Apolipoproteins prevent glomerular albumin permeability induced in vitro by serum from patients with focal segmental glomerulosclerosis

State of the art in childhood nephrotic syndrome: concrete discoveries and unmet needs

Nephrotic syndrome (NS) is a clinical entity characterized by proteinuria, hypoalbuminemia, and peripheral edema. NS affects about 2-7 per 100,000 children aged below 18 years old yearly and is classified, based on the response to drugs, into steroid sensitive (SSNS), steroid dependent, (SDNS), multidrug dependent (MDNS), and multidrug resistant (MRNS). Forms of NS that are more difficult to treat are associated with a worse outcome with respect to renal function. In particular, MRNS commonly progresses to end stage renal failure requiring renal transplantation, with recurrence of the original disease in half of the cases. Histological presentations of NS may vary from minimal glomerular lesions (MCD) to focal segmental glomerulosclerosis (FSGS) and, of relevance, the histological patterns do not correlate with the response to treatments. Moreover, around half of MRNS cases are secondary to causative pathogenic variants in genes involved in maintaining the glomerular structure. The pathogenesis of NS is still poorly understood and therapeutic approaches are mostly based on clinical experience. Understanding of pathogenetic mechanisms of NS is one of the 'unmet needs' in nephrology and represents a significant challenge for the scientific community. The scope of the present review includes exploring relevant findings, identifying unmet needs, and reviewing therapeutic developments that characterize NS in the last decades. The main aim is to provide a basis for new perspectives and mechanistic studies in NS.

Primary Focal Segmental Glomerulosclerosis Plasmas Increase Lipid Droplet Formation and Perilipin-2 Expression in Human Podocytes

Many patients with primary focal segmental glomerulosclerosis (FSGS) develop recurrence of proteinuria after kidney transplantation. Several circulating permeability factors (CPFs) responsible for recurrence have been suggested, but were never validated. We aimed to find proteins involved in the mechanism of action of CPF(s) and/or potential biomarkers for the presence of CPF(s). Cultured human podocytes were exposed to plasma from patients with FSGS with presumed CPF(s) or healthy and disease controls. Podocyte proteomes were analyzed by LC-MS. Results were validated using flow cytometry, RT-PCR, and immunofluorescence. Podocyte granularity was examined using flow cytometry, electron microscopy imaging, and BODIPY staining. Perilipin-2 protein expression was increased in podocytes exposed to presumed CPF-containing plasmas, and correlated with the capacity of plasma to induce podocyte granularity, identified as lipid droplet accumulation. Elevated podocyte perilipin-2 was confirmed at protein and mRNA level and was also detected in glomeruli of FSGS patients whose active disease plasmas induced podocyte perilipin-2 and lipid droplets. Our study demonstrates that presumably, CPF-containing plasmas from FSGS patients induce podocyte lipid droplet accumulation and perilipin-2 expression, identifying perilipin-2 as a potential biomarker. Future research should address the mechanism underlying CPF-induced alterations in podocyte lipid metabolism, which ultimately may result in novel leads for treatment.

Focal segmental glomerulosclerosis: Risk for recurrence and interventions to optimize outcomes following recurrence

BACKGROUND

FSGS is a common indication for kidney transplant with a high-risk of posttransplant recurrence.

METHODS

In this review, we summarize current knowledge about FSGS recurrence after kidney transplantation, including epidemiology, pretransplant planning, posttransplant management, and investigational treatments.

RESULTS

FSGS recurs in 14%-60% of first transplants, likely associated with a circulating permeability factor. Pretransplant counseling regarding recurrence is critical, and patients with FSGS should undergo pretransplant genetic screening. Rapid progression to ESKD, initial steroid responsiveness, younger age at diagnosis, race/ethnicity, and mesangial hypercellularity or minimal change histology on native biopsy may be associated with recurrence. Living donation is not contraindicated but does not result in improved graft survival relative to deceased donation. Pretransplant nephrectomy may be performed for a variety of reasons, but does not decrease recurrence. Pretransplant therapy with rituximab and/or PE is understudied but not clearly effective at preventing recurrence. Patients with FSGS typically present early with rapid-onset severe proteinuria. Diagnosis can be confirmed by biopsy showing foot process effacement; typical FSGS lesions are not seen on light microscopy in the early stages. There is no established effective treatment for recurrent FSGS, but renin-angiotensin-aldosterone system inhibition and extracorporeal therapies, including PE and IA, are most commonly used. Adjunct or alternative therapies may include rituximab, lipopheresis, and cyclosporine.

A critical re-analysis of cases of post-transplantation recurrence in genetic nephrotic syndrome

BACKGROUND

Genetic defects in podocyte proteins account for up to 30% of steroid-resistant nephrotic syndrome (SRNS) in the paediatric population. Most children with genetic SRNS are resistant to immunosuppression and at high risk of progression to stage 5 chronic kidney disease. Kidney transplantation is often the treatment of choice. The possibility of post-transplantation disease recurrence in genetic SRNS remains controversial, and poses fundamental questions about disease biology.

METHODS

We critically evaluated the published cases of post-transplantation recurrence in genetic patients, particularly testing 'mutations' against the most recent population variant databases, in order to clarify the diagnoses, and compare the clinical courses and responses to therapy.

RESULTS

Biallelic pathogenic variants in NPHS1 leading to a complete absence of nephrin were the most commonly reported and best understood instance of nephrotic syndrome occurring post-transplantation. This is an immune-mediated process driven by antibody production against the novel nephrin protein in the allograft. We also identified a number of plausible reported cases of post-transplantation recurrence involving pathogenic variants in NPHS2 (8 patients, biallelic), one in WT1 (monoallelic) and one in NUP93 (biallelic). However, the mechanism for recurrence in these cases remains unclear. Other instances of recurrence in genetic disease were difficult to interpret due to differing clinical criteria, inclusion of patients without true pathogenic variants or the influence of other factors on renal outcome.

CONCLUSIONS

Overall, post-transplantation recurrence remains very rare in patients with genetic SRNS. It appears to occur later after transplantation than in other patients and usually responds well to plasmapheresis with a good renal outcome.

Novel in vitro assays to detect circulating permeability factor(s) in idiopathic focal segmental glomerulosclerosis

BACKGROUND

Many patients with idiopathic focal segmental glomerulosclerosis (FSGS) develop recurrence of proteinuria after kidney transplantation (TX). Although several circulating permeability factors (CPFs) responsible for recurrence have been suggested, there is no consensus. To facilitate CPF identification and predict recurrence after TX, there is a need for robust methods that demonstrate the presence of CPFs.

METHODS

Cultured human podocytes (hPods) and human and mouse glomerular endothelial cells (ciGEnC, mGEnC) were exposed to plasmas of FSGS patients with presumed CPFs, and of (disease) controls. A visual scoring assay and flow cytometry analysis of side scatter were used to measured changes in cellular granularity after exposure to plasma.

