis was used to differentiate idiopathic FSGS from secondary FSGS. Median foot process width, determined morphometrically by electron microscopy, was 3236 nm in 17 patients with idiopathic FSGS, 1098 nm in 7 patients with secondary FSGS, and 1725 nm in 15 patients with MCNS, as compared to 562 nm in 12 control patients. Multivariate analysis showed that foot process width did not correlate with proteinuria or serum albumin levels but was significantly associated as an independent factor with the type of disease. Foot process width over 1500 nm differentiated idiopathic from secondary FSGS with high sensitivity and specificity. Our results show that quantitative analysis of foot processes may offer a potential tool to distinguish idiopathic from secondary FSGS.

How does proteinuria cause progressive renal damage?

The possibility that proteinuria may accelerate kidney disease progression to end-stage renal failure has received support from the results of increasing numbers of experimental and clinical studies. Evidence indicating that this process occurs through multiple pathways, including induction of tubular chemokine expression and complement activation that lead to inflammatory cell infiltration in the interstitium and sustained fibrogenesis, is reviewed. Macrophages are prominent in the interstitial inflammatory infiltrate. This cell type mediates progression of renal injury to the extent that macrophage numbers in renal biopsy predict renal survival in patients with chronic renal disease. Chemoattractants and adhesive molecules for inflammatory cells are upregulated by excess ultrafiltered protein load of proximal tubular cells via activation of NF-kappaB-dependent and NF-kappaB-independent pathways. This mechanism is a potential target for therapeutic approaches, as shown by beneficial effects of manipulations with inhibitory molecules of NF-kappaB activation or of chemokine receptors in experimental studies. Targeting complement synthesis or activation in proximal tubule might offer novel therapeutic opportunities. Finally, proximal tubular cell receptors for uptake of plasma proteins that are under investigation may provide activation signals on excess tubular protein handling.

The autoimmune response to vimentin after renal transplantation in nonhuman primates is immunosuppression dependent

BACKGROUND

Chronic allograft nephropathy (CAN) is a common late complication of kidney transplantation. Antibodies to both human leukocyte antigen and nonhuman leukocyte antigen antigens have been implicated in the development of this condition. Here we investigated the presence of antivimentin antibodies in nonhuman primate recipients of kidney allografts as a possible predictor of CAN and the effects of immunosuppression.

METHODS

Thirty seven rhesus monkeys received a kidney allograft to study the potency of several different immunosuppressive regimens (conventional immunosuppression, n=19, vs. costimulatory blockade, n=18). Monkeys were tested for antivimentin antibody by enzyme-linked immunosorbent assay and for anti-donor antibody by staining donor spleen cells with recipient serum. The appearance of antibodies was correlated with the graft pathology in biopsy and necropsy material.

RESULTS

Antivimentin antibodies were found in 31 of 37 animals, whereas only 15 of 32 animals made anti-donor antibodies. Conventional immunosuppression did not prevent antivimentin antibody formation. Costimulation blockade, in particular blocking CD40 and CD86, significantly delayed or prevented antivimentin antibody formation, but did not prevent CAN. Antivimentin antibodies were not significantly associated with development of CAN.

CONCLUSIONS

We postulate that vimentin acts as an autoantigen after renal transplantation; it elicits an autoimmune response that is not regulated by cyclosporine. This autoimmune response may be part of the complex immunologic events occurring posttransplantation and may contribute to the development of CAN, but cannot be considered as a major cause of CAN because this condition also develops without antivimentin antibodies.

Pathways to nephron loss starting from glomerular diseases-insights from animal models

