The role of podocyte injury in the pathogenesis of focal segmental glomerulosclerosis

Does baseline nephrotic range proteinuria determine the long-term outcomes of membranous lupus nephritis patients?: Nephrotic Range Proteinuria in Membranous Lupus Nephritis

OBJECTIVES

Management strategies for membranous lupus nephritis (MLN) are generally based on the severity of proteinuria. However, long-term outcomes comparing subnephrotic and nephrotic range proteinuria remain understudied. We explored whether baseline proteinuria level, subnephrotic or nephrotic, impacts long-term outcomes.

METHODS

We conducted a retrospective study identifying patients with biopsy-proven MLN. Patients were categorized based on baseline proteinuria: subnephrotic (<3.5 g/day) or nephrotic (≥3.5 g/day). Long-term outcomes, including an adverse composite outcome (end-stage kidney disease, sustained ≥30 % decline in eGFR, or death) and LN flares, were analyzed. Time-to-event outcomes were assessed using Kaplan-Meier curves, and associations were evaluated using Cox regression.

RESULTS

88 patients were included, with 49 (55.7 %) in the subnephrotic group (median 1.5 g/day) and 39 (44.3 %) in the nephrotic group (median 4.7 g/day). At baseline, the subnephrotic group had a longer time to LN onset, less frequent hyperlipidemia, higher serum albumin, less diffuse podocyte effacement, and less frequent cyclophosphamide treatment. No significant differences were noted in kidney function, urine sediment abnormalities, or histopathology. 38 patients (43.2 %) experienced the adverse composite outcome, with no difference between groups (40.8 % in the subnephrotic group vs. 46.2 % in the nephrotic group, p = 0.78]. Flares occurred in 35 patients (39.8 %), with no difference between groups (38.8 % in the subnephrotic group vs. 41.0 % in the nephrotic group, p = 1.00).

CONCLUSIONS

No significant differences in renal disease characteristics or long-term outcomes were found between MLN patients with nephrotic and subnephrotic baseline proteinuria. These findings challenge current practices, suggesting a need for more individualized immunosuppressive treatment in MLN.

Cell cycle disorders in podocytes: an emerging and increasingly recognized phenomenon

Proteinuria is observed in various kidney diseases and is frequently associated with a compromised glomerular filtration barrier. Podocytes, as a crucial component of this barrier, play an essential role in preserving the kidney's normal filtration function. Podocytes are terminally differentiated cells that typically do not proliferate. However, certain harmful stimuli can trigger podocytes to re-enter the cell cycle. Due to its unique cytoskeletal structure, podocytes are unable to maintain the structure of the foot process and complete cell division at the same time, eventually form binucleated or multinucleated podocytes. Studies have found that podocytes re-entering the cell cycle are more susceptible to injury, and are prone to detachment from the basement membrane or apoptosis, which are accompanied by the widening of foot processes. This eventually leads to podocyte mitotic catastrophe and the development of proteinuria. Podocyte cell cycle disorders have previously been found mainly in focal segmental glomerulosclerosis and IgA nephropathy. In recent years, this phenomenon has been frequently identified in diabetic kidney disease and lupus nephritis. An expanding body of research has begun to investigate the mechanisms underlying podocyte cell cycle disorders, including cell cycle re-entry, cell cycle arrest, and mitotic catastrophe. This review consolidates the existing literature on podocyte cell cycle disorders in renal diseases and summarizes the molecules that trigger podocyte re-entry into the cell cycle, thereby providing new drug targets for mitigating podocyte damage. This is essential for alleviating podocyte injury, reducing proteinuria, and delaying the progression of kidney diseases.

Developmental Causes of Focal Segmental Glomerulosclerosis

Background

Focal segmental glomerulosclerosis (FSGS) is a histological pattern of glomerular damage that includes idiopathic conditions as well as genetic and non-genetic forms. Among these various etiologies, different phenotypes within the spectrum of congenital anomalies of the kidney and urinary tract (CAKUT) have been associated with FSGS.

Summary

Until recently, the main pathomechanism of how congenital kidney and urinary tract defects lead to FSGS was attributed to a reduced number of nephrons, resulting in biomechanical stress on the remaining glomeruli, detachment of podocytes, and subsequent inability to maintain normal glomerular architecture. The discovery of deleterious single-nucleotide variants in PAX2, a transcription factor crucial in normal kidney development and a known cause of papillorenal syndrome, in individuals with adult-onset FSGS without congenital kidney defects has shed new light on developmental defects that become evident during podocyte injury.

Key Message

In this mini-review, we challenge the assumption that FSGS in CAKUT is caused by glomerular hyperfiltration alone and hypothesize a multifactorial pathogenesis that includes overlapping cellular mechanisms that are activated in both damaged podocytes as well as nephron progenitor cells.

WWC1 promotes podocyte survival via stabilizing slit diaphragm protein dendrin

Previous studies have indicated that glomerular podocyte injury serves a crucial role in proteinuria during the process of chronic kidney disease. The slit diaphragm of podocytes forms the final barrier to proteinuria. Dendrin, a constituent of the slit diaphragm protein complex, has been observed to relocate from the slit diaphragm to the nuclei in injured podocytes and promote podocyte apoptosis. However, the exact mechanism for nuclear relocation of dendrin remains unclear. The expression of WWC1 in podocyte injury induced by lipopolysaccharides (LPS) or adriamycin (ADR) was detected by reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR), western blotting and the immunofluorescence assay. The role of WWC1 in podocyte apoptosis was detected by knockdown of WWC1 and flow cytometry. The mRNA and protein expression levels of apoptosis‑associated genes Bcl‑2‑associated X (Bax) and Bcl‑2 were measured by RT‑qPCR and western blotting. The impact of WWC1 on dendrin nucleus relocation in vitro in podocytes was further evaluated by knockdown of WWC1. Expression of WWC1 significantly decreased in injured podocytes in vitro. The loss‑of‑function assay indicated that knockdown of WWC1 gene in vitro promoted podocyte apoptosis, accompanied with increased levels of the pro‑apoptotic protein Bax and decreased levels of the anti‑apoptotic protein Bcl‑2. Furthermore, the relocation of dendrin protein was significantly promoted by knockdown of the WWC1 gene. In conclusion, the study indicated that loss of WWC1 may contribute to podocyte apoptosis by inducing nuclear relocation of dendrin protein, which provided novel insight into the molecular events in podocyte apoptosis.