RESULTS

Nine out of 13 active disease plasmas of 10 FSGS patients with presumed CPFs induced granularity in hPod in a dose- and time-dependent manner. Corresponding remission plasmas induced no or less granularity in hPod. Similar results were obtained with ciGEnC and mGEnC, although induced granularity was less compared with hPod. Notably, foetal calf serum, healthy plasma and a remission plasma partially blocked FSGS plasma-induced hPod granularity.

CONCLUSIONS

We developed a novel assay in which active disease, presumably CPF-containing, FSGS plasmas induced granularity in cultured hPod. Our results may indicate the presence of CPF inhibitor(s) in healthy and remission plasma. We suggest the presence of a delicate balance between CPF and a CPF inhibitory factor, which is disturbed in patients with active disease. Our novel assays can be applied in future research to identify CPF and CPF inhibitors, and possibly to predict recurrence after TX.

A misprocessed form of Apolipoprotein A-I is specifically associated with recurrent Focal Segmental Glomerulosclerosis

Apolipoprotein A-Ib (ApoA-Ib) is a high molecular weight form of Apolipoprotein A-I (ApoA-I) found specifically in the urine of kidney-transplanted patients with recurrent idiopathic focal segmental glomerulosclerosis (FSGS). To determine the nature of the modification present in ApoA-Ib, we sequenced the whole APOA1 gene in ApoA-Ib positive and negative patients, and we also studied the protein primary structure using mass spectrometry. No genetic variations in the APOA1 gene were found in the ApoA-Ib positive patients that could explain the increase in its molecular mass. The mass spectrometry analysis revealed three extra amino acids at the N-Terminal end of ApoA-Ib that were not present in the standard plasmatic form of ApoA-I. These amino acids corresponded to half of the propeptide sequence of the immature form of ApoA-I (proApoA-I) indicating that ApoA-Ib is a misprocessed form of proApoA-I. The description of ApoA-Ib could be relevant not only because it can allow the automated analysis of this biomarker in the clinical practice but also because it has the potential to shed light into the molecular mechanisms that cause idiopathic FSGS, which is currently unknown.

Effect of human very low-density lipoproteins on cardiotrophin-like cytokine factor 1 (CLCF1) activity

The cytokines CLCF1 and CNTF are ligands for the CNTF receptor and the apolipoprotein E (ApoE) receptor sortilin. Both share structural similarities with the N-terminal domain of ApoE, known to bind CNTF. We therefore evaluated whether ApoE or ApoE-containing lipoproteins interact with CLCF1 and regulate its activity. We observed that CLCF1 forms complexes with the three major isoforms of ApoE in co-immunoprecipitation and proximity assays. FPLC analysis of mouse and human sera mixed with CLCF1 revealed that CLCF1 co-purifies with plasma lipoproteins. Studies with sera from ApoE^-/- mice indicate that ApoE is not required for CLCF1-lipoprotein interactions. VLDL- and LDL-CLCF1 binding was confirmed using proximity and ligand blots assays. CLCF1-induced STAT3 phosphorylation was significantly reduced when the cytokine was complexed with VLDL. Physiological relevance of our findings was asserted in a mouse model of oxygen-induced retinopathy, where the beneficial anti-angiogenic properties of CLCF1 were abrogated when co-administrated with VLDL, indicating, that CLCF1 binds purified lipoproteins or lipoproteins in physiological fluids such as serum and behave as a "lipocytokine". Albeit it is clear that lipoproteins modulate CLCF1 activity, it remains to be determined whether lipoprotein binding directly contributes to its neurotrophic function and its roles in metabolic regulation.

Soluble CD40 ligand directly alters glomerular permeability and may act as a circulating permeability factor in FSGS

CD40/CD40 ligand (CD40L) dyad, a co-stimulatory bi-molecular complex involved in the adaptive immune response, has also potent pro-inflammatory actions in haematopoietic and non-haematopoietic cells. We describe here a novel role for soluble CD40L (sCD40L) as modifier of glomerular permselectivity directly acting on glomerular epithelial cells (GECs). We found that stimulation of CD40, constitutively expressed on GEC cell membrane, by the sCD40L rapidly induced redistribution and loss of nephrin in GECs, and increased albumin permeability in isolated rat glomeruli. Pre-treatment with inhibitors of CD40-CD40L interaction completely prevented these effects. Furthermore, in vivo injection of sCD40L induced a significant reduction of nephrin and podocin expression in mouse glomeruli, although no significant increase of urine protein/creatinine ratio was observed after in vivo injection. The same effects were induced by plasma factors partially purified from post-transplant plasma exchange eluates of patients with focal segmental glomerulosclerosis (FSGS), and were blocked by CD40-CD40L inhibitors. Moreover, 17 and 34 kDa sCD40L isoforms were detected in the same plasmapheresis eluates by Western blotting. Finally, the levels of sCD40Lwere significantly increased in serum of children both with steroid-sensitive and steroid-resistant nephrotic syndrome (NS), and in adult patients with biopsy-proven FSGS, compared to healthy subjects, but neither in children with congenital NS nor in patients with membranous nephropathy.

Our results demonstrate that sCD40L directly modifies nephrin and podocin distribution in GECs. Moreover, they suggest that sCD40L contained in plasmapheresis eluates from FSGS patients with post-transplant recurrence may contribute, presumably cooperating with other mediators, to FSGS pathogenesis by modulating glomerular permeability.

Multiple Targets for Novel Therapy of FSGS Associated with Circulating Permeability Factor

A plasma component is responsible for altered glomerular permeability in patients with focal segmental glomerulosclerosis. Evidence includes recurrence after renal transplantation, remission after plasmapheresis, proteinuria in infants of affected mothers, transfer of proteinuria to experimental animals, and impaired glomerular permeability after exposure to patient plasma. Therapy may include decreasing synthesis of the injurious agent, removing or blocking its interaction with cells, or blocking signaling or enhancing cell defenses to restore the permeability barrier and prevent progression. Agents that may prevent the synthesis of the permeability factor include cytotoxic agents or aggressive chemotherapy. Extracorporeal therapies include plasmapheresis, immunoadsorption with protein A or anti-immunoglobulin, or lipopheresis. Oral or intravenous galactose also decreases P_alb activity. Studies of glomeruli have shown that several strategies prevent the action of FSGS sera. These include blocking receptor-ligand interactions, modulating cell reactions using indomethacin or eicosanoids 20-HETE or 8,9-EET, and enhancing cytoskeleton and protein interactions using calcineurin inhibitors, glucocorticoids, or rituximab. We have identified cardiotrophin-like cytokine factor 1 (CLCF-1) as a candidate for the permeability factor. Therapies specific to CLCF-1 include potential use of cytokine receptor-like factor (CRLF-1) and inhibition of Janus kinase 2. Combined therapy using multiple modalities offers therapy to reverse proteinuria and prevent scarring.

