The actin cytoskeleton of kidney podocytes is a direct target of the antiproteinuric effect of cyclosporine A

Management of idiopathic membranous nephropathy

IMPORTANCE OF THE FIELD

Idiopathic membranous nephropathy (IMN) can have a variable natural course. Treatments able to induce remission can improve the long-term prognosis. However, the optimal therapy for IMN remains controversial.

AREA COVERED IN THIS REVIEW

We reviewed the historical and current literature from 1979 to 2010 regarding the natural course of IMN and the possible treatments giving special emphasis to randomized controlled trials and to more recent approaches.

WHAT THE READER WILL GAIN

The reader will gain a comprehensive review of the available treatments of IMN. A personal therapeutic algorithm for nephrotic patients with IMN is also provided.

TAKE HOME MESSAGE

At least five different treatments showed efficacy in many (but not all) patients with IMN.

Dystroglycan does not contribute significantly to kidney development or function, in health or after injury

Dystroglycan (DG or DAG1) is considered a critical link between the basement membrane and the cytoskeleton in multiple tissues. DG consists of two subunits, an extracellular α-subunit that binds laminin and other basement membrane components, and a transmembrane β-subunit. DG-null mouse embryos die during early embryogenesis because DG is required for Reichert's membrane formation. DG also forms an integral part of the dystrophin-glycoprotein complex in muscle. Although no human DG mutations have been reported, multiple forms of muscular dystrophy have been linked to DG glycosylation defects, and targeted deletion of muscle DG causes muscular dystrophy in mice. Moreover, DG is widely distributed in endothelial and epithelial cells, including those in the kidney. There has therefore been significant interest in DG's role in the kidney, especially in podocytes. Previous reports suggested that DG's disturbance in podocytes might cause glomerular filtration barrier abnormalities. To fully understand DG's contribution to nephrogenesis and kidney function, we used a conditional DG allele and a variety of Cre mice to systematically delete DG from podocytes, ureteric bud, metanephric mesenchyme, and then from the whole kidney. Surprisingly, none of these conditional deletions resulted in significant morphological or functional abnormalities in the kidney. Furthermore, DG-deficient podocytes did not show increased susceptibility to injury, and DG-deficient kidneys did not show delayed recovery. Integrins are therefore likely the primary extracellular matrix receptors in renal epithelia.

Effect of cyclosporin A on proteinuria in the course of glomerulopathy associated with WT1 mutations

Denys-Drash syndrome (DDS) is characterized by progressive glomerulopathy caused by diffuse mesangial sclerosis (DMS), genitourinary defects, and a higher risk of developing Wilms' tumor. It is commonly assumed that the DMS is unresponsive to any medications. In this report, we present a patient with Denys-Drash syndrome, in whom the cyclosporine A (CsA) was found to induce total remission. This observation and observations of other authors confirm that in genetic forms of nephrotic syndrome, the proteinuric effect of CsA may be due to a non-immunologic mechanism. We confirm the beneficial effect of CsA treatment in DDS; however, the potential nephrotoxicity of this drug will probably not allow long-term use.

A case of membranous nephropathy with primary biliary cirrhosis and cyclosporine-induced remission

We report a case of membranous nephropathy with primary biliary cirrhosis (PBC). A 76-year-old Japanese man had been treated for PBC and was referred to our department because of acute severe proteinuria (10 g/day). Renal biopsy was performed and was compatible with a diagnosis of membranous nephropathy. The patient was treated with cyclosporine A (CsA; Neoral®), which was followed by normalization of liver function and partial remission of proteinuria (0.68 g/day) within six months without any significant side effects. This case suggests that CsA monotherapy is effective for the treatment of both liver dysfunction and membranous nephropathy associated with PBC.

A novel mutation in NPHS2 causing nephrotic syndrome in a Saudi Arabian family

We report a consanguineous family from Saudi Arabia with three affected children presenting with infantile nephrotic syndrome. In order to provide a molecular diagnosis, a genome-wide SNP analysis of the affected patients was performed. We identified a region of homozygosity on chromosome 1, containing the NPHS2 gene. Direct sequencing, by exon PCR, of NPHS2 identified a homozygous nucleotide change 385C > T within exon 3 in the three affected children, leading to a premature stop codon (Q129X). This homozygous truncating mutation in NPHS2 is novel and was associated with a severe clinical phenotype. Additional mutations in related genes NPHS1, PLCE1 and NEPH1 were not identified, excluding tri-allelism within these genes in this family.

Good outcomes with mycophenolate-cyclosporine-based induction protocol in children with severe proliferative lupus nephritis

The outcomes of children with severe proliferative lupus nephritis (LN) were examined using a new mycophenolate and cyclosporine-based (MMF-CSA) induction protocol. Sixteen children with LN (WHO class III and IV), 31.3% of whom required dialysis at induction, were retrospectively studied. Median MMF dose was 942 mg/m( 2)/day. Thirteen patients (81%) with persistent proteinuria received CSA. Clinical and laboratory parameters were compared at pre-induction, 6 and 12 months. Treatment outcome was defined by Systemic Lupus Erythematosus Disease Activity Index (SLEDAI), renal function, haematuria, proteinuria and serological markers (complements C3, C4 and anti-dsDNA). Comparing these parameters at induction, 6 months and 12 months, respectively, SLEDAI (25.4 +/- 8.7 versus 3.2 +/- 2.9 versus 2.9 +/- 2.8), serum C3 (47 +/- 21 versus 107 +/- 27 versus 111 +/- 38 mg/dl), C4 (12 +/- 14 versus 23 +/- 14 versus 22 +/- 11 mg/dl) and urine protein (6.97 +/- 7.09 versus 0.98 +/- 1.56 versus 0.21 +/- 0.13 g/ day/1. 73 m(2)) improved significantly (p < 0.05). Anti-dsDNA titres decreased in 73% by 6 and 12 months (p < 0.05). Complete renal remission was achieved in 7/16 (43.8%) at 6 months and 12/16 (75%) at 12 months, the rest achieving partial remission with no treatment failures. In conclusion, a combination MMF-CSA protocol is an effective therapeutic alternative for induction of children with severe proliferative LN, resulting in significant clinical and serological improvement with minimal adverse effects.