Recurrent focal segmental glomerulosclerosis after kidney transplantation

Focal segmental glomerulosclerosis (FSGS) is an important cause of glomerular disease in children and adolescents and nearly 50 % of affected patients will progress to end-stage kidney disease over a 5 to 10-year period. Unfortunately, there is no established treatment for disease in the native kidney. Moreover, up to 55 % of patients develop recurrent disease after receiving a kidney transplant, with a substantially higher risk in patients who have already experienced recurrent disease in a prior transplant. A number of clinical and laboratory factors have been identified as risk factors for this complication. In addition, new investigations into podocyte biology and circulating permeability factors have shed light on the cause of recurrent the disease. While a number of novel therapeutic agents have been applied in the management of this problem, there still is no proven treatment. In this review, we summarize recent advances in the epidemiology, pathophysiology, and treatment of recurrent FSGS in pediatric patients who have received a kidney transplant.

Costimulatory blockade: A novel approach to the treatment of glomerular disease?

Costimulatory pathways (Cluster of differentiation 28, tumor necrosis factor-related, adhesion and T Cell Ig- and mucin-domain molecules) regulating the interactions between receptors on the T cells and their ligands expressed on several cell types, have a key role in controlling many immunological and non immunological processes. Indeed, accumulating evidence indicate that these molecules are involved in the pathogenesis of numerous conditions, such as allograft rejection, atherosclerosis, rheumatoid arthritis, psoriasis and renal diseases, including glomerulonephritis. Primary or secondary (i.e., associated with infections, drugs or systemic diseases, such as systemic lupus erythematosus, diabetes, etc.) glomerulonephritis represent a group of heterogeneous diseases with different pathogenic mechanisms. Since costimulatory molecules, in particular CD80 and CD40, have been found to be expressed on podocytes in the course of different experimental and clinical glomerulonephritis, costimulation has been thought as a new therapeutic target for patients with glomerular diseases. However, although experimental data suggested that the blockade of costimulatory pathways is effective and safe in the prevention and treatment of glomerular diseases, clinical trials reported contrasting results. So, at this moment, there is not a strong evidence for the general use of costimulatory blockade as an alternative treatment strategy in patients with primary or secondary glomerulonephritis. Here, we critically discuss the current data and the main issues regarding the development of this innovative therapeutic approach.

Characterization of a Trpc6 Transgenic Mouse Associated with Early Onset FSGS

Rationale:

Mutations in Transient Receptor Potential Channel 6 (TRPC6) gene are associated with autosomal dominant focal and segmental glomerulosclerosis (FSGS). The majority of the identified mutations affect the ion channel function. Since calcium channels are promising candidate drug targets, there is an an urgent need for a mouse model to assess new therapeutic drugs and to help delineate the pathogenic process leading to FSGS. We have previously reported the generation of three independent transgenic mouse lines carrying different Trpc6 mutations that display a glomerular disease comparable to the phenotype presented by individuals with FSGS. However, the utility of these models for drug testing is dampened by the late-onset of the presentation and the mild phenotypic manifestations.

Methodology:

In order to obtain a time-effective mouse model for Trpc6-associated FSGS we generated a new transgenic mutant Trpc6 mouse model emulating the amino acid change carried by the first pediatric patient of FSGS associated with a TRPC6 mutation: M132T.

Results:

Mice carrying the orthologous Trpc6 M131T transgene showed early onset proteinuria and early signs of FSGS. When exploring molecular consequences of the overexpression of this mutated form of Trpc6 in podocytes, differences in expression levels of Axin2 and β-catenin were found in glomeruli from transgenic Trpc6 M131T mice. These data supports the proposed molecular mechanisms related to the activation of calcineurin-NFAT/Wnt signaling, as outcome of the increased calcium influx caused by the mutated form of Trpc6.

Conclusion:

Given that the Trpc6 M131T mouse develops an early onset of FSGS-like phenotypes it represents a promising model for studying the pathogenesis of FSGS caused by TRpC6, facilitating the assessment of new drugs as treatments and allowing further studies to understand underlying molecular pathways involved in the development of the TRPC6 mediated disease.

Epithelial cell foot process effacement in podocytes in focal and segmental glomerulosclerosis: a quantitative analysis

BACKGROUND/AIM

Effacement of the epithelial cell foot processes of glomerular podocytes are thought to be diffuse in the primary form of focal and segmental glomerulosclerosis (FSGS). In contrast, effacements that occur in the secondary form of FSGS are thought to be focal. To evaluate this theory, the extent of epithelial cell foot process effacement was analyzed and compared in podocytes from cases of primary and secondary FSGS.

METHODS

Consecutive cases of adult primary and secondary FSGS that were diagnosed between January 1997 and December 2010 were retrospectively retrieved and analyzed. The electron microscopy materials of all specimens were reviewed. Glomerular podocytes from each sample were counted, recorded and analyzed for the percentage of diffuse versus focal epithelial foot process effacement.