Soluble Urokinase Receptors in Focal Segmental Glomerulosclerosis: A Review on the Scientific Point of View

Focal segmental glomerulosclerosis (FSGS) is one of the primary glomerular disorders in both children and adults which can progress to end-stage renal failure. Although there are genetic and secondary causes, circulating factors have also been regarded as an important factor in the pathogenesis of FSGS, because about 40% of the patients with FSGS have recurrence after renal transplantation. Soluble urokinase-type plasminogen activator receptor (suPAR) is a soluble form of uPAR, which is a membrane-bound protein linked to GPI in various immunologically active cells, including podocytes. It has recently been suggested as a potential circulating factor in FSGS by in vitro podocyte experiments, in vivo mice models, and human studies. However, there have also been controversies on this issue, because subsequent studies showed conflicting results. suPAR levels were also increased in patients with other glomerular diseases and were inversely correlated with estimated glomerular filtration rate. Nevertheless, there has been no balanced review on this issue. In this review, we compare the conflicting data on the involvement of suPAR in the pathogenesis of FSGS and shed light on interpretation by taking into account many points and the potential variables and confounders influencing serum suPAR levels.

Permeability factors in idiopathic nephrotic syndrome: historical perspectives and lessons for the future

The term idiopathic nephrotic syndrome (iNS) traditionally covers minimal change disease and primary focal segmental glomerulosclerosis (FSGS), now thought to be separate disease entities. Clinical and experimental evidence suggest that circulating permeability factors are involved in their pathogenesis. In the past four decades, many investigators have searched for the responsible factors, thus far with little success. The recent report of the soluble urokinase plasminogen activator receptor as a causative factor in FSGS has received much attention, but again the initially promising findings were not confirmed. We describe the history of the search for permeability factors, discuss the pitfalls that are likely responsible for the lack of success and propose criteria that should be used in future studies when evaluating candidate permeability factors.

Predictive urinary biomarkers for steroid-resistant and steroid-sensitive focal segmental glomerulosclerosis using high resolution mass spectrometry and multivariate statistical analysis

Background

Focal segmental glomerulosclerosis (FSGS) is a glomerular scarring disease diagnosed mostly by kidney biopsy. Since there is currently no diagnostic test that can accurately predict steroid responsiveness in FSGS, prediction of the responsiveness of patients to steroid therapy with noninvasive means has become a critical issue. In the present study urinary proteomics was used as a noninvasive tool to discover potential predictive biomarkers.

Methods

Urinary proteome of 10 patients (n = 6 steroid-sensitive, n = 4 steroid-resistant) with biopsy proven FSGS was analyzed using nano-LC-MS/MS and supervised multivariate statistical analysis was performed.

Results

Twenty one proteins were identified as discriminating species among which apolipoprotein A-1 and Matrix-remodeling protein 8 had the most drastic fold changes being over- and underrepresented, respectively, in steroid sensitive compared to steroid resistant urine samples. Gene ontology enrichment analysis revealed acute inflammatory response as the dominant biological process.

Conclusion

The obtained results suggest a panel of predictive biomarkers for FSGS. Proteins involved in the inflammatory response are shown to be implicated in the responsiveness. As a tool for biomarker discovery, urinary proteomics is especially fruitful in the area of prediction of responsiveness to drugs. Further validation of these biomarkers is however needed.

A form of apolipoprotein a-I is found specifically in relapses of focal segmental glomerulosclerosis following transplantation

Recurrence of idiopathic focal segmental glomerulosclerosis (FSGS) following kidney transplantation occurs in a large percentage of patients. Accurate prediction of recurrence and elucidation of its pathogenesis are major therapeutic goals. To detect differential proteins related to FSGS recurrence, proteomic analysis was performed on plasma and urine samples from 35 transplanted idiopathic FSGS patients, divided into relapsing and nonrelapsing. Several proteins were detected increased in urine of relapsing FSGS patients, including a high molecular weight form of apolipoprotein A-I, named ApoA-Ib, found exclusively in relapsing patients. This finding was verified by Western blot individually in the 35 patients and validated in an independent group of 40 patients with relapsing or nonrelapsing FSGS, plus two additional groups: FSGS-unrelated patients showing different proteinuria levels (n = 30), and familial FSGS transplanted patients (n = 14). In the total of 119 patients studied, the ApoA-Ib form was detected in 13 of the 14 relapsing FSGS patients, and in one of the 61 nonrelapsing patients. Only one of the 30 patients with FSGS-unrelated proteinuria tested positive for ApoA-Ib, and was not detected in familial patients. Urinary ApoA-Ib is associated with relapses in idiopathic FSGS and warrants additional investigation to determine its usefulness as biomarker of relapse following transplantation.

Target organ damage in African American hypertension: role of APOL1

Apolipoprotein L1 (APOL1) gene association studies and results of the African American Study of Kidney Disease and Hypertension are disproving the longstanding concept that mild to moderate essential hypertension contributes substantially to end-stage renal disease susceptibility in African Americans. APOL1 coding variants underlie a spectrum of kidney diseases, including that attributed to hypertension (labeled arteriolar or hypertensive nephrosclerosis), focal segmental glomerulosclerosis, and HIV-associated nephropathy. APOL1 nephropathy risk variants persist because of protection afforded from the parasite that causes African sleeping sickness. This breakthrough will lead to novel treatments for hypertensive African Americans with low-level proteinuria, for whom effective therapies are lacking. Furthermore, APOL1 nephropathy risk variants contribute to racially variable allograft survival rates after kidney transplantation and assist in detecting nondiabetic forms of nephropathy in African Americans with diabetes. Discovery of APOL1-associated nephropathy was a major success of the genetics revolution, demonstrating that secondary hypertension is typically present in nondiabetic African Americans with nephropathy.

The renaissance of corticotropin therapy in proteinuric nephropathies

Refractory nephrotic syndrome continues to be a therapeutic challenge despite advances in immunosuppression and blockade of the renin-angiotensin-aldosterone cascade. Adrenocorticotropic hormone (ACTH), a pituitary neuroimmunoendocrine polypeptide, was widely used in the 1950s as an effective therapy for childhood nephrotic syndrome, but has since been replaced by synthetic glucocorticoid analogues. In addition to controlling steroidogenesis, ACTH also acts as an important physiological agonist of the melanocortin system. Clinical and experimental evidence now suggests that ACTH has antiproteinuric, lipid-lowering and renoprotective properties, which are not fully explained by its steroidogenic effects. ACTH therapy is effective in inducing remission of nephrotic syndrome in patients with a variety of proteinuric nephropathies, even those resistant to steroids and other immunosuppressants. This Perspectives article describes the biophysiology of ACTH, with an emphasis on its melanocortin actions, particularly in renal parenchymal cells, which could potentially explain the therapeutic effects of ACTH in nephrotic glomerulopathies.