Tacrolimus is an alternative therapeutic option for the treatment of refractory lupus nephritis

We conducted an open-labeled, prospective study to determine the efficacy and safety of tacrolimus as an alternative therapeutic option for those patients with refractory lupus nephritis. The study population comprised one male and eight female patients with diffuse proliferative lupus nephritis. All patients had failed to respond to sufficient intravenous cyclophosphamide therapy with proteinuria of >or=1 g/day and active urinary sediments. Tacrolimus (0.1 mg/kg/day) was administered for 1 year with adjusting drug level (4-10 microg/l). The mean serum creatinine level and spot urine protein creatinine ratio (UPCR) at baseline were 1.39 mg/dl and 2.27, respectively. After the treatment, proteinuria reduced significantly from median UPCR value of 2.19 (range, 1.19-3.34) to 0.44 (range, 0.12-2.13) (p < 0.05). Seven (78%) of the nine patients showed a complete clinical response, which was defined as stabilization in the disease-activity markers and serum creatinine level with reduction of >or=50% in UPCR; two patients showed complete remission with UPCR <0.2. One patient showed treatment failure because of the disease progression. No serious adverse effects were observed during the study. This study demonstrates that tacrolimus can show a significant therapeutic response in cases that are refractory to the standard regimen for diffuse proliferative lupus nephritis.

Posttransplant recurrence of primary glomerulonephritis

All forms of primary GN may recur after kidney transplantation and potentially jeopardize the survival of the graft. IgA nephritis (IgAN) may recur in approximately one third of patients, more frequently in younger patients and in those with a rapid progression of the original disease. However, with the exception of few patients with rapid progression, there is no evidence that recurrence of IgAN has a deleterious effect on graft survival at least up to 10 years. Recurrence of focal segmental glomerulosclerosis (FSGS) is often associated with nephrotic proteinuria and is more frequent in children, in patients with rapid progression of the original disease, and in those who lost a previous transplant from recurrence. The natural course of recurrent FSGS is usually unfavorable. Early and intensive plasmapheresis may obtain complete or partial response in several patients. Good results have also been reported with rituximab. Idiopathic membranous nephropathy (IMN) may recur in 30% to 40% of patients. The graft survival in patients with IMN is not different than that of patients with other renal diseases. Good results with rituximab have been reported. Membranoproliferative GN (MPGN) may recur in 27% to 65% of patients. The recurrence is more frequent and the prognosis is more severe in type II MPGN. Although recurrent GN is relatively frequent and may worsen the outcome of renal allografts in some patients, its effect is diluted by several other risk-factors that may have a greater effect than recurrent GN on the long-term graft survival.

Antagonistic regulation of actin dynamics and cell motility by TRPC5 and TRPC6 channels

The Rho family of small guanosine triphosphatases (Rho GTPases: RhoA, Cdc42, and Rac1) regulates many aspects of cell behavior, including actin dynamics and cell migration. The generation of calcium ion (Ca(2+)) microdomains is critical in promoting cell migration because they control the localized activity of Rho GTPases. We identified receptor-activated TRPC5 and TRPC6 (transient receptor potential canonical type 5 and 6) channels as antagonistic regulators of actin remodeling and cell motility in fibroblasts and kidney podocytes. We show that TRPC5 is in a molecular complex with Rac1, whereas TRPC6 is in a molecular complex with RhoA. TRPC5-mediated Ca(2+) influx induces Rac1 activation, thereby promoting cell migration, whereas TRPC6-mediated Ca(2+) influx increases RhoA activity, thereby inhibiting cell migration. Our data unveil antagonistic Ca(2+) influx pathways as a conserved signaling mechanism for the integrated regulation of cell migration.

Circulating permeability factors in idiopathic nephrotic syndrome and focal segmental glomerulosclerosis

Circulating permeability factors may be important in idiopathic nephrotic syndrome (INS) including focal segmental glomerulosclerosis (FSGS) and in recurrence after renal transplantation. Evidence for plasma factors includes posttransplant recurrence of proteinuria and its response to plasmapheresis or immunoadsorption and induction of proteinuria in experimental animals by infusion of patient plasma or its fractions. The authors and other investigators have used proteomic techniques to seek pathogenic molecules. The authors have recently proposed cardiotrophin-like cytokine-1 (CLC-1) as an active factor in FSGS. Other potential permeability factors include hemopexin and vascular permeability factor in minimal change nephrotic syndrome (MCNS) and soluble urokinase receptor in FSGS. In the authors' studies, in vitro plasma permeability activity is blocked by diverse substances that may decrease levels of active molecules or block the effects of circulating permeability factors. It has been shown that the simple sugar galactose blocks the effect of FSGS serum on albumin permeability in vitro and decreases permeability activity when administered to patients. Because identities of permeability factors and their mechanisms of action are not well defined, therapy of INS/FSGS is empiric. Corticosteroids are the mainstay of initial therapy whereas calcineurin inhibitors such as cyclosporine A (CsA) and immunosuppressive medications provide adjunctive therapy. Nonspecific therapies such as blocking the renin-angiotensin system and controlling blood pressure and plasma lipids may also diminish proteinuria and slow progression. Identification of molecules that initiate proteinuria and application of findings from in vitro studies may lead to development of new treatments to arrest progression and prevent recurrence after transplantation.