RESULTS

Seventeen primary and 16 secondary FSGS specimens were reviewed. In the 17 primary FSGS cases, 35.1% of the podocytes were focally fused, and 64.9% were diffusely fused. In the 16 secondary FSGS cases, 72.1% of the podocytes were focally fused. There were significantly more focally fused podocytes in secondary FSGS than in primary FSGS. In contrast, significantly more diffusely fused podocytes were observed in primary FSGS than in secondary FSGS.

CONCLUSION

This is the first study to perform a quantitative analysis of the extent of epithelial cell foot process effacement in podocytes in primary and secondary FSGS. This study also confirms that the electron microscopic characteristics of the podocyte effacement process can be used to facilitate the diagnosis of primary versus secondary FSGS.

Dysfunction of the PGC-1α-mitochondria axis confers adriamycin-induced podocyte injury

Adriamycin (ADR)-induced nephropathy in animals is an experimental analog of human focal segmental glomerulosclerosis, which presents as severe podocyte injury and massive proteinuria and has a poorly understood mechanism. The present study was designed to test the hypothesis that the peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α-mitochondria axis is involved in ADR-induced podocyte injury. Using MPC5 immortalized mouse podocytes, ADR dose dependently induced downregulation of nephrin and podocin, cell apoptosis, and mitochondrial dysfunction based on the increase in mitochondrial ROS production, decrease in mitochondrial DNA copy number, and reduction of mitochondrial membrane potential and ATP content. Moreover, ADR treatment also remarkably reduced the expression of PGC-1α, an important regulator of mitochondrial biogenesis and function, in podocytes. Strikingly, PGC-1α overexpression markedly attenuated mitochondrial dysfunction, the reduction of nephrin and podocin, and the apoptotic response in podocytes after ADR treatment. Moreover, downregulation of PGC-1α and mitochondria disruption in podocytes were also observed in rat kidneys with ADR administration, suggesting that the PGC-1α-mitochondria axis is relevant to in vivo ADR-induced podocyte damage. Taken together, these novel findings suggest that dysfunction of the PGC-1α-mitochondria axis is highly involved in ADR-induced podocyte injury. Targeting PGC-1α may be a novel strategy for the treatment of ADR nephropathy and human focal segmental glomerulosclerosis.

B cell suppression in primary glomerular disease

Membranous nephropathy, focal segmental glomerulosclerosis (FSGS), and minimal change disease (MCD) are the most common causes of idiopathic nephrotic syndrome. For many years prednisone, alkylating agents, and calcineurin inhibitors have been the standard of therapy for these patients. More effective or better tolerated treatment modalities are needed. B cell targeted therapy was recently introduced in clinical practice. In this review, we briefly summarize the current standard therapy and discuss the efficacy of B cell targeted therapy in primary glomerular diseases. Observational, short-term studies suggest that rituximab is effective and comparable to standard therapy in maintaining remissions in patients with frequently relapsing or steroid-dependent MCD or FSGS. In contrast, response is limited in patients with steroid-resistant nephrotic syndrome. Rituximab also induces remissions in patients with membranous nephropathy. Controlled clinical trials on kidney endpoints are urgently needed to position B cell targeted therapy in clinical practice.

CD2AP is associated with end-stage renal disease in patients with type 1 diabetes

CD2-associated protein (CD2AP) is essential for podocyte function. CD2AP mutations have been found in patients with focal segmental glomerulosclerosis, a disease histologically resembling diabetic nephropathy and often progressing to end-stage renal disease (ESRD). We hypothesised that variations in the CD2AP gene may contribute to susceptibility to glomerular injury in diabetes and investigated if single-nucleotide polymorphisms (SNPs) in CD2AP are associated with diabetic nephropathy in patients with type 1 diabetes. The discovery cohort consisted of 2,251 Finnish patients with type 1 diabetes. SNPs were selected from the HapMap database to cover the CD2AP gene. The associations between genotyped SNPs and diabetic nephropathy or ESRD were analysed with the chi-squared test and logistic regression. Three SNPs were selected for replication in cohorts from Denmark, Italy, the United Kingdom and Ireland. None of the 15 successfully genotyped SNPs were associated with diabetic nephropathy when compared to patients with normal albumin excretion rate. However, when genotype frequencies in patients with ESRD were compared with all other patients, two CD2AP SNPs, rs9369717 and rs9349417, were found to be associated with ESRD. The meta-analysis of the original and two additional European cohorts resulted in significant p values <0.01 for these SNPs. A third SNP, rs6936632, was suggestively associated with ESRD in the Finnish patients and in the meta-analysis of four cohorts. CD2AP gene variants may contribute to susceptibility to ESRD in patients with type 1 diabetes.

Focal segmental glomerulosclerosis after renal transplantation

Focal segmental glomerulosclerosis (FSGS) is a clinicopathologic syndrome of proteinuria, usually of nephrotic range, associated with focal and segmental sclerotic glomerular lesions. Therefore, FSGS is diagnosed by clinical features and histopathological examination of renal biopsy. The natural history of the condition varies, and although it may respond to treatment, FSGS is an important disease in the etiology of end-stage renal disease (ESRD). Furthermore, after kidney transplantation, approximately 30% of patients with FSGS develop recurrent FSGS. The risk factors for recurrence of FSGS include childhood onset and age <15 yr, rapid progression of the primary FSGS to ESRD, recurrence of FSGS in a previous allograft, diffuse mesangial hypercellularity in the native kidney, collapsing FSGS, and podocin gene mutation. In addition, after kidney transplantation, de novo FSGS also develops in approximately 10-20% of allografts, associated with a complication of hyperfiltration injury, chronic transplant glomerulopathy, and calcineurin inhibitor toxicity. FSGS is considered a podocyte disease, and the pathology is characterized by segmental FSGS lesion with glomerular epithelial hypercellularity. The pathological diagnosis of FSGS is based on the 2004 Columbia classification system. In the present minireview, we discuss the pathology of recurrence and de novo FSGS after kidney transplantation.