The population genetics of chronic kidney disease: insights from the MYH9-APOL1 locus

Many rare kidney disorders exhibit a monogenic, Mendelian pattern of inheritance. Population-based genetic studies have identified many genetic variants associated with an increased risk of developing common kidney diseases. Strongly associated variants have potential clinical uses as predictive markers and may advance our understanding of disease pathogenesis. These principles are elegantly illustrated by a region within chromosome 22q12 that has a strong association with common forms of kidney disease. Researchers had identified DNA sequence variants in this locus that were highly associated with an increased prevalence of common chronic kidney diseases in people of African ancestry. Initial research concentrated on MYH9 as the most likely candidate gene; however, population-based whole-genome analysis enabled two independent research teams to discover more strongly associated mutations in the neighboring APOL1 gene. The powerful evolutionary selection pressure of an infectious pathogen in West Africa favored the spread of APOL1 variants that protect against a lethal form of African sleeping sickness but are highly associated with an increased risk of kidney disease. We describe the data sources, process of discovery, and reasons for initial misidentification of the candidate gene, as well as the lessons that can be learned for future population genetics research.

The APOL1 gene and allograft survival after kidney transplantation

Coding variants in the apolipoprotein L1 gene (APOL1) are strongly associated with nephropathy in African Americans (AAs). The effect of transplanting kidneys from AA donors with two APOL1 nephropathy risk variants is unknown. APOL1 risk variants were genotyped in 106 AA deceased organ donors and graft survival assessed in 136 resultant kidney transplants. Cox-proportional hazard models tested for association between time to graft failure and donor APOL1 genotypes. The mean follow-up was 26.4 ± 21.8 months. Twenty-two of 136 transplanted kidneys (16%) were from donors with two APOL1 nephropathy risk variants. Twenty-five grafts failed; eight (32%) had two APOL1 risk variants. A multivariate model accounting for donor APOL1 genotype, overall African ancestry, expanded criteria donation, recipient age and gender, HLA mismatch, CIT and PRA revealed that graft survival was significantly shorter in donor kidneys with two APOL1 risk variants (hazard ratio [HR] 3.84; p = 0.008) and higher HLA mismatch (HR 1.52; p = 0.03), but not for overall African ancestry excluding APOL1. Kidneys from AA deceased donors harboring two APOL1 risk variants failed more rapidly after renal transplantation than those with zero or one risk variants. If replicated, APOL1 genotyping could improve the donor selection process and maximize long-term renal allograft survival.

Missense mutations in the APOL1 gene are highly associated with end stage kidney disease risk previously attributed to the MYH9 gene

MYH9 has been proposed as a major genetic risk locus for a spectrum of nondiabetic end stage kidney disease (ESKD). We use recently released sequences from the 1000 Genomes Project to identify two western African-specific missense mutations (S342G and I384M) in the neighboring APOL1 gene, and demonstrate that these are more strongly associated with ESKD than previously reported MYH9 variants. The APOL1 gene product, apolipoprotein L-1, has been studied for its roles in trypanosomal lysis, autophagic cell death, lipid metabolism, as well as vascular and other biological activities. We also show that the distribution of these newly identified APOL1 risk variants in African populations is consistent with the pattern of African ancestry ESKD risk previously attributed to MYH9.

Mapping by admixture linkage disequilibrium (MALD) localized an interval on chromosome 22, in a region that includes the MYH9 gene, which was shown to contain African ancestry risk variants associated with certain forms of ESKD (Kao et al. 2008; Kopp et al. 2008). MYH9 encodes nonmuscle myosin heavy chain IIa, a major cytoskeletal nanomotor protein expressed in many cell types, including podocyte cells of the renal glomerulus. Moreover, 39 different coding region mutations in MYH9 have been identified in patients with a group of rare syndromes, collectively termed the Giant Platelet Syndromes, with clear autosomal dominant inheritance, and various clinical manifestations, sometimes also including glomerular pathology and chronic kidney disease (Kopp 2010; Sekine et al. 2010). Accordingly, MYH9 was further explored in these studies as the leading candidate gene responsible for the MALD signal. Dense mapping of MYH9 identified individual single nucleotide polymorphisms (SNPs) and sets of such SNPs grouped as haplotypes that were found to be highly associated with a large and important group of ESKD risk phenotypes, which as a consequence were designated as MYH9-associated nephropathies (Bostrom and Freedman 2010). These included HIV-associated nephropathy (HIVAN), primary nonmonogenic forms of focal segmental glomerulosclerosis, and hypertension affiliated chronic kidney disease not attributed to other etiologies (Bostrom and Freedman 2010). The MYH9 SNP and haplotype associations observed with these forms of ESKD yielded the largest odds ratios (OR) reported to date for the association of common variants with common disease risk (Winkler et al. 2010). Two specific MYH9 variants (rs5750250 of S-haplotype and rs11912763 of F-haplotype) were designated as most strongly predictive on the basis of Receiver Operating Characteristic analysis (Nelson et al. 2010). These MYH9 association studies were then also extended to earlier stage and related kidney disease phenotypes and to population groups with varying degrees of recent African ancestry admixture (Behar et al. 2010; Freedman et al. 2009a, b; Nelson et al. 2010), and led to the expectation of finding a functional African ancestry causative variant within MYH9. However, despite intensive efforts including re-sequencing of the MYH9 gene no suggested functional mutation has been identified (Nelson et al. 2010; Winkler et al. 2010). This led us to re-examine the interval surrounding MYH9 and to the detection of novel missense mutations with predicted functional effects in the neighboring APOL1 gene, which are significantly more associated with ESKD than all previously reported SNPs in MYH9.

Podocyte glutamatergic signaling contributes to the function of the glomerular filtration barrier

Podocytes possess the complete machinery for glutamatergic signaling, raising the possibility that neuron-like signaling contributes to glomerular function. To test this, we studied mice and cells lacking Rab3A, a small GTPase that regulates glutamate exocytosis. In addition, we blocked the glutamate ionotropic N-methyl-d-aspartate receptor (NMDAR) with specific antagonists. In mice, the absence of Rab3A and blockade of NMDAR both associated with an increased urinary albumin/creatinine ratio. In humans, NMDAR blockade, obtained by addition of ketamine to general anesthesia, also had an albuminuric effect. In vitro, Rab3A-null podocytes displayed a dysregulated release of glutamate with higher rates of spontaneous exocytosis, explained by a reduction in Rab3A effectors resulting in freedom of vesicles from the actin cytoskeleton. In addition, NMDAR antagonism led to profound cytoskeletal remodeling and redistribution of nephrin in cultured podocytes; the addition of the agonist NMDA reversed these changes. In summary, these results suggest that glutamatergic signaling driven by podocytes contributes to the integrity of the glomerular filtration barrier and that derangements in this signaling may lead to proteinuric renal diseases.