The podocyte as a direct target of immunosuppressive agents

Podocytes play a key role in maintaining the blood-urine barrier for high-molecular-weight proteins. They are considered to be terminally differentiated, and podocyte loss cannot be compensated by regenerative proliferation. Various diseases leading to podocyte damage and loss result in proteinuria and cause nephrotic syndrome. Therefore, direct therapeutical strategies to protect podocytes in disease situations are a logical concept to prevent disease or to delay disease progression. Acquired podocytopathies like idiopathic focal segmental glomerulosclerosis and minimal change disease are historically considered as immunological diseases. Therefore, immunosuppressive agents such as steroids and calcineurin inhibitors are the commonly used treatment strategies. However, the causative disease mechanisms behind these treatment strategies remain elusive. Recent evidence shows that immunosuppressive agents, in addition to the effect on the immune system, directly influence the unique structure and function of podocytes. In this context, the actin cytoskeleton of the podocyte and cytokines such as vascular endothelial growth factor play a pivotal role. In this review, we summarize the direct effects on podocytes obtained in vivo and in vitro after treatment with calcineurin inhibitors, mTOR inhibitors and glucocorticoids. These direct effects could play a key role in the treatment concepts of podocytopathies with an important impact on the long-term renal function in patients with pharmacological immunosuppression.

Specialized roles for cysteine cathepsins in health and disease

Cathepsins were originally identified as proteases that act in the lysosome. Recent work has uncovered nontraditional roles for cathepsins in the extracellular space as well as in the cytosol and nucleus. There is strong evidence that subspecialized and compartmentalized cathepsins participate in many physiologic and pathophysiologic cellular processes, in which they can act as both digestive and regulatory proteases. In this review, we discuss the transcriptional and translational control of cathepsin expression, the regulation of intracellular sorting of cathepsins, and the structural basis of cathepsin activation and inhibition. In particular, we highlight the emerging roles of various cathepsin forms in disease, particularly those of the cardiac and renal systems.

Frasier syndrome: early gonadoblastoma and cyclosporine responsiveness

Frasier syndrome is characterized by progressive glomerulopathy that is unresponsive to corticosteroids, male pseudohermaphroditism, and an increased risk of genitourinary tumors. Of 21 girls with steroid-resistant nephrotic syndrome secondary to focal segmental glomerulosclerosis (FSGS) who were screened for mutations in the WT1 gene, two showed Frasier syndrome. Both patients had donor splice-site mutations in intron 9 of the WT1 gene and a male karyotype (46, XY). Long-term therapy with cyclosporine resulted in partial remission in both cases. One patient showed foci of gonadoblastoma in the excised dysgenetic gonads. This report highlights the need for screening for mutations in the WT1 gene in girls with steroid-resistant FSGS. Patients with Frasier syndrome might benefit from early gonadectomy.

Podocyte-specific overexpression of wild type or mutant trpc6 in mice is sufficient to cause glomerular disease

Mutations in the TRPC6 calcium channel (Transient receptor potential channel 6) gene have been associated with familiar forms of Focal and Segmental Glomerulosclerosis (FSGS) affecting children and adults. In addition, acquired glomerular diseases are associated with increased expression levels of TRPC6. However, the exact role of TRPC6 in the pathogenesis of FSGS remains to be elucidated. In this work we describe the generation and phenotypic characterization of three different transgenic mouse lines with podocyte-specific overexpression of the wild type or any of two mutant forms of Trpc6 (P111Q and E896K) previously related to FSGS. Consistent with the human phenotype a non-nephrotic range of albuminuria was detectable in almost all transgenic lines. The histological analysis demonstrated that the transgenic mice developed a kidney disease similar to human FSGS. Differences of 2-3 folds in the presence of glomerular lesions were found between the non transgenic and transgenic mice expressing Trpc6 in its wild type or mutant forms specifically in podocytes. Electron microscopy of glomerulus from transgenic mice showed extensive podocyte foot process effacement. We conclude that overexpression of Trpc6 (wild type or mutated) in podocytes is sufficient to cause a kidney disease consistent with FSGS. Our results contribute to reinforce the central role of podocytes in the etiology of FSGS. These mice constitute an important new model in which to study future therapies and outcomes of this complex disease.

Immunosuppression and renal outcome in congenital and pediatric steroid-resistant nephrotic syndrome

BACKGROUND AND OBJECTIVES

Mutations in podocyte genes are associated with steroid-resistant nephrotic syndrome (SRNS), mostly affecting younger age groups. To date, it is unclear whether these patients benefit from intensified immunosuppression with cyclosporine A (CsA). The aim of this study was to evaluate the influence of podocyte gene defects in congenital nephrotic syndrome (CNS) and pediatric SRNS on the efficacy of CsA therapy and preservation of renal function.

DESIGN, SETTINGS, PARTICIPANTS, & MEASUREMENTS

Genotyping was performed in 91 CNS/SRNS patients, irrespective of age at manifestation or response to CsA.

RESULTS

Mutations were identified in 52% of families (11 NPHS1, 17 NPHS2, 11 WT1, 1 LAMB2, 3 TRPC6). Sixty-eight percent of patients with nongenetic SRNS responded to CsA, most of them achieved complete remission. In contrast, none of the patients with genetic CNS/SRNS experienced a complete remission and only two (17%) achieved a partial response, both affected by a WT1 mutation. Preservation of renal function was significantly better in children with nongenetic disease after a mean follow-up time of 8.6 years (ESRD in 29% versus 71%).

CONCLUSIONS

The mutation detection rate in our population was high (52%). Most patients with genetic CNS/SRNS did not benefit from CsA with significantly lower response rates compared with nongenetic patients and showed rapid progression to end-stage renal failure. These data strongly support the idea not to expose CNS/SRNS patients with inherited defects related to podocyte function to intensified immunosuppression with CsA.

Oral cyclophosphamide is on the verge of extinction as therapy for severe autoimmune diseases (especially lupus): should nephrologists care?