Focal segmental glomerulosclerosis plays a major role in the progression of IgA nephropathy. II. Light microscopic and clinical studies

It is well known that lesions morphologically identical with focal segmental glomerulosclerosis (FSGS) may appear in IgA nephropathy (IgAN). Capsular adhesions without underlying abnormalities in the tuft, often the first sign of FSGS, are frequent in IgAN. In this retrospective study, a new cohort of 128 adult patients with IgAN was used to validate the new Oxford classification system of IgAN, and shown to have highly significant associations with clinical and outcome parameters. We then used these patients to determine the extent to which IgAN could be accounted for in terms of FSGS. Some form of lesion consistent with FSGS, notably hyalinosis and collapsing glomerulopathy, was found in 101 of these patients. No glomerular lesions were found in 16 patients, and 11 had mild lesions not definable as FSGS. Those with FSGS had significantly worse renal survival at 80 months than those without. Comparison of pure forms of FSGS (excluding collapsing glomerulopathy) with cases of FSGS having other glomerular lesions (mesangial hyperplasia, endocapillary hypercellularity, glomerular necroses, extracapillary proliferation) revealed that those with FSGS and other superimposed lesions did significantly worse than cases of pure FSGS at 80 months following diagnosis. Importantly, patients with pure FSGS had relatively poor survival even without other superimposed glomerular abnormalities. Thus, the majority of cases of IgAN can be interpreted as representing one or another variant of FSGS. Hence, interpreting IgAN in terms of FSGS emphasizes the role that podocyte lesions may play in the pathogenesis and progression of this disease.

Focal segmental glomerulosclerosis plays a major role in the progression of IgA nephropathy. I. Immunohistochemical studies

IgA nephropathy (IgAN) often shows lesions morphologically identical with those of focal segmental glomerulosclerosis (FSGS). In order to determine the possible role of FSGS in IgAN lesions, we measured glomerular capsular adhesions, often the first step toward FSGS, in biopsies from 127 patients with IgAN, 100 with lupus nephritis, and 26 with primary FSGS. Capsular adhesions with no lesions in the underlying tuft, consistent with podocyte abnormality or loss, were found regularly in FSGS and IgAN, but infrequently in lupus. Fifteen biopsies of patients with IgAN were studied immunohistochemically using markers for podocytes, Bowman's parietal epithelial cells, proliferating cells, and macrophages. Cytokeratins CK-8 and C2562 differentiated normal podocytes (negative) from parietal epithelial cells (variably positive). There was focal loss of the podocyte markers synaptopodin, glomerular epithelial protein 1 (GLEPP-1), nephrin, and vascular endothelial growth factor (VEGF), particularly at sites of capsular adhesions in otherwise histologically normal glomeruli. Cells displaying the parietal epithelial cell markers PAX2 (paired box gene 2) and the cytokeratins were also positive for the proliferating cell marker, proliferating cell nuclear antigen. These cells gathered at sites of adhesion, and in response to active lesions in the tuft, grew inward along the adhesion onto the tuft, forming a monolayer positive for parietal markers and the podocyte marker Wilms tumor protein-1 (WT-1). These cells deposited a layer of collagen over the sclerosing tuft. Thus, all biopsies of patients with IgAN had changes basically identical to those classically described in FSGS. Hence, our study strongly suggests that podocytopathy of a type similar to that in primary FSGS occurs frequently in IgAN.

Association of phospholipase A2 receptor 1 polymorphisms with idiopathic membranous nephropathy in Chinese patients in Taiwan

Background

Idiopathic membranous nephropathy (IMN) is one of the most common forms of autoimmune nephritic syndrome in adults. The purpose of this study is to evaluate whether polymorphisms of PLA2R1 affect the development of IMN.

Methods

Taiwanese-Chinese individuals (129 patients with IMN and 106 healthy controls) were enrolled in this study. The selected single nucleotide polymorphisms (SNPs) in PLA2R1 were genotyped by real-time polymerase chain reaction using TaqMan fluorescent probes, and were further confirmed by polymerase chain reaction-restriction fragment length polymorphism. The roles of the SNPs in disease progression were analyzed.

Results

Genotype distribution was significantly different between patients with IMN and controls for PLA2R1 SNP rs35771982 (p = 0.015). The frequency of G allele at rs35771982 was significantly higher in patients with IMN as compared with controls (p = 0.005). In addition, haplotypes of PLA2R1 may be used to predict the risk of IMN (p = 0.004). Haplotype H1 plays a role in an increased risk of IMN while haplotype H3 plays a protective role against this disease. None of these polymorphisms showed a significant and independent influence on the progression of IMN and the risk of end-stage renal failure and death (ESRF/death). High disease progression in patients having C/T genotype at rs6757188 and C/G genotype at rs35771982 were associated with a low rate of remission.

Conclusions

Our results provide new evidence that genetic polymorphisms of PLA2R1 may be the underlying cause of IMN, and the polymorphisms revealed by this study warrant further investigation.