Focal Segmental Glomerulosclerosis and Renal Transplantation

Primary focal segmental glomerulosclerosis (FSGS) is a major cause of nephrotic syndrome and eventual end-stage renal disease. It is known to be due to an abnormality of the visceral epithelial cells (podocytes) of the glomerulus. The morphological hallmark of primary FSGS is diffuse effacement of podocyte foot processes. The etiology of the podocyte damage is not been clearly established. FSGS can also be a secondary process due to underlying conditions including obesity and heroin use. In the secondary processes, the mechanism appears to be a decreased ratio of podocytes to the glomerular filtration surface area. Familial forms of FSGS also exist due to alterations of several different podocyte proteins. Primary FSGS is an increasing cause of end-stage renal disease. Recurrence of severe FSGS in renal allograft recipients presents a major challenge to transplant physicians. The incidence of recurrence is generally accepted to be between 20% and 30%. Risk factors for and characteristics of recurrence include a rapid progression of the primary disease to end-stage renal failure, early onset of nephrotic range proteinuria after allografting, frequent loss of the allograft, a high frequency of recurrence in subsequent allografts, and children less than 15 years of age. Some investigators have identified a circulating factor called the FSGS factor that appears to be associated with recurrence after transplantation. This factor has been shown to be a protein between 30 and 50 kd molecular weight. Logically, the possibility of a circulating factor associated with recurrence of FSGS led investigators to treat patients with plasmapheresis. Several studies have been reported with varying success. The response of patients to plasmapheresis seems to be completely individual. Other studies have added cyclophosphamide and/or mycophenolate mofetil to the plasmapheresis protocol. Again success in these studies has been variable. However, because some patients show complete recovery with plasmapheresis, individuals who develop recurrent FSGS after transplantation usually are given a trial of plasmapheresis therapy.

HIV-1 Tat reduces nephrin in human podocytes: a potential mechanism for enhanced glomerular permeability in HIV-associated nephropathy

OBJECTIVE

To determine whether HIV-1 Tat may directly alter glomerular permeability in HIV-associated nephropathy (HIVAN).

DESIGN

Heavy proteinuria is a hallmark of HIVAN. The slit diaphragm is the ultimate glomerular filtration barrier critical for maintaining the efficiency of the ultrafiltration unit of the kidney. In this study, we evaluated the direct effect of Tat protein on the permeability of isolated glomeruli and on the expression of nephrin, the main slit diaphragm component, by human cultured podocytes.

METHODS

Permeability was studied by measuring the permeability to albumin in isolated rat glomeruli. We also evaluated the expression of nephrin in human cultured podocytes by using immunofluorescence and Western blot.

RESULTS

We found that Tat increased albumin permeability in isolated glomeruli, and rapidly induced the redistribution and loss of nephrin in cultured podocytes. Pretreatment of glomeruli and podocytes with blocking antibodies showed that Tat reduced nephrin expression by engaging vascular endothelial growth factor receptors types 2 and 3 and the integrin alphavbeta3. Pre-incubation of podocytes with two platelet-activating factor (PAF) receptor antagonists prevented the loss and redistribution of nephrin induced by Tat, suggesting that PAF is an intracellular mediator of Tat action. Tat induced a rapid PAF synthesis by podocytes. When podocytes transfected to overexpress PAF-acetylhydrolase, the main catabolic enzyme of PAF, were stimulated with Tat, the redistribution and loss of nephrin was abrogated.

CONCLUSION

The present results define a mechanism by which Tat may reduce nephrin expression in podocytes, thus increasing glomerular permeability. This provides new insights in the understanding of HIVAN pathogenesis.

Podocin-Related Mechanisms in Posttransplantation Recurrence of Focal Segmental Glomerulsclerosis

Posttransplantation recurrence of focal segmental glomerulosclerosis (FSGS) is one of the most disarming events in human pathology with important social and psychological consequences. It usually occurs in 30% to 50% of patients affected by the primary form of the disease with an abrupt onset in the majority of cases occurring within 1 month of the transplantation. Prediction of recurrent cases and early therapy with plasmapheresis are the main goals of the therapy. Although the mechanism of posttransplantation recurrence is still obscure, it has been proposed to be of a multifactorial origin, in which plasma factors determine the shedding of proteins of the slit-diaphragm, such as nephrin and podocin, with structural alterations of the ultra-filtering unit of the glomerulus. Low resynthesis of podocin and/or haplo-insufficiency due to heterozygous mutations should represent significant predisposing factors to proteinuria. In this review, the role of podocin in posttransplantation recurrence will be evaluated focusing on the possibility that resynthesis of the protein could represent a key step also for stable normalization of the renal filter. The recent characterization of the podocin promoter cis- and trans- acting elements and the possibility to characterize low- and high-podocin producer haplotypes offer opportunities to evaluate the capacity for podocin resynthesis in the donor kidney. A review of the literature on posttransplantation recurrence of FSGS in patients originally carrying homozygous and/or heterozygous NPHS2 mutations supports the general idea of a multifactorial origin of the primary disease that can be extended to the pathogenesis of posttransplantation recurrence.

Recurrence of proteinuria following renal transplantation in congenital nephrotic syndrome of the Finnish type

We report a Caucasian boy of Italian descent with congenital nephrotic syndrome of the Finnish type (NPHS1, CNF, MIM 256300) who developed recurrence of proteinuria and hypoalbuminemia on the seventh post-operative day following living related renal transplantation from his paternal aunt. The allograft biopsy was normal except for effacement of podocyte foot processes on electron microscopy. He was treated by the substitution of mycophenolate mofetil with cyclophosphamide for 12 weeks, in addition to cyclosporine, prednisone and daclizumab. His proteinuria resolved quickly following the initiation of cyclophosphamide treatment, and he remains in remission 4 years after receiving his transplant. His native and allograft kidneys were evaluated for nephrin expression by immunohistochemistry, DNA analysis for the NPHS1 mutation, serum for the presence of auto-antibodies to nephrin by both enzyme-linked immunosorbent assay (ELISA) and fetal glomeruli immunofluorescence assay, and serum for glomerular permeability to albumin (Palb) activity using a functional in vitro assay for Palb. Nephrin expression was completely absent in the native kidney, while it was decreased in the allograft compared with normal. DNA analysis of the NPHS1 gene revealed mutations 3248G>T and 3250delG in exon 24, causing G1083V and 1084Vfs, respectively, inherited from his father, and 3478C>T in exon 27, that leads to R1160X, inherited from his mother. Serum was negative for auto-antibodies to nephrin. Interestingly, the Palb activity was increased at the time of recurrence of proteinuria following transplantation (Palb 0.73+/-0.10) and remained elevated when retested more than 3 years later (Palb 0.54+/-0.09). This is the first report of increased Palb activity in recurrence of proteinuria following transplantation in NPHS1. We speculate the role of increased Palb activity in the recurrence of proteinuria following transplantation in NPHS1.