Studies of glomerular diseases in animal models show that progression toward nephron loss starts with extracapillary lesions, whereby podocytes play the central role. If injuries remain bound within the endocapillary compartment, they will undergo recovery or be repaired by scaring. Degenerative, inflammatory and dysregulative mechanisms leading to nephron loss are distinguished. In addition to several other unique features, the dysregulative mechanisms leading to collapsing glomerulopathy are particular in that glomeruli and tubules are affected in parallel. In contrast, in degenerative and inflammatory diseases, tubular injury is secondary to glomerular lesions. In both of the latter groups of diseases, the progression starts in the glomerulus with the loss of the separation between the tuft and Bowman's capsule by forming cell bridges (parietal cells and/or podocytes) between the glomerular and the parietal basement membranes. Cell bridges develop into tuft adhesions to Bowman's capsule, which initiate the formation of crescents, either by misdirected filtration (proteinaceous crescents) or by epithelial cell proliferation (cellular crescents). Crescents may spread over the entire circumference of the glomerulus and, via the glomerulotubular junction, may extend onto the tubule. Two mechanisms concerning the transfer of a glomerular injury onto the tubulointerstitium are discussed: (1) direct encroachment of extracapillary lesions and (2) protein leakage into tubular urine, resulting in injury to the tubule and the interstitium. There is evidence that direct encroachment is the crucial mechanism. Progression of chronic renal disease is underlain by a vicious cycle which passes on the damage from lost and/or damaged nephrons to so far healthy nephrons. Presently, two mechanisms are discussed: (1) the loss of nephrons leads to compensatory mechanisms in the remaining nephrons (glomerular hypertension, hyperfiltration, hypertrophy) which increase their vulnerability to any further challenge (overload hypothesis); and (2) a proteinuric glomerular disease leads, by some way or another, to tubulointerstitial inflammation and fibrosis, accounting for the further deterioration of renal function (fibrosis hypothesis). So far, no convincing evidence has been published that in primary glomerular diseases fibrosis is harmful to healthy nephrons. The potential of glomerular injuries to regenerate or to be repaired by scaring is limited. The only option for extracapillary injuries with tuft adhesion is repair by formation of a segmental adherent scar (i.e., segmental glomerulosclerosis).

Predictive value of urinary micro-cholesterol (mCHO) levels in patients with progressive glomerular disease

BACKGROUND

Trace amounts of lipids are present in the urine of patients with glomerular disease, raising the possibility that the excess lipids reabsorbed by tubule cells may be toxic to these cells. In the present study, we assessed the prognostic value of micro-cholesterol (mCHO) levels in patients with chronic glomerular disease.

METHODS

The urinary mCHO levels of healthy subjects and patients with chronic kidney disease were measured by the enzymatic cholesterol cycling (ECC) method with a minimum detection level of 0.10 x 10(-3) mmol/L. First, the urinary mCHO levels of healthy subjects and 320 patients with various glomerular diseases with proteinuria >1000 mg/gCr were measured. Second, correlations of urinary mCHO levels with those of various other molecules, including albumin, IgG, IgM, transferrin, phospholipid, alpha1-microglobulin (alpha1MG), Apo A1, Apo A2, and Apo B, and urinary fatty body counts, were determined. Third, urinary mCHO, total protein (TP), albumin, and N-acetyl-beta-D-glucosaminidase (NAG) levels were measured longitudinally over 12 months (20.5 +/- 5.8 months) in 68 nondiabetic patients with impaired renal function [serum creatinine (Cr) > or = 1.5 mg/dL]. Correlations of the concentrations of urinary parameters in the initial 3-month period with the slopes of the reciprocal of creatinine versus time for the entire follow-up period were assessed by the ROC method and multiple regression analysis.

RESULTS

Urinary mCHO levels of the healthy subjects were 0.06 to 0.72 mg/gCr for males and 0.16 to 2.34 mg/gCr for females. Urinary mCHO levels in subjects with minimal change nephrotic syndrome were significantly lower than those in the patients with other glomerular diseases with massive proteinuria. Urinary mCHO levels correlated significantly with Apo A1 and Apo A2 levels, but not with urinary Apo B levels, in the latter subjects. The correlation coefficient of urinary fatty body counts (a marker of lipoprotein loading tubulopathy) with mCHO was higher than those with TP, albumin, IgG, IgM, and alpha1MG. The urinary mCHO elevation was significantly greater in patients who had a nonselective index of proteinuria than in those with a highly or moderately selective index. In nondiabetic patients with impaired renal function, the urinary mCHO level had a higher predictive value for rapid decline of renal function than TP, albumin, or NAG.

CONCLUSION

The urinary cholesterol level corresponds to the magnitude of urinary HDL excretion, and correlates with the degree of lipoprotein loading tubulopathy. Measurement of urinary mCHO by the ECC method is a simple and useful tool for predicting progression of chronic glomerular disease.

Vimentin and heat shock protein expression are induced in the kidney by angiotensin and by nitric oxide inhibition

BACKGROUND

Angiotensin II (Ang II) infusion and nitric oxide synthesis (NOS) inhibition with Nomega-nitro-l-arginine-methyl-ester (l-NAME) are experimental models of hypertension associated with renal inflammation and oxidative stress. To gain insight into the nature of the tubulointerstitial injury induced in these models, we studied lectin-binding specificities, vimentin expression, and heat shock protein (HSP) 60 and 70 in these experimental models.