Recurrence of focal segmental glomerulosclerosis in renal allograft: an in-depth review

Focal segmental glomerulosclerosis is a major cause of chronic kidney disease requiring transplantation in children. Recurrence rate in the renal allograft transplantation is as high as 50%. Recurrence of FSGS is associated with renal dysfunction and early graft loss. To date, there is no established therapy for recurrent FSGS after renal transplant. We have reviewed the current English literature in order to summarize current practices with emphasis on graft outcome. We conclude that despite multiple approaches to the post transplant management of recurrent FSGS, none have been shown to be consistently beneficial. Currently, pheresis combined with high dose anti-calcineurin with or without rituximab seems to be the most promising. Further controlled studies are needed to define the optimal therapeutic regimens to treat recurrent of FSGS.

Fibrinogen that appears in Bowman's space of proteinuric kidneys in vivo activates podocyte Toll-like receptors 2 and 4 in vitro

BACKGROUND

Composition of nonselective proteinuria includes several endogenous ligands of Toll-like receptors (TLRs) not normally present in Bowman's space, thus raising the possibility that TLRs are involved in proteinuria-mediated podocyte injury.

METHODS

Kidneys of NEP25 mice, a model of glomerular sclerosis induced by podocyte-specific injury, were immunohistochemically evaluated for the presence of fibrin/fibrinogen, which are potent ligands for TLRs. A podocyte cell line was treated with fibrinogen or lipopolysaccharides and examined for expression of cytokines. siRNAs were used to knockdown components of TLR signaling.

RESULTS

We found deposits of fibrin/fibrinogen only in the damaged podocytes of proteinuric kidneys, indicating that podocytes are exposed to these potent TLR ligands in proteinuric state. In cultured podocytes, we confirmed mRNA expressions of TLR2, TLR4, as well as their major TLR signal transducer, MyD88. Fibrinogen and lipopolysaccharides dose-dependently upregulated mRNA expressions of MCP-1, TNF-alpha and TLR2 in podocytes as well as increased the MCP-1 protein in the medium. Knockdown of TLR2 and TLR4 inhibited the fibrinogen-induced MCP-1 mRNA upregulation. Knockdown of MyD88 also inhibited the upregulation.

CONCLUSION

These results suggest that plasma macromolecules that appear in Bowman's space in proteinuric conditions have the capacity to induce podocyte cytokines through TLRs, and thereby accelerate podocyte injury.

Combination of Immunosuppressive Agents in Treatment of Steroid-Resistant Minimal Change Disease and Primary Focal Segmental Glomerulosclerosis

BACKGROUND

Minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS) are the most prevalent histopathological lesions in idiopathic nephrotic syndrome (INS). The latter is associated with high morbidity and mortality due to symptomatic anasarca, bacterial infections, venous and arterial thromboembolism, and potential progression to end-stage renal disease in the case of FSGS. Traditionally, most patients are treated with corticosteroids, cyclophosphamide (CTX) or calcineurin-inhibitors (C-I). Unfortunately, many patients become steroid or C-I dependent, with the inherent risk of long-term side effects, or are resistant to both. The aim of this paper is to report on our experience with a new protocol of a combination of immunosuppressive agents added sequentially to improve the response of steroid and C-I refractory or resistant-INS and to minimize the long-term side-effects of single-agent treatment.

METHODS

Twenty-one patients with corticosteroid-resistant and C-I refractory INS (6 with MCD and 15 with FSGS) were treated prospectively over 6 and a half years. Our protocol consisted of an initial regimen of C-I followed by the addition of mycophenolate mofetil (MMF) and then by monthly intravenous CTX for 3 consecutive months. Dose reduction of C-I or/and MMF was attempted afterwards at 4-months intervals. Patients who remained refractory to the previously mentioned protocol were treated with an additional course of pulse Solu-Medrol given for 3 days followed by oral corticosteroids tapered over 6 months in addition to a second course of intravenous CTX given for 3 consecutive months.

RESULTS

With the initial regimen, two patients with MCD, remained in complete remission (CR) without any therapy after the course of CTX. Fifteen patients had variable response to C-I and MMF, but they achieved CR after CTX and their initial dosage of C-I and MMF were reduced to nearly one half. The remaining four patients had refractory form of FSGS even after the initial regimen, yet responded with CR to the additional course of steroid/CTX. However, no success with dose-reduction, in C-I and MMF, was achieved in the latter four patients.

CONCLUSION

Our study represents the first clinical trial with prospective and adequate follow-up of combination therapy of immunosuppressive agents in INS. This method is effective and safe for treatment of patients who are refractory to the conventional single-agent therapy.

Epidermal growth factor activates Na(+/)H(+) exchanger in podocytes through a mechanism that involves Janus kinase and calmodulin

Sodium-proton exchanger type 1 (NHE-1) is ubiquitously expressed, is activated by numerous growth factors, and plays significant roles in regulating intracellular pH and cellular volume, proliferation and cytoskeleton. Despite its importance, little is known about its regulation in renal glomerular podocytes. In the current work, we studied the regulation of NHE-1 activity by the epidermal growth factor receptor (EGFR) in cultured podocytes. RT-PCR demonstrated mRNAs for NHE-1 and NHE-2 in differentiated podocytes, as well as for EGFR subunits EGFR/ErbB1, Erb3, and ErbB4. EGF induced concentration-dependent increases in proton efflux in renal podocytes as assessed using a Cytosensor microphysiometer, were diminished in the presence of 5-(N-methyl-N-isobutyl) amiloride or in a sodium-free solution. Furthermore, pharmacological inhibitors of Janus kinase (Jak2) and calmodulin (CaM) attenuated EGF-induced NHE-1 activity. Co-immunoprecipitation studies determined that EGF induced formation of complexes between Jak2 and CaM, as well as between CaM and NHE-1. In addition, EGF increased levels of tyrosine phosphorylation of Jak2 and CaM. The EGFR kinase inhibitor, AG1478, blocked activation of NHE-1, but did not block EGF-induced phosphorylation of Jak2 or CaM. These results suggest that EGF induces NHE-1 activity in podocytes through two pathways: (1) EGF-->EGFR-->Jak2 activation (independent of EGFR tyrosine kinase activity)-->tyrosine phosphorylation of CaM-->CaM binding to NHE-1-->conformational change of NHE-1-->activation of NHE-1; and (2) EGF-->EGFR-->EGFR kinase activation-->association of CaM with NHE-1 (independent of Jak2)-->conformational change of NHE-1-->activation of NHE-1.