Recurrence of focal-segmental glomerulosclerosis in children after renal transplantation: clinical and genetic aspects

Focal segmental glomerulosclerosis (FSGS) is the primary renal disease in approximately one-tenth of pediatric patients receiving a renal allograft. Recurrence of proteinuria after renal transplantation is observed in approximately 30% of patients and negatively impacts graft survival. Risk factors for recurrence are a chronological age <15 years at onset of the nephrotic syndrome and a rapid progression of the disease in the native kidneys leading to end-stage renal disease in less than 3 years. Mesangial proliferation in the native kidneys is also an important negative predictive factor for disease recurrence. With rapid recurrence of FSGS and loss of the allograft, further renal transplants also carry a high likelihood of FSGS recurrence. Different pathogenic factors have been discussed for the recurrence of proteinuria/FSGS in the transplanted kidney, especially the involvement of a proteinuric circulating factor, whose production seems to follow T-cell dysfunction. In the last decade, mutations in genes encoding podocyte proteins have been identified in different forms of hereditary FSGS. Mutations of NPHS2 were detected in 26-38% of familial autosomal recessive steroid-resistant NS (SRNS), 6-19% of sporadic cases of SRNS, and in few adult patients with FSGS. Large multicenter studies demonstrated that patients with two pathogenic NPHS2 mutations have a very low risk of recurring FSGS after renal transplantation, whereas patients with only one mutation presumably have a risk comparable to non-NPHS2 FSGS patients. The management of FSGS following renal transplantation remains controversial. Following the assumption of a putative permeability factor, several studies have suggested the efficacy of plasmapheresis in inducing remission, preferably in conjunction with high-dose cyclosporine A or cyclophosphamide. Prospective studies will be necessary to better evaluate different therapeutic approaches.

Preemptive plasmapheresis and recurrence of FSGS in high-risk renal transplant recipients

Recurrent focal segmental glomerulosclerosis (FSGS) following transplantation is ascribed to the presence of a circulating FSGS permeability factor (FSPF). Plasmapheresis (PP) can induce remission of proteinuria in recurrent FSGS. This study addressed the efficacy of pre-transplant PP in decreasing the incidence of recurrence in high-risk patients. Ten patients at high-risk for FSGS recurrence because of rapid progression to renal failure (n = 4) or prior transplant recurrence of FSGS (n = 6) underwent a course of 8 PP treatments in the peri-operative period. Recurrences were identified by proteinuria >3 g/day and confirmed by biopsy. Seven patients, including all 4 with first grafts and 3 of 6 with prior recurrence, were free of recurrence at follow-up (238-1258 days). Final serum creatinine in 8 patients with functioning kidneys averaged 1.53 mg/dL. FSGS recurred within 3 months in 3 patients, each of whom had lost prior transplants to recurrent FSGS. Two of these progressed to end-stage renal disease (ESRD) and the third has significant renal dysfunction. Based on inclusion criteria, recurrence rates of 60% were expected if no treatment was given. Therefore, PP may decrease the incidence of recurrent FSGS in high-risk patients. Definitive conclusions regarding optimal management can only be drawn from larger, randomized, controlled studies.

Complete remission of post-transplant FSGS recurrence by long-term plasmapheresis

Focal segmental glomerulosclerosis (FSGS) is known to recur in approximately 30% of renal allografts with graft loss in about half of these cases. The exact etiology remains unclear, though a putative circulating permeability factor or loss of inhibitory substances is being discussed. Different therapeutic approaches have been used. We report on a 10-year-old Arabian boy with a recurrence of FSGS immediately after transplantation. In addition to intensifying immunosuppressive therapy with high-dose cyclosporin A and cyclophosphamide, plasmapheresis was initiated and remission was achieved after 8 months. Three weeks after cessation of plasmapheresis a relapse occurred. Plasmapheresis was resumed and remission was achieved again after four additional sessions. The interval between plasmapheresis treatments was then gradually increased and fourteen months after transplantation plasmapheresis was stopped again. Since then (1.5 years after cessation of treatment) the patient has been in complete remission without any further episode of proteinuria. In conclusion, complete and sustained remission with stable renal function was achieved in our patient by long-term plasmapheresis in combination with intensified immunosuppression. Therefore, continuation of plasmapheresis treatment should be considered even in the situation of initial non-response.

NPHS2 (Podocin) mutations in nephrotic syndrome. Clinical spectrum and fine mechanisms

Nephrotic syndrome (NS) is the most frequent cause of proteinuria in children and is emerging as a leading cause of uremia. Molecular studies in families with recessive NS have led to the discovery of specialized molecules endowed in podocytes that play a role in proteinuria. This review focalizes the key position of podocin (NPHS2 gene) in this rapidly evolving field and furnishes a compendium to those involved in clinics and genetics of NS. Screening for NPHS2 mutations have been done in sporadic NS and familial cases with recessive inheritance, documenting a mutation detection rate of 45-55% in families and 8-20% in sporadic NS according to the different groups and considering all the clinical phenotypes. Almost 50 NPHS2 mutations have been reported and variants and/or non silent polymorphisms potentially involved in proteinuria were recognized. Personalized data on clinical aspects related to responsiveness to drugs, evolution to end stage renal failure and post-transplant outcome are reported. Functional studies and cell sorting experiments demonstrated retention in the endoplasmic reticulum of most mutants involving the stomatin domain. Pull-down experiments with the common R229Q polymorphism demonstrated an altered interaction with nephrin that affects the stability of the functional unit. Overall, data are here presented that underscore a major role of inherited defects of NPHS2 in NS in children (including a relevant impact in sporadic cases) and give the functional rationale for the association. A practical compendium is also given to clinicians involved in the management of NS that should modify the classic therapeutic approach.