METHODS

Sprague-Dawley rats received Ang II infusion (435 ng/kg/min) for 2 weeks by subcutaneous minipumps (Ang II group, N = 5) or l-NAME in the drinking water (70 mg/100 mL) for 3 weeks (l-NAME group N = 7). The control group consisted of 10 rats. Systolic blood pressure (tail-cuff plethysmography), serum creatinine, and proteinuria were determined weekly. At the end of the treatment period, rats were sacrificed and kidneys studied. Binding specificities of fluorescein-labeled lectins were examined in frozen sections, and cellular infiltrates were identified by immunohistology and expression of vimentin and HSP 60 and 70 with immunohistochemistry and computer image analysis.

RESULTS

Tubulointerstitial accumulation of macrophages, lymphocytes, and Ang II-positive cells were present in the Ang II group and l-NAME group. Vimentin, HSP 60, and HSP 70 were increased 8 to 20 times in the cortex of the rats of the Ang II group and the l-NAME groups. Neoexpression of vimentin and HSPs was found primarily in proximal tubular cells.

CONCLUSION

Ang II infusion and NOS inhibition induce tubular injury with epithelial cell transdifferentiation and expression of stress proteins. The role of these changes in the accumulation and activation of the interstitial inflammatory infiltrate merits further investigation.

Retinoids as a potential treatment for experimental puromycin-induced nephrosis

1 Puromycin aminonucleoside (PAN)-induced nephrosis is a model of human minimal change disease. In rats, PAN induces nephrotic-range proteinuria, renal epithelial cell (podocyte) damage, infiltration of mononuclear leukocytes, and apoptosis of several renal cell types. 2 Retinoic acid (RA) modulates a wide range of biological processes, such as inflammation and apoptosis. Since renal damage by PAN is characterized by inflammatory infiltration and epithelial cell death, the effect of treatment with all-trans RA (tRA) was examined in the PAN nephrosis model and in the cultured differentiated podocyte. 3 Treatment with tRA 4 days after PAN injection did not inhibit the proteinuria peak but reversed it significantly. However, treatment with tRA both before and 2 days after the injection of PAN protected the glomerular epithelial cells, diminishing the cellular edema and diffuseness of the foot process effacement. Preservation of the podocyte architecture correlated with the inhibition of proteinuria. The anti-inflammatory effect of tRA was evidenced by the inhibition of PAN-induced interstitial mononuclear cell infiltration and the decreased renal expression of two molecules involved in monocyte infiltration: fibronectin and monocyte chemoattractant protein-1. TUNEL assays showed that tRA inhibited the PAN-induced apoptosis of cultured differentiated mouse podocytes. 4 We conclude that tRA treatment may prevent proteinuria by protecting the podocytes from injury and diminishing the interstitial mononuclear infiltrate in the model of PAN nephrosis. Retinoids are a potential new treatment for kidney diseases characterized by proteinuria and mononuclear cell infiltration.

Risk factors for cyclosporine-induced tubulointerstitial lesions in children with minimal change nephrotic syndrome

BACKGROUND

Cyclosporine (CsA) is effective for the treatment of children with steroid-dependent and -resistant nephrotic syndrome (NS), but it can result in chronic CsA nephrotoxicity including CsA-induced tubulointerstitial lesions. The factors responsible for the development of CsA-induced tubulointerstitial lesions are unknown.

METHODS

To identify the risk factors for the development of CsA-induced tubulointerstitial lesions in children with minimal change NS who had been treated with long-term moderate-dose CsA, we compared several clinical and laboratory factors of 37 patients with and without CsA-induced tubulointerstitial lesions by the Mann-Whitney U test, Fisher's exact test, and stepwise logistic-regression analysis.