Lessons from studies on focal segmental glomerulosclerosis: an important role for parietal epithelial cells?

Glomerular diseases are caused by multiple mechanisms. Progressive glomerular injury is characterized by the development of segmental or global glomerulosclerosis independent of the nature of the underlying renal disease. Most studies on glomerular disease focus on the constituents of the filtration barrier (podocytes, glomerular basement membrane (GBM), endothelial cells) or the mesangial cells. Little attention is given to the epithelial cells lining Bowman's capsule, the so called parietal epithelial cells (PECs). This 'lack of attention' is partly explained by the presumed 'passive' function of PECs, which are large, flattened cells that cover Bowman's capsule in a single cell layer and form a barrier between the ultrafiltrate and the periglomerular interstitium, in normal glomerular physiology. A more important reason has been the lack of an established primary role for the parietal epithelium in glomerular diseases. However, in recent years, several studies have demonstrated that PECs are involved in extracapillary proliferation. In addition, PECs can become highly active, proliferating cells, expressing many growth factors, chemokines, cytokines, and their receptors. It was recently demonstrated that PECs also play a part in the development of focal segmental glomerulosclerosis (FSGS). This review summarises current knowledge of the PEC, with emphasis on the role of PECs in the development of FSGS.

FSGS-associated alpha-actinin-4 (K256E) impairs cytoskeletal dynamics in podocytes

Mutations in the ACTN4 gene, encoding the actin crosslinking protein alpha-actinin-4, are associated with a familial form of focal segmental glomerulosclerosis (FSGS). Mice with podocyte-specific expression of K256E alpha-actinin-4 develop foot process effacement and glomerulosclerosis, highlighting the importance of the cytoskeleton in podocyte structure and function. K256E alpha-actinin-4 exhibits increased affinity for F-actin. However, the downstream effects of this aberrant binding on podocyte dynamics remain unclear. Wild-type and K256E alpha-actinin-4 were expressed in cultured podocytes via adenoviral infection to determine the effect of the mutation on alpha-actinin-4 subcellular localization and on cytoskeletal-dependent processes such as adhesion, spreading, migration, and formation of foot process-like peripheral projections. Wild-type alpha-actinin-4 was detected primarily in the Triton-soluble fraction of podocyte lysates and localized to membrane-associated cortical actin and focal adhesions, with some expression along stress fibers. Conversely, K256E alpha-actinin-4 was detected predominantly in the Triton-insoluble fraction, was excluded from cortical actin, and localized almost exclusively along stress fibers. Both wild-type and K256E alpha-actinin-4-expressing podocytes adhered equally to an extracellular matrix (collagen-I). However, podocytes expressing K256E alpha-actinin-4 showed a reduced ability to spread and migrate on collagen-I. Lastly, K256E alpha-actinin-4 expression reduced the mean number of actin-rich peripheral projections. Our data suggest that aberrant sequestering of K256E alpha-actinin-4 impairs podocyte spreading, motility, and reduces the number of peripheral projections. Such intrinsic cytoskeletal derangements may underlie initial podocyte damage and foot process effacement encountered in ACTN4-associated FSGS.

Protection of transplant-induced renal ischemia-reperfusion injury with carbon monoxide

Carbon monoxide (CO), a product of heme metabolism by heme oxygenases, is known to impart protection against oxidative stress. We hypothesized that CO would protect ischemia-reperfusion (I/R) injury of transplanted organs, and the efficacy of CO was studied in the rat kidney transplantation model. A Lewis rat kidney graft, preserved in University of Wisconsin solution at 4 degrees C for 24 h, was orthotopically transplanted into syngeneic rats. Recipients were maintained in room air or exposed to CO (250 ppm) in air for 1 h before and 24 h after transplantation. Animals were killed 1, 3, 6, and 24 h after transplantation to assess efficacy of inhaled CO. Rapid upregulation of mRNA for IL-6, IL-1beta, TNF-alpha, ICAM-1, heme oxygenase-1, and inducible nitric oxide synthase was observed within 3 h after transplantation in the control grafts of air-exposed recipients, associating with histopathological evidences of acute tubular necrosis, interstitial hemorrhage, and edema. In contrast, the increase of inflammatory mediators was markedly inhibited in kidney grafts of CO-treated recipients, which correlated with improved renal cortical blood flow. Further detailed morphological analyses revealed that CO preserved the glomerular vascular architecture and podocyte viability with less apoptosis of tubular epithelial cells and less ED1(+) macrophage infiltration. CO inhalation resulted in improved serum creatinine levels and clearance, and animal survival was significantly improved with CO to 60.5 from 25 days in untreated controls. The study demonstrates that exposure of kidney graft recipients to CO at a low concentration can impart significant protective effects against renal I/R injury and improve function of renal grafts.