Renal glomerular permselectivity and vascular endothelium

The glomerular barrier is the kidney's physical block to the unrestricted flow of molecules from the plasma into the urinary space. Its exquisite selectivity allows solutes and water in the glomerular capillaries to pass through but it prevents the bulk of plasma proteins, most notably albumin, from crossing. Classically the barrier consists of three components: glomerular endothelium, glomerular basement membrane and glomerular epithelium (podocytes) with slit diaphragm. A lot of investigations are experimental but they are sufficient to show the pivotal role of endothelium in glomerular proteinuria. In this study the author discuss glomerular endothelium with particular emphasis on the barrier presumed to be imparted by endothelium-glomerular basement membrane-podocyte interactions. By specialized glomerular endothelial structure (caveolae, tight junctions, endothelium glycocalyx) and by circulating permeability factors (albumin, orosomucoid, apolipoproteins, Amadori's products). Concluding remarks underline the significance to study the glomerular vascular endothelial dysfunction.

Glomerular albumin permeability as an in vitro model for characterizing the mechanism of focal glomerulosclerosis and predicting post-transplant recurrence

The putative mechanisms of proteinuria in idiopathic focal glomerulosclerosis and of its post-transplant recurrence are discussed. It is proposed that a balance between circulating factors with permeability activity on glomeruli and putative inhibitors play a key role. The characterization of inductors is currently in progress; most inhibitors appear to be apolipoproteins (mainly apoJ and apo E) but we cannot exclude other substances. The goal is now to evaluate the concentration of both inducers and inhibitors of glomerular permeability in vivo. Permeability activity in plasma of patients with FSGS with and without recurrence of the disease may be evaluated by an in vitro functional essay with isolated glomeruli. Published data on permeability activity evaluated with this method in different proteinuric states gave, however, controversial results and this test cannot be readily considered of clear clinical utility. Only the definitive characterization and quantification in vivo of the different molecules that play a role in FSGS may furnish adequate answer.

Sera from patients with collapsing focal segmental glomerulosclerosis increase albumin permeability of isolated glomeruli

The collapsing variant of focal segment glomerulosclerosis (FSGS) is characterized by heavy proteinuria and rapid progression to renal failure. Its cause is not known. We have characterized a substance in the circulation of patients with classic FSGS that increases in vitro permeability of glomeruli to albumin (P(alb)) and causes proteinuria when injected into rats. Inclusion of normal serum prevents the increase in P(alb) caused by this FSGS factor. We investigated the effect of sera from patients with collapsing FSGS on P(alb), as well as the effect of inclusion of normal serum. Isolated glomeruli were incubated with serum from each of 11 patients with collapsing FSGS (1:50 dilution) or with patient serum and an equal volume of pooled normal serum. P(alb) was determined on the basis of changes in glomerular volume in response to an oncotic gradient. Sera from 10 of the 11 patients with collapsing FSGS increased P(alb) of isolated glomeruli to a value of 0.5 or greater. In each of the 5 cases tested, inclusion of normal serum abolished the increase in P(alb). Sera of patients with collapsing FSGS increased glomerular P(alb). Our finding that the increase in P(alb) is abolished by normal serum suggests that the substance and its mechanism of action are similar or identical to the FSGS factor we have isolated from the plasma of patients with recurrent FSGS. The presence of a circulating factor in collapsing FSGS has implications for prognosis and treatment in primary and recurrent collapsing FSGS.

The focal segmental glomerulosclerosis permeability factor: biochemical characteristics and biological effects

Focal segmental glomerulosclerosis (FSGS) is characterized by steroid resistant nephrotic syndrome and progression to end-stage renal disease. Proteinuria in certain patients with FSGS may be caused by a circulating factor (FSGS permeability factor [FSPF]). The current report documents the biochemical characteristics and the biological and molecular effects of 70% ammonium sulfate supernatant of plasma from patients with recurrence of FSGS after transplantation (FSGS 70% supernatant). FS permeability activity, defined as the capacity of plasma from patients with FSGS to increase albumin permeability (P(alb)) of isolated glomeruli, was assessed in vitro. Permeability activity was not affected by lyophilization. FSPF bound strongly to matrices containing Mono-Q anion exchanger or protein A. It eluted from matrix-bound Cibacron blue F3GA over a wide range of salt concentrations, indicating a potential binding with other proteins, such as albumin. FSPF caused a maximal increase in P(alb) within 2 mins of incubation in vitro. Cellular proteins isolated from glomeruli with increased P(alb) showed decreased tyrosine phosphorylation of focal adhesion kinase, paxillin, and other proteins. Tyrosine phosphatase ]inhibition prevented the increase in P(alb). Intravenous administration of as little as 3 mg protein in FSGS 70% supernatant increased P(alb), while 9 mg or more were required to produce proteinuria. We conclude that FSPF is a low-molecular-weight protein, carries an anionic charge, and binds to protein A. Effects of FSPF on the glomerular permeability barrier are rapid and dose dependent and involve signaling through altered phosphorylation of cellular proteins. Identification of these biochemical and biological characteristics may be used to design strategies for removing FSPF from circulation and for purification and identification of this factor.

Nephrotic syndrome in childhood

Childhood nephrotic syndromes are most commonly caused by one of two idiopathic diseases: minimal-change nephrotic syndrome (MCNS) and focal segmental glomerulosclerosis (FSGS). A third distinct type, membranous nephropathy, is rare in children. Other causes of isolated nephrotic syndrome can be subdivided into two major categories: rare genetic disorders, and secondary diseases associated with drugs, infections, or neoplasia. The cause of idiopathic nephrotic syndrome remains unknown, but evidence suggests it may be a primary T-cell disorder that leads to glomerular podocyte dysfunction. Genetic studies in children with familial nephrotic syndrome have identified mutations in genes that encode important podocyte proteins. Patients with idiopathic nephrotic syndrome are initially treated with corticosteroids. Steroid-responsiveness is of greater prognostic use than renal histology. Several second-line drugs, including alkylating agents, ciclosporin, and levamisole, may be effective for complicated and steroid-unresponsive MCNS and FSGS patients. Nephrotic syndrome is associated with several medical complications, the most severe and potentially fatal being bacterial infections and thromboembolism. Idiopathic nephrotic syndrome is a chronic relapsing disease for most steroid-responsive patients, whereas most children with refractory FSGS ultimately develop end-stage renal disease. Research is being done to further elucidate the disorder's molecular pathogenesis, identify new prognostic indicators, and to develop better approaches to treatment.