Some day we will have powerful targeted therapies for autoimmune diseases. Remission will be induced efficiently. Side effects will be mere ripples. Unfortunately, that day is not imminent. Current therapies are powerful but with unintended targets and side effects that can be equivalent to a sea change. For SLE, the current competition to select the 'gold standard' immunosuppressant has come down to two regimens: intravenous cyclophosphamide (IVCY, standard NIH protocol or its variations) versus oral mycophenolate (MMF). Until recently, IVCY reigned as the gold standard, a title it achieved through a curious journey that did not involve rigorous head-to-head competition. Oral cyclophosphamide (POCY) has not been invited to the current competition to select the gold standard immunosuppressant despite the substantial evidence that POCY can perform at least as well as IVCY or mycophenolate, and compared to IVCY, is far less expensive, easier for the patient, and maybe more effective in African-Americans. Here, we state the case for POCY as therapy for severe autoimmune diseases. We suggest that if POCY is allowed to compete, it will not disappoint.

Establishment of protein delivery systems targeting podocytes

Background

Podocytes are uniquely structured cells that are critical to the kidney filtration barrier. Their anatomic location on the outer side of the glomerular capillaries expose podocytes to large quantities of both plasma and urinary components and thus are reachable for drug delivery. Recent years have made clear that interference with podocyte-specific disease pathways can modulate glomerular function and influence severity and progression of glomerular disease.

Methodology/Principal Findings

Here, we describe studies that show efficient transport of proteins into the mammalian cells mouse 3T3 fibroblasts and podocytes, utilizing an approach termed profection. We are using synthetic lipid structures that allow the safe packing of proteins or antibodies resulting in the subsequent delivery of protein into the cell. The uptake of lipid coated protein is facilitated by the intrinsic characteristic of cells such as podocytes to engulf particles that are physiologically retained in the extracellular matrix. Profection of the restriction enzyme MunI in 3T3 mouse fibroblasts caused an increase in DNA degradation. Moreover, purified proteins such as β-galactosidase and the large GTPase dynamin could be profected into podocytes using two different profection reagents with the success rate of 95–100%. The delivered β-galactosidase enzyme was properly folded and able to cleave its substrate X-gal in podocytes. Diseased podocytes are also potential recipients of protein cargo as we also delivered fluorophore labeled IgG into puromycin treated podocytes. We are currently optimizing our protocol for in vivo profection.

Conclusions

Protein transfer is developing as an exciting tool to study and target highly differentiated cells such as podocytes.

Activation of NFAT signaling in podocytes causes glomerulosclerosis

Mutant forms of TRPC6 can activate NFAT-dependent transcription in vitro via calcium influx and activation of calcineurin. The same TRPC6 mutants can cause FSGS, but whether this involves an NFAT-dependent mechanism is unknown. Here, we generated mice that allow conditional induction of NFATc1. Mice with NFAT activation in nascent podocytes in utero developed proteinuria and glomerulosclerosis postnatally, resembling FSGS. NFAT activation in adult mice also caused progressive proteinuria and FSGS. Ultrastructural studies revealed podocyte foot process effacement and deposition of extracellular matrix. NFAT activation did not initially affect expression of podocin, synaptopodin, and nephrin but reduced their expression as glomerular injury progressed. In contrast, we observed upregulation of Wnt6 and Fzd9 in the mutant glomeruli before the onset of significant proteinuria, suggesting a potential role for Wnt signaling in the pathogenesis of NFAT-induced podocyte injury and FSGS. These results provide in vivo evidence for the involvement of NFAT signaling in podocytes, proteinuria, and glomerulosclerosis. Furthermore, this study suggests that NFAT activation may be a key intermediate step in the pathogenesis of mutant TRPC6-mediated FSGS and that suppression of NFAT activity may contribute to the antiproteinuric effects of calcineurin inhibitors.

Non-immunologic mechanisms of calcineurin inhibitors explain its antiproteinuric effects in genetic glomerulopathies

It has been reported (this issue Pediatric Nephrology) that cyclosporine A (CyA) therapy in combination with corticosteroids, angiotensin-converting enzyme inhibitor, and an angiotensin receptor blocker decreased proteinuria in three patients with nephrotic syndrome (NS) due to WT1 mutations. Treatment with calcineurin inhibitors were found to induce a partial remission of proteinuria in several other children with genetic forms of NS, such as mutation in the podocine and in the phospholipase C epsilon gene. CyA therapy has also been reported to be beneficial to patients with Alport syndrome. Recent data have shown that the antiproteinuric effect of CyA in these cases may be due to a non-immunologic mechanism. CyA exerts an antiproteinuria effect by preventing the degradation of the actin organizing protein synaptodpodin and by a downregulation of TRPC6. This mechanism leads to the stabilization of the actin cytoskeleton in the kidney podocytes. This beneficial effect of CyA is interesting, but long-term results regarding function and nephrotoxicity are still missing.

Successful treatment of steroid-resistant nephrotic syndrome associated with WT1 mutations

The Wilms' tumor suppressor gene 1 (WT1) encodes a transcription factor involved in kidney and gonadal development. WT1 is also a key regulator of podocyte functions and mutations have been found in a small percentage of children with isolated or syndromal steroid-resistant nephrotic syndrome. It is commonly assumed that the nephrotic syndrome (NS) in patients with WT1 mutations is unresponsive to therapy and characterized by rapid progression to end-stage renal disease. We report long-term observations in 3 children with focal-segmental glomerulosclerosis associated with WT1 mutations and NS (2 cases) or nephrotic range proteinuria (1 case). All patients showed a favorable response to an intensified therapy consisting of cyclosporin A (CyA) in combination with induction therapy with intravenous and oral prednisone. Treatment with angiotensin-converting enzyme inhibitors and angiotensin receptor blockers was added to the regimen at various times. As shown both by the short-term response and during long-term follow-up, this treatment resulted in clinical remission of the NS and/or significant reduction of proteinuria, while normal renal function could be maintained over many years. Thus, glomerular diseases in selected patients with mutations in genes regulating renal development and podocyte function may respond to combination therapy with CyA and corticosteroids.