Eluate derived by extracorporal antibody-based immunoadsorption elevates the cytosolic Ca2+ concentration in podocytes via B2 kinin receptors

BACKGROUND/AIM

Patients with idiopathic focal segmental glomerulosclerosis (FSGS) often develop a recurrence of the disease after kidney transplantation. In a number of FSGS patients, plasmapheresis and immunoadsorption procedures have been shown to transiently reduce proteinuria and are thought to do this by eliminating a circulating factor. Direct cellular effects of eluates from immunoadsorption procedures on podocytes, the primary target of injury in FSGS, have not yet been reported.

METHODS

Eluates were derived from antibody-based immunoadsorption of a patient suffering from primary FSGS, a patient with systemic lupus erythematosus, and a healthy volunteer. The cytosolic free Ca(2+) concentration ([Ca(2+)](i)) of differentiated podocytes was measured by single-cell fura-2 microfluorescence measurements. Free and total immunoreactive kinin levels were measured by radioimmunoassay.

RESULTS

FSGS eluates increased the [Ca(2+)](i) levels concentration dependently (EC(50) 0.14 mg/ml; n = 3-19). 1 mg/ml eluate increased the [Ca(2+)](i) values reversibly from 82 +/- 12 to 1,462 +/- 370 nmol/l, and then they returned back to 100 +/- 16 nmol/l (n = 19). The eluate-induced increase of [Ca(2+)](i) consisted of an initial Ca(2+) peak followed by a Ca(2+) plateau which depended on the extracellular Ca(2+) concentration. The eluate-induced increase of [Ca(2+)](i) was inhibited by the specific B(2) kinin receptor antagonist Hoe 140 in a concentration-dependent manner (IC(50) 2.47 nmol/l). In addition, prior repetitive application of bradykinin desensitized the effect of eluate on [Ca(2+)](i). A colonic epithelial cell line not reacting to bradykinin did not respond to eluate either (n = 6). Similar to FSGS eluates, the eluate preparations of both the systemic lupus patient and the healthy volunteer led to a biphasic, concentration-dependent [Ca(2+)](i) increase in podocytes which again was inhibited by Hoe 140. Free kinins were detected in all eluate preparations.

CONCLUSION

The procedure of antibody-based immunoadsorption leads to kinin in the eluate which elevates the [Ca(2+)](i) level of podocytes via B(2) kinin receptors.

E pluribus unum: The riddle of focal segmental glomerulosclerosis

A recent consensus conference proposed a new classification for focal segmental glomerulosclerosis (FSGS). Five patterns have been defined: FSGS not otherwise specified, perihilar variant, cellular variant, tip variant, and collapsing variant. In light of the multiplicity of classification schemes in use, the promise of a rational and uniform scheme for FSGS pathology is most welcome. This approach has worked extremely well for the classification of lupus nephritis. It does not necessarily mean, however, that this new classification scheme will help to select treatment protocols according to histopathologic subsets of FSGS. In fact, one renal biopsy examination may show multiple variants and this classification, despite many merits, still lumps categories that should be split and splits categories that should be lumped together. It has become clear that despite its histologic diversity FSGS begins as a podocyte disease that progresses from a cellular to a scar lesion. Recent years have brought about astonishing insight into the complex molecular array of proteins forming the slit diaphragm between podocyte foot processes, a narrow space essential for restricting glomerular permeability to albumin. Concentrating on the podocyte rather than on the glomerular tuft is helpful for abolishing the classic distinction between primary versus secondary forms of FSGS, a distinction that crumbles away with each new evidence of genetic, ischemic, or viral etiologies of FSGS, despite similar lesions. In fact, recent studies focusing on the podocyte changes that occur in various subsets of FSGS have unraveled the striking phenomena of podocyte dedifferentiation and transdifferentiation along with differential expression of cyclin-dependent kinase inhibitors. Interestingly, the latter showed that expression of cyclin-dependent kinase inhibitors p21 and proliferation marker Ki-67 are the same in cellular FSGS, collapsing glomerulopathy, and human immunodeficiency virus-associated FSGS. Taken together these findings lead to a reassuring unitary interpretation of the pluralistic appearance of FSGS by histopathology. Clearly, further studies of the podocyte will lead to improved understanding of FSGS and to improved classification schemes that are grounded in molecular understanding of glomerular injury and that will guide the clinician in the choice of treatment and prognosis.

HIV-1 Nef induces proliferation and anchorage-independent growth in podocytes

HIV-associated nephropathy (HIVAN) is now the third leading cause of end-stage renal disease in the African American population. HIV-1 infects renal tubular and glomerular epithelial cells or podocytes, cells that are a critical part of the filtration barrier. HIV-1 infection induces the loss of podocyte differentiation markers and increases podocyte proliferation. It has been previously shown that HIV-infection induces loss of contact inhibition. Here, the HIV-1 gene responsible for proliferative changes is identified by using cultured podocytes in vitro. The HIV-1 proviral construct, pNL4-3 was rendered noninfectious by replacing the HIV-1 gag/pol sequences with an EGFP reporter gene (pNL4-3: DeltaG/P-EGFP). This construct was then pseudotyped with VSV.G envelope to infect podocytes that were conditionally immortalized with SV-40 T antigen. In addition, mutated constructs were engineered with premature stop codons in the HIV-1 env, vif, vpr, vpu, nef, or rev genes. The parental construct and all the other mutated constructs, with the exception of nef, induced proliferation under nonpermissive conditions and anchorage-independent growth (colony formation in soft agar) under permissive conditions. In contrast, deletion of nef markedly reduced proliferation and colony formation. Although tat alone, or tat plus rev induced marginal levels of anchorage-independent growth, coexpression with nef significantly increased colony formation. Finally, stable expression of Nef in a retroviral vector, pBabe-puro, was sufficient to induce increased proliferation and colony formation. Moreover, nef induced saturation density and loss of contact inhibition. These data indicate that Nef induces multiple proliferative effects in podocytes in culture and that nef may therefore be an important gene in the pathogenesis of HIVAN in vivo.