RESULTS

Thirteen patients had CsA-induced tubulointerstitial lesions and 24 patients had none. Among clinical and laboratory factors, the duration of CsA treatment (P = 0.003) and the duration of heavy proteinuria during CsA treatment (P = 0.024) were related to the development of CsA-induced tubulointerstitial lesions as determined by the univariate analyses. Indeed, CsA-induced tubulointerstitial lesions were found in 2 of 18 (11%) patients who had been treated with CsA for less than 24 months, but in 11 of 19 patients (58%) who had been treated for more than 24 months (P = 0.005). They were also found in 4 of 23 patients (17%) who had heavy proteinuria for less than 30 days during CsA treatment, but in 9 of 14 patients (64%) who had heavy proteinuria for more than 30 days (P = 0.006). Stepwise logistic-regression analysis revealed that the duration of CsA treatment for more than 24 months (chi2 = 6.203, P = 0.013) and the duration of heavy proteinuria during CsA treatment for more than 30 days (chi2 = 5.871, P = 0.015) were independent risk factors for the development of CsA-induced tubulointerstitial lesions.

CONCLUSIONS

Duration of the CsA treatment and the duration of heavy proteinuria during CsA treatment were independent significant risk factors for the development of CsA-induced tubulointerstitial lesions in children with MCNS who had been treated with long-term moderate-dose CsA.

Renal handling of albumin: a critical review of basic concepts and perspective

Biochemical and physiological processes that underlie the mechanism of albuminuria are completely reassessed in this article in view of recent discoveries that filtered proteins undergo rapid degradation during renal passage and the resulting excreted peptide fragments are not detected by conventional urine protein assays. This means that filtered protein and/or albumin levels in urine have been seriously underestimated. The concept that albuminuria is a result of changes in glomerular permeability is questioned in light of these findings and also in terms of a critical examination of charge selectivity, shunts, or large-pore formation and hemodynamic effects. The glomerulus appears to function merely in terms of size selectivity alone, and for albumin, this does not change significantly in disease states. Intensive albumin processing by a living kidney occurs through cellular processes distal to the glomerular basement membrane. Failure of this cellular processing primarily leads to albuminuria. This review brings together recent data about urinary albumin clearance and current knowledge of receptors known to process albumin in both health and disease states. We conclude with a discussion of topical and controversial issues associated with the proposed new understanding of renal handling of albumin.

Podocyte injuries exacerbate mesangial proliferative glomerulonephritis

BACKGROUND

From the observations of morphology seen in early phases of the experimental models of the irreversible mesangial proliferative glomerulonephritis, we hypothesized that podocyte injury is one of the important factors in bringing upon irreversible glomerular alterations. To verify this hypothesis, we investigated whether podocyte injury induced by puromycin aminonucleoside (PAN) injection affects the mesangial alterations of anti-Thy 1.1 glomerulonephritis.

METHODS

Female Wistar rats were injected with 0.5 mg monoclonal antibody (mAb) 1-22-3 five days after the injection of 10 mg or 5 mg/100 g body weight (BW) of puromycin aminonucleoside (PAN), and sacrificed at 7 days or 8 weeks after the mAb 1-22-3 injection.

RESULTS

Consecutive injections of 10 mg/100 g BW of PAN and mAb 1-22-3 caused the irreversible mesangial alteration with persistent proteinuria (at week 8, proteinuria 100.3 +/- 57.8 mg/24 h, matrix score 1.13 +/- 0.52, collagen type I score 2.04 +/- 0.53, mRNA for collagen type I 227 +/- 79% to the group with a single injection of 1-22-3). Although single injection of 5 mg/100 g BW of PAN was not capable of inducing abnormal proteinuria, consecutive injections of 5 mg/100 g BW of PAN and mAb 1-22-3 also caused irreversible mesangial alteration and persistent proteinuria.

CONCLUSIONS

Podocyte injury might be an important factor that exacerbates mesangial proliferation and mesangial matrix expansion. The irreversible mesangial alterations caused by consecutive injections of PAN and mAb 1-22-3 may be a novel model that could be used to analyze the mechanism of progressive mesangial alteration.

Tubulointerstitial renal disease

Tubulointerstitial damage, in progressive chronic renal disease of all types, arises because of a complex interplay between factors in the tubular lumen, tubular epithelial cells, peritubular capillaries, resident and infiltrating interstitial cells and extracellular matrix. Particularly in proteinuric renal disease, tubular epithelial cells play a central role in orchestrating these events. In response to mediators arising systemically, in the tubular lumen or from other renal cells, tubular epithelial cells undergo a complex series of structural and functional changes and produce a bewildering number of soluble and fixed mediators, which in turn lead to interstitial inflammation and fibrosis. Knowledge of these interactions has increased exponentially over the past decade, and has defined a number of new targets for treatment. Both expansion and consolidation of this knowledge is needed to determine which of these targets holds the most promise for future treatment.