Cyclosporine A treatment in patients with Alport syndrome: a single-center experience

Limited and discordant data are available on cyclosporine A (CsA) treatment for proteinuria in Alport syndrome (AS). To address this lack of consistent data, we have studied 15 AS patients (14 males; mean age 15.3 +/- 6.0 years) treated with CsA. Patient selection criteria included a urinary protein/creatinine ratio > or =1 mg/mg and a creatinine clearance >40 ml/min/1.73 m(2). CsA treatment was started at an initial dose of 5 mg/kg/day and subsequently adjusted to reach target C2 levels of 500 ng/ml. Renal function, proteinuria, and blood pressure were monitored. Blood pressure was treated to avoid the administration of angiotensin converting enzyme or angiotensin receptor blockers for the first 2 years of therapy. The average follow-up was 3.5 years. Five patients had chronic renal failure at the beginning of treatment, of whom three and one reached end-stage renal failure within 1 and 3 years, respectively. In the remaining 11 patients, the glomerular filtration rate declined by 11 +/- 6% within 6 months, but remained stable thereafter. Proteinuria decreased by 63 +/- 21% from baseline, but returned nearly to baseline after 2.5 years of follow-up. Based on these results, we suggest that CsA is effective in reducing proteinuria in patients with Alport syndrome but that this effect is temporary. Our data do not support the use of CsA therapy for proteinuric patients with AS, particularly if they have chronic renal failure.

Molecular make-up of the glomerular filtration barrier

The glomerular filtration barrier is composed of glomerular endothelial cells, the glomerulus basement membrane and the podocyte cell layer. The filtration barrier is a target of injury in several systemic and renal diseases, and this often leads to progressive renal disease and kidney failure. Therefore, it is essential to understand the molecular biology of the glomerulus. During the last two decades, a lot of new information about molecular components of the glomerulus filtration barrier has been generated. Many of the key discoveries have been obtained through studies on the genetic background of inherited glomerular diseases. These studies have emphasized the role of podocytes in the filtration barrier function. During the last decade, the use of knockout mouse technology has become more available and given important new insights into the functional significance of glomerular components. Large-scale approaches, such as microarray profiling, have also given data about molecules involved in the biology and pathology of the glomerulus. In the coming decade, the use of global expression profiling platforms, transgenic mouse lines, and other in vivo gene delivery methods will rapidly expand our understanding of biology and pathology of the glomerular filtration barrier, and hopefully expose novel target molecules for therapy in progressive renal diseases.

Planar cell polarity pathway regulates actin rearrangement, cell shape, motility, and nephrin distribution in podocytes

Glomerular podocytes are highly polarized cells characterized by dynamic actin-based foot processes (FPs). Neighboring FPs form specialized junctions, slit diaphragms (SDs), which prevent passage of proteins into the ultrafiltrate. The SD protein complex is linked to cytoskeletal actin filaments and mutations in SD proteins lead to a dramatic change in cell morphology; proteinuria is accompanied by FP retraction and loss of SD structure. Thus, organization of the podocyte cytoskeleton is tightly linked to filtration barrier function. In a variety of cell systems, cytoskeleton arrangement is regulated by the planar cell polarity (PCP) pathway. PCP signals lead to the appearance of highly organized cellular structures that support directional cell movement and oriented cell division. Derangement of the PCP pathway causes neural tube defects and cystic kidney disease in mice. Here, we establish that the PCP pathway regulates the cytoskeleton of podocytes. We identify expression of core PCP proteins in mouse kidney sections and of PCP transcripts in murine and human cultured podocytes. The pathway is functional since Wnt5a causes redistribution of PCP proteins Dishevelled and Daam1. We also show that Wnt5a treatment changes podocyte morphology, alters nephrin distribution, increases the number of stress fibers, and increases cell motility. In reciprocal experiments, siRNA depletion of the core PCP gene Vangl2 reduced the number of cell projections and decreased stress fibers and cell motility. Finally, we demonstrate direct interactions between Vangl2 and the SD protein, MAGI-2. This suggests that the PCP pathway may be directly linked to organization of the SD as well as to regulation of podocyte cytoskeleton. Our observations indicate that PCP signaling may play an important role both in podocyte development and FP cytoskeleton dynamics.

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.

Long-term outcome of children with steroid-resistant nephrotic syndrome treated with tacrolimus

We report the outcome of our single-center, long-term follow-up study of tacrolimus therapy in children with steroid-resistant nephrotic syndrome (SRNS). All cases of nephrotic syndrome (NS) with kidney biopsies treated at our center between January 2000 and July 2008 were reviewed. Children with systemic lupus erythematosus and steroid-dependent NS were excluded. Nineteen children with SRNS received tacrolimus. Histopathological analysis of the biopsy revealed the underlying conditions of these 19 patients to be focal segmental glomerulosclerosis (ten patients), C1q nephropathy (four), membranous nephropathy (two), minimal change disease (one), membranoproliferative glomerulonephritis (one), and immunoglobulin A nephropathy (one). The mean follow-up was 55 months, and the median age of the patient cohort was 10 years. We observed complete remission in 11 (58%) patients, partial remission in six (32%), and failure to respond in two (9%). The median time to response was 8 weeks. Side effects were mild and transient (one case of acute kidney injury and three cases of hyperglycemia). The initial rate for combined partial and complete remission of the NS in children with SRNS was 81%, which was sustained in 58% of the patients on follow-up. Among children with FSGS, the sustained remission rate was 50%, while 40% progressed to end-stage renal disease (ESRD) (mean time 52 months). Based on the results of this study, we conclude that tacrolimus is an effective and well-tolerated therapeutic option for the treatment of SRNS in children. However, the occurrence of relapses of the NS with progression to ESRD during the long-term follow-up indicates the need for careful monitoring of such patients.