Carolina rinse solution minimizes kidney injury and improves graft function and survival after prolonged cold ischemia

BACKGROUND

Kidney damage caused by cold ischemia-reperfusion injury promotes adverse outcomes after renal transplantation. The purpose of this study was to determine whether Carolina rinse solution (CRS) used at the end of cold ischemic storage decreases kidney injury and improves graft function and survival.

METHODS

Inbred male Lewis rats were used as donors and recipients. Left kidneys were removed from donor rats, infused with cold University of Wisconsin solution, and stored for 24, 30, or 48 hr at 0-1 degrees C. Just before implantation, kidneys were flushed with either Ringer's solution or CRS at 35-37 degrees C or were not treated. Kidneys were then transplanted into recipient rats with removal of both native kidneys.

RESULTS

Survival and renal function were analyzed over a 14-day postoperative period. Among rats receiving kidneys after 24-hr cold storage, creatinine clearance was 75% greater in rats transplanted with kidneys flushed with CRS compared with Ringer's solution or nontreatment. In animals receiving kidneys after 30-hr cold storage, recipient survival after CRS was significantly higher than with Ringer's solution or without flushing (80% vs. 25% and 17%, respectively). However, CRS failed to prevent renal graft failure after 48 hr of cold storage (14% survival with CRS vs. 0% with Ringer's solution). In separate ex vivo studies, nonviable cell nuclei were labeled by trypan blue after cold preservation and brief warm reperfusion. CRS decreased podocyte and peritubular endothelial cell killing associated with cold ischemia-reperfusion injury.

CONCLUSION

Flushing renal explants with warm CRS before implantation diminishes cold ischemia-reperfusion injury and improves the function and survival of transplanted kidneys.

Modulation of podocyte phenotype in collapsing glomerulopathies

Podocytes are well-differentiated postmitotic cells whose function is largely based on their complex cytoskeletal architecture. In diseases with proteinuria, podocytes undergo morphologic changes. Podocytes react to an injurious stimulus by a reorganization of their foot process architecture that is independent of the primary injury and the cause of the proteinuria. Collapsing glomerulopathies, including the idiopathic and secondary forms due to HIV infection, have been previously considered a part of the focal sclerosing glomerulosclerosis (FSGS) spectrum. However, in contrast to FSGS, both forms of collapsing glomerulopathy are characterized by segmental and global collapse of the glomerular basement membrane (GBM) and by characteristic ultrastructural alterations in podocytes. These alterations include loss of the actin-based cytoskeleton, a dysregulated/dedifferentiated phenotype, cellular hypertrophy, and cell proliferation. These observations raise the following questions: 1) What mechanism causes glomerular collapse and do podocytes have a role? We recently proposed that in collapsing glomerulopathies the composition of the GBM is altered and contains more immature forms of collagen IV. These observations suggest that dedifferentiated/dysregulated podocytes may participate in remodeling the GBM composition, producing fetal collagen isoforms. 2) What is the pathomechanism underlying podocyte dysregulation? Although it is still unclear which etiologic factors are responsible for the idiopathic forms of collapsing glomerulopathy, in situ hybridization studies in a transgenic mouse model of HIV-associated collapsing glomerulopathy and on renal biopsies of patients with HIV-associated collapsing glomerulopathy demonstrated the presence of the HIV-1 RNA in podocytes and tubular epithelial cells. These findings suggest a direct link between viral gene expression and the dysregulation of the podocyte phenotype. 3) Another open question is how podocytes become infected in HIV-associated collapsing glomerulopathy. HIV-1 typically uses CD4 and a co-receptor such as CCR5 or CXCR4 to enter cells. So far, there is no demonstration of the expression of these receptors in podocytes. These negative findings, however, do not exclude the possibility that in the kidney another, CD4 independent, co-receptor may be used for viral cell entry. Finally, is it important to mention that collapsing glomerulopathies have a high prevalence in black patients, suggesting a link between racial background and the virus-related podocyte injury.

Different patterns of renal cell killing after warm and cold ischemia

Kidneys preserved for transplantation surgery sustain injuries caused by cold ischemia during storage. Additionally, kidneys harvested from non-heart-beating donors encounter the stress of warm ischemia. The aim of this study was to determine the specific cell types losing viability after warm and cold ischemia. In warm ischemia studies, the pedicles of left kidneys of Lewis rats were cross-clamped for up to 90 min. In cold ischemia studies, kidneys were flushed with cold University of Wisconsin solution and stored up to 48h at 0-1 degrees C. After warm or cold ischemia, kidneys were perfused via the renal arteries with Krebs-Henseleit bicarbonate (KHB) buffer at 37 degrees C, followed by trypan blue to label the nuclei of nonviable cells. Warm ischemia for 90 min caused renal failure and led to injury of proximal tubular cells, e.g., loss of brush borders, cast formation and trypan blue labeling. Cold ischemia for 48 h also caused renal failure but, unlike warm ischemia, caused trypan blue labeling of glomerular podocytes and peritubular endothelial cells. In warm ischemia-induced injury, electron microscopy showed shedding of microvilli and marked swelling of proximal tubular cells, microvilli and mitochondria. In cold ischemia-induced injury, podocytes were blebbed and swollen, and their pedicels were detached from the basement membrane, but disruption in proximal tubules was milder. In conclusion, warm ischemia triggers injury primarily to proximal tubular cells, whereas cold ischemia damages glomerular podocytes and peritubular endothelial cells in addition to proximal tubules.