Tacrolimus combined with corticosteroids in treatment of nephrotic idiopathic membranous nephropathy: a multicenter randomized controlled trial

BACKGROUND

Idiopathic membranous nephropathy (IMN), a common cause of nephrotic syndrome in adults, is usually treated with corticosteroids in combination with cyclophosphamide or cyclosporine. A recent placebo-controlled study suggested that tacrolimus monotherapy was effective in IMN. However, the effectiveness of tacrolimus versus classic regimen and its potential nephrotoxicity remain inconclusive. This study evaluated the efficacy and safety of tacrolimus plus prednisone in patients with nephrotic IMN.

METHODS

Seventy-three patients with nephrotic IMN were recruited in this multicenter randomized controlled trial, 39 receiving tacrolimus and prednisone, while 34 receiving cyclophosphamide and prednisone. Tacrolimus was given at 0.1 mg/kg/d initially and adjusted to a blood trough level at 5 to 10 ng/mL for 6 months and then reduced to 2 to 5 ng/mL in the subsequent 3 months.

RESULTS

Intention-to-treat analysis suggested that the remission rate at the end of the sixth month was significantly higher in tacrolimus group than that in cyclophosphamide group (85% versus 65%, P < 0.05). The decrease of proteinuria was significantly greater in tacrolimus group. At the end of the 12th month, the remission rates were comparable between these 2 groups. Patients treated with tacrolimus were more likely to develop glucose intolerance (or diabetes mellitus), infection, and hypertension. No obvious nephrotoxicity of calcineurin inhibitor was found in repeat renal biopsy.

CONCLUSIONS

Tacrolimus plus corticosteroids is an alternative therapeutic regimen for nephrotic IMN. The short-term efficacy might be better than cyclophosphamide plus prednisone.

Toward the development of podocyte-specific drugs

Most kidney diseases that ultimately lead to end-stage renal failure originate within the glomerulus and are associated with proteinuria. Treatment options are unspecific and offer partial cures at best because available therapies do not primarily treat glomerular cells but rather act systemically and thus cause many side effects. Most glomerulopathies directly stem from injury to podocytes, cells that have a key role in the maintenance of the glomerular filter. Thus, these cells constitute an obvious and promising target for the development of novel kidney-protective drugs. During the last decade, enormous advances have been made in the understanding of podocyte structure and function. A number of pathways that are altered during glomerular diseases may be targeted by novel small- and large-molecule drugs as well as biologicals that have been identified in nephrology and other areas of drug development. Cultured podocytes provide a valuable model for high-throughput drug screening assays. Furthermore, podocytes have been shown to possess many features that make them particularly good target cells for renal protection. This mini-review discusses some of the most recent promising data related to potential drug therapy for proteinuria and kidney disease through direct podocyte targeting.

Role of altered insulin signaling pathways in the pathogenesis of podocyte malfunction and microalbuminuria

PURPOSE OF REVIEW

In diabetic nephropathy, insulin resistance and hyperinsulinemia correlate with the development of albuminuria. The possibility that altered insulin signaling in glomerular cells and particularly podocytes contributes to the development of diabetic nephropathy will be discussed.

RECENT FINDINGS

Whereas normal podocytes take up glucose in response to insulin, diabetic podocytes become insulin resistant in experimental diabetic nephropathy prior to the development of significant albuminuria. Both clinical and experimental data suggest that insulin sensitizers may be renoprotective independent of their systemic effects on the metabolic control of diabetes.

SUMMARY

We will review the clinical and experimental evidence that altered insulin signaling correlates with the development of diabetic nephropathy in both type 1 and type 2 diabetes, and that insulin sensitizers may be superior to other hypoglycemic agents in the prevention of diabetic nephropathy. We will then review potential mechanisms by which altered podocyte insulin signaling may contribute to the development of diabetic nephropathy. Understanding the role of podocytes in glucose metabolism is important because it may lead to the discovery of novel pathogenetic mechanisms of diabetic nephropathy, it may affect current strategies for prevention and treatment of diabetic nephropathy, and it may allow the identification of novel therapeutic targets.

VEGF signalling inhibition-induced proteinuria: Mechanisms, significance and management

Proteinuria is a dose-related side-effect occurring after inhibition of vascular endothelial growth factor (VEGF) signalling and may reflect severe glomerular damage. The inhibition of the VEGF signalling axis induces downexpression or suppression of nephrin, an important protein for the maintenance of the glomerular slit diaphragm, sometimes leading to nephritic syndrome and/or glomerular thrombotic microangiopathy, the main-associated kidney disease. A MEDLINE search was carried out using the following criteria: (1) all MEDLINE listings as of 01-01-2000 with abstracts; (2) English language; and (3) Humans. The following phrases were used to query the database: (proteinuria) AND (anti-VEGF OR VEGF inhibition OR bevacizumab OR sunitinib OR sorafenib OR VEGF Trap OR axitinib OR pazopanib OR AZ 2171). The references of each article identified were carefully reviewed for additional reference. The incidence of mild and asymptomatic proteinuria ranges from 21% up to 63%, but heavy proteinuria has been reported in up to 6.5% of renal cell carcinoma patients. Although discontinuation of anti-VEGF agent induced significant reduction, persistence of proteinuria is common. Although angiotensinconverting-enzyme inhibitors and/or angiotensin receptor blockers seem to be preferred, no specific recommendation for an antiproteinuric agent can be made in this context because there are no controlled studies addressing the subject. Periodic monitoring of urinary protein should be carried out in anti-VEGF-treated patients and patients showing proteinuria need special referral to nephrologists.

The opposing roles of estradiol on synaptic protein expression in hippocampal cultures

Estrogen-induced synaptic plasticity was frequently shown by an increase of spines at apical dendrites of CA1 pyramidal neurons after systemic application of estradiol to ovariectomized rats. Surprisingly, exogenous application of estradiol to hippocampal cultures had no effect on spines and on spine synapses, although quantitative immunohistochemistry revealed an upregulation of spinophilin and of synaptophysin, in these cultures. The role of synaptophysin as a presynaptic marker and of spinophilin as a postsynaptic marker, appears questionable from these discrepancies. In contrast, synaptopodin, a marker protein of "mature" mushroom-shaped spines, was downregulated after treatment of hippocampal cultures with estradiol. Synaptopodin is strongly associated to the spine apparatus, a spine-specific cell organelle, which is present in 80% of all mushroom-shaped spines. Consistently, we found a reduction in the number of spines, containing a spine apparatus in response to estradiol, suggesting that the presence of a spine apparatus in many but not all spines is very likely a result of their dynamic character. In summary, synaptic proteins appear to be regulated by estradiol, independent of its function on spine and spine synapse formation.

Efficacy and safety of cyclosporine A in patients with refractory systemic lupus erythematosus in a daily clinical practice

We investigated the efficacy and safety of cyclosporine A (CsA; targeted serum trough level: 80-150 ng/ml) in a daily clinical practice for treating patients with systemic lupus erythematosus (SLE), who had been, or were expected to be, refractory to glucocorticoids (GCs) and other immunosuppressants. Fifty-nine patients with SLE receiving CsA were observed for at least 6 months (21.5 months on average). A significant reduction of proteinuria was noted 2 weeks after initiation of treatment in patients with nephritis, resulting in a clinical response in five of eight patients in the GC dose-up group and 11 of 18 patients in the stable GC dose group, respectively. Notably, the mean score for disease activity on the SLE Disease Activity Index decreased significantly from 8.6 +/- 5.3 to 4.4 +/- 2.5 after CsA treatment in patients in the stable GC dose group (n = 40). Moreover, the mean flare rate decreased by approximately 60% with CsA. Side effects of CsA appeared in 32.2% of patients and all of them subsided through dose reduction or discontinuation (n=8) of CsA. Consequently, the cumulative 2-year survival rate of CsA was 75%. The results suggest that CsA should be considered for patients with SLE refractory to GCs.

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.

Molecular genetic analysis of podocyte genes in focal segmental glomerulosclerosis--a review

This review deals with podocyte proteins that play a significant role in the structure and function of the glomerular filter. Genetic linkage studies has identified several genes involved in the development of nephrotic syndrome and contributed to the understanding of the pathophysiology of glomerular proteinuria and/or focal segmental glomerulosclerosis. Here, we describe already well-characterized genetic diseases due to mutations in nephrin, podocin, CD2AP, alpha-actinin-4, WT1, and laminin beta2 chain, as well as more recently identified genetic abnormalities in TRPC6, phospholipase C epsilon, and the proteins encoded by the mitochondrial genome. In addition, the role of the proteins which have shown to be important for the structure and functions by gene knockout studies in mice, are also discussed. Furthermore, some rare syndromes with glomerular involvement, in which molecular defects have been recently identified, are briefly described. In summary, this review updates the current knowledge of genetic causes of congenital and childhood nephrotic syndrome and provides new insights into mechanisms of glomerular dysfunction.

Idiopathic membranous nephropathy: management strategies

Treatment of idiopathic membranous nephropathy is based on a 'symptomatic' therapy that includes ACE inhibitors or angiotensin II receptor antagonists, and on an 'aetiological' therapy aimed at modulating underlying immunological mechanisms. The role of the latter is still debated given the usually indolent course of disease; furthermore, traditional immunosuppressants would not have an impact on patient and renal survival according to a systematic review of literature. However, up to 40% of untreated patients eventually develop end-stage renal disease and remission of nephrotic syndrome protects patients from related life-threatening complications and is the strongest positive prognostic factor for long-term kidney function. Therefore, immunosuppressive therapy seems to be rational in high-risk patients with nephrotic syndrome or deteriorating renal function. This article outlines a possible role for each 'aetiological' therapy on the basis of available evidence in order to provide some practical recommendations. The first-line therapy is based on a 6-month regimen of alternating corticosteroids and an alkylating agent ('Ponticelli' regimen), whereas oral ciclosporin and intramuscular corticotrophin (adrenocorticotrophic hormone) are alternatives that provide comparable results in terms of remission of proteinuria, with a different adverse effect profile. New drugs are emerging as potential treatments, such as mycophenolate mofetil, tacrolimus, intravenous immunoglobulins and rituximab. Specific settings, such as chronic renal failure or elderly age, require a careful balance between benefits and toxicity of immunosuppression. The tailor-made use of this repertoire of drugs can provide a tool to achieve remission of proteinuria and modify the natural course of idiopathic membranous nephropathy.

Partial remission with cyclosporine A in a patient with nephrotic syndrome due to NPHS2 mutation

Autosomal recessive steroid-resistant nephrotic syndrome (NS) is a rare, genetically determined nephropathy caused mainly by a mutation in the NPHS2 gene. This type of NS is usually resistant to other immunosuppressive therapy as well, but a few cases of cyclosporine A-induced partial remission of inherited NS have been reported. We present a boy that developed NS at the age of 18 months. There was no decrease of proteinuria on standard prednisolone therapy, and a diagnosis of steroid-resistant NS was established. However, the proteinuria decreased significantly following the initiation of cyclosporine A therapy (from 1280 to 380 mg/m(2) per day) without any negative effects on renal function (stable glomerular filtration rate 130-150 ml/min per 1.73 m(2)). The molecular genetic test revealed a homozygous R138Q mutation in the NPHS2 gene. Our case demonstrates that cyclosporine A can induce partial remission in patients with genetic forms of NS without influencing the glomerular filtration rate. However, its long-term effect and safety in children with hereditary forms of nephrotic syndrome have yet to be investigated.

Glomerular filtration barrier

PURPOSE OF REVIEW

In 2008, more than 376 papers were published on the glomerular barrier. Most of them dealt with the podocyte and its role in kidney disease.

RECENT FINDINGS

There is new information on signaling pathways that are utilized in podocytes during proteinuria. Interestingly, the glomerular endothelium, with its fenestrae and glycocalyx, seems to be important for the maintenance of an intact glomerular barrier. All new advances at the molecular level are compatible with a highly size and charge-selective glomerular membrane and refute the concept of a 'leaky' glomerular barrier with tubular retrieval of intact albumin. Still, the hypothesis has its advocates, keeping a stimulating 'charge debate' alive.

SUMMARY

Glomerular diseases account for 90% of chronic kidney disease requiring dialysis and transplantation at an annual cost of $20 billion in the USA. In clinical practice, we lack specific treatment of these diseases, giving us plenty of room for improvement. Future research should be directed toward deeper understanding of the signaling pathways involved in different conditions of proteinuria, the cross-talk between cell types in the glomerulus, and the identification of novel targets for treatment of acquired kidney disease.

The glomerular filter: an imperfect barrier is required for perfect renal function

PURPOSE OF REVIEW

There is currently a major debate on the mechanisms of albuminuria, and this review appraises recent studies in this area.

RECENT FINDINGS

The traditional view of albuminuria is that it is the result of damage to an essentially impermeable glomerular barrier. However, over the years, critical evidence for this traditional model has been shown to be flawed. An alternative explanation has evolved in which the glomerular filter governs albumin permeability by size selectivity alone. This means that the filter offers a significant barrier to albumin, but it is imperfect - the barrier leaks albumin. The virtue of this leakage is that it endows the filter an in-built anticlogging mechanism. The filtered albumin, if not rescued, would be excreted at nephrotic levels in the urine. There is evidence that proximal tubular cells participate in retrieving most of this filtered albumin to return it back to the blood supply intact. A small amount of the filtered albumin is not retrieved but directed toward lysosomal degradation, and the peptide products are exocytosed into the tubular lumen and excreted.

SUMMARY

In acquired and chemically induced kidney disease, albuminuria is the result of dysfunction in proximal tubular cell processing of albumin rather than alterations in glomerular permeability.

Efficacy and safety of tacrolimus for lupus nephritis: a placebo-controlled double-blind multicenter study

We evaluated the efficacy and safety of tacrolimus in patients receiving glucocorticoid therapy for lupus nephritis. Patients with persistent nephritis were randomized to receive 28 weeks of double-blind treatment with tacrolimus (3 mg/day) or placebo. The primary endpoint was the change in the lupus nephritis disease activity index (LNDAI) calculated from scores for daily urinary protein excretion, urinary red cells, serum creatinine, anti-double-stranded DNA antibody, and serum complement. Statistical analysis was performed using the full analysis set. The LNDAI was decreased by 32.9 +/- 31.0% (mean +/- SD) in the tacrolimus group (n = 28) and was increased by 2.3 +/- 38.2% in the placebo group (n = 35) at final evaluation. There was significant improvement in the tacrolimus group. Daily urinary protein excretion showed a significant decrease in the tacrolimus group (p < 0.001). The complement (C3) level showed a significant increase in the tacrolimus group (p = 0.001). Treatment-related adverse events occurred in 92.9% of the tacrolimus group and 80.0% of the placebo group, but the difference was not significant. In patients on glucocorticoid therapy for lupus nephritis, addition of tacrolimus to basal therapy achieved significant improvement compared with placebo. Tacrolimus may therefore be a useful alternative treatment for lupus nephritis.

Genetics of nephrotic syndrome: new insights into molecules acting at the glomerular filtration barrier

Nephrotic syndrome is caused by increased permeability of the glomerular filtration barrier for macromolecules. The identification of mutations of various podocyte-expressed proteins as causes of familial nephrotic syndrome has significantly contributed to shedding light into the molecular pathogenesis of nephrotic proteinuria and into the physiology of the glomerular sieve. More recent findings have changed our conception of the glomerular filtration barrier from a relatively static structure to a highly dynamic one. Both the multiprotein slit diaphragm complex around nephrin and the integrin receptor complex that mediates binding of the podocyte to the glomerular basement membrane, may translate outside-inside signaling and lead to podocyte actin cytoskeleton rearrangement. This may enable the podocyte network to adapt to environmental changes and respond to injury. Disturbance in these processes may not only be involved in the pathogenesis of hereditary nephrotic syndrome but also in that of more common acquired proteinuric diseases. Elucidation of the molecular mechanisms involved will possibly open the way to new therapeutic approaches.

Risk factors for cyclosporin A nephrotoxicity in children with steroid-dependant nephrotic syndrome

BACKGROUND AND OBJECTIVES

Cyclosporin A (CsA) is a well-established treatment for steroid-dependent nephrotic syndrome (SDNS) that may, however, cause chronic ischemic renal lesions. The objective of the study was to assess the prevalence of CsA nephrotoxicity (CsAN) in protocol biopsies of children with SDNS.

DESIGN, SETTINGS, PARTICIPANTS, & MEASUREMENTS

From 1990 through 2008, we performed 71 renal biopsies in 53 patients with SDNS. The mean CsA C2 levels were 466 +/- 134 ng/ml, and the mean duration of treatment was 4.7 +/- 2.0 yr before biopsy (range 2.9 to 12.7 yr).

RESULTS

CsAN was observed in 22 (31%) of 71 renal biopsies. Of these, 11 corresponded to isolated vascular or tubular lesions, and 11 corresponded to combined vascular and tubular lesions. The majority of CsAN lesions were mild (17 of 22). In no cases were lesions graded as severe. By regression analysis, CsAN was positively associated with the use of angiotensin-converting enzyme inhibitors (ACEIs) or angiotensin II receptor blockers (ARBs) and with hyperuricemia and negatively associated with minimal-change lesions. By multivariate analysis, only association with the use of ACEIs or ARBs retained significance. Stratification of the population according to CsA C2 levels showed increased risk for CsAN for C2 levels >600 ng/ml.

CONCLUSIONS

Mild to moderate CsAN occurs in approximately one third of patients who have SDNS and are treated with CsA for >3 yr. Our data suggest that patients who require high dosages of CsA or treatment for hypertension, in particular when ACEIs/ARBs are used, are at higher risk for CsAN.