Pathobiochemistry of nephrotic syndrome

Nephrotic syndrome is a clinical and laboratory syndrome caused by the increased permeability of the glomerular capillary wall for macromolecules. Nephrotic syndrome is a potentially life-threatening state and persistent nephrotic syndrome has a poor prognosis with a high risk of progression to end-stage renal failure and a high risk of cardiovascular complications due to severe hyperlipidemia. Pathogenesis of increased glomerular permeability in different glomerular diseases has not been fully elucidated. Recently, identification of the mutated genes for some podocyte proteins (nephrin, podocin, alpha-actinin-4) in rare familial forms of nephrotic syndrome shed has new light on the molecular mechanisms of glomerular permselectivity. Gradually it becomes apparent that sporadic mutations of podocyte proteins (e.g., podocin) may be present even in some patients with acquired nephrotic syndrome. Expression of other podocyte proteins may change during the course of experimental nephrotic syndrome, possibly as a response to podocyte damage resulting either in apoptosis or stimulation of proliferation and some form of repair, including glomerular sclerosis. Better understanding of these mechanisms could clearly also have therapeutic implications. Glomerular permeability factors are believed to play a role in some noninflammatory glomerular diseases, mainly minimal change disease and focal segmental glomerulosclerosis, but their molecular identification remains elusive, possibly due to the nonhomogeneous nature of the underlying diseases. As an example, focal segmental glomerulosclerosis possibly can be caused by the sporadic mutation of some genes for podocyte proteins, increased production of glomerular permeability factor (possibly by T lymphocytes), or the loss of inhibitors of glomerular permeability factors in nephrotic urine. Clearly the factors causing increased glomerular permeability and factors perpetuating glomerular sclerosis are not necessarily the same. Proteinuria does not seem to be only the consequence of glomerular damage, but it may possibly cause tubular damage and initiate interstitial fibrosis and thus contribute to the progression of chronic renal failure in proteinuric renal diseases. Recent insights into the mechanisms of tubular protein reabsorption may give new tools for preventing the progression of chronic renal disease. Cubilin inhibitors could potentially ameliorate tubular and interstitial damage in patients with heavy proteinuria refractory to treatment. Nephrotic hyperlipidemia is accompanied with increased risk of cardiovascular complications and should be treated in all patients with persistent nephrotic syndrome. The putative positive effect of hypolipidemic drugs (namely statins) on the cardiovascular risk and potentially also on the rate of progression of chronic renal failure remains to be demonstrated in prospective controlled studies. Recent progress in understanding podocyte biology in rare inherited glomerular diseases gives the chance to understand in the near future the molecular pathogenesis of increased glomerular permeability in the much more common acquired forms of nephrotic syndrome.

Other immunosuppressive agents for focal segmental glomerulosclerosis

A prolonged course with corticosteroids represents the first therapeutic approach for nephrotic patients with focal segmental glomerulosclerosis (FSGS). In patients with contraindications to steroids or in those who do not respond to steroids or cyclosporine, cytotoxic agents, mycophenolate mofetil (MMF), plasmapheresis, and low-density lipoprotein (LDL) apheresis have been tried as alternative treatments. A short-term treatment with cytotoxic agents often is ineffective in steroid-resistant patients However, an aggressive and prolonged treatment with cytotoxic agents combined with corticosteroids proved to be effective in more than half of steroid-resistant children. In adults, the response to cytotoxic agents was good in steroid-responsive patients, but was poor in steroid-resistant patients. Better results were observed when cytotoxic therapy was prolonged for several months. The problem with these drugs is that long-term immunosuppression may be complicated by severe side effects including a major risk for cancer. Uncontrolled studies reported that MMF can induce some reduction of proteinuria, but complete remission of proteinuria was rare and no data on long-term follow-up evaluation with this drug are available. Good results have been reported with plasmapheresis, immunoadsorption, and lipopheresis. However, all the reports were uncontrolled, small sized, and with short-term follow-up evaluation. In conclusion, there are several therapeutic options for patients who respond to steroids and have further relapses of nephrotic syndrome, but how to treat steroid-resistant patients is still a matter of debate. Nevertheless, a 6-month trial with cytotoxic agents or MMF can be offered to steroid-resistant patients to identify the few patients who respond to these agents. The preliminary results with plasmapheresis or lipopheresis are promising but further studies are needed to assess the role of these treatments.

Permeability factors in focal segmental glomerulosclerosis

The pathologic diagnosis of focal segmental glomerulosclerosis (FSGS) is associated with a syndrome of steroid-resistant nephrotic syndrome and progressive renal insufficiency. The incidence of FSGS has increased in recent years. Known causes of FSGS include genetic abnormalities, viral infections, decreased nephron number, and hyperperfusion/hyperfiltration. The etiology is unknown in the majority of cases. FSGS recurs after initial renal transplantation in as many as 30% to 50% of patients. Recent studies have verified the hypothesis that plasma of patients with FSGS contains a factor or factors that increase permeability of glomerular capillaries and cause proteinuria after injection into rats. Patients who experience posttransplant recurrence of FSGS and those with rapidly progressive disease exhibit this activity. Permeability activity has been verified in functional assays and defined by measurement of albumin permeability (P(alb)) or glomerular volume variation (GVV). Permeability activity is decreased by plasmapheresis or immunoadsorption and can be recovered from discarded plasma or eluate from adsorption materials. Studies from our laboratory indicate that permeability activity is carried by small, highly glycosylated, hydrophobic protein(s)/peptide(s). Normal plasma contains substances capable of blocking or inactivating the FSGS permeability factor. Pharmacologic agents including cyclosporine, indomethacin, and derivatives of Trypterigium wilfordii also block permeability activity in vitro. The observation that permeability activity can be blocked by diverse agents raises hope that specific therapy may be designed for FSGS. Future investigations will permit identification of the active FSGS permeability factor, of mechanisms that initiate and perpetuate proteinuria, and of interventions to prevent renal failure in native kidneys and recurrence of disease in renal allografts.

Apolipoprotein E in idiopathic nephrotic syndrome and focal segmental glomerulosclerosis

BACKGROUND

Hyperlipemia characterizes nephrotic syndrome (NS) and contributes to the progression of the underlying nephropathy. The data in the literature support an implication of apolipoprotein E (apoE) in both hyperlipemia and focal segmental glomerulosclerosis (FSGS), a malignant condition associated with NS.

METHODS

The apoE genotype was determined in 209 nephrotic patients, who were classified according to age and their response to steroids as resistant children (N = 96) and adults (43), and steroid dependent (33) and steroid responder (37) children. A total of 123 presented the histological features of FSGS. In a subgroup of 28 patients, serum and urinary levels of apoE and renal deposits were evaluated by immunofluorescence.

RESULTS

The allelic frequencies of the three major haplotypes epsilon2, epsilon3, and epsilon4 were the same in nephrotic patients versus controls, and homozygosity for epsilon3epsilon3 was comparably the most frequent genotype (70 vs. 71%) followed by epsilon3epsilon4, epsilon2epsilon3, epsilon2epsilon4, epsilon4epsilon4. Serum levels of apoE were fivefold higher in NS and in FSGS patients than in controls, with a direct correlation with hypercholesterolemia and proteinuria. ApoE genotypes did not influence serum levels. Urinary levels were 1/10,000 of serum with an increment in nephrotic urines. Finally, immunofluorescence demonstrated the absence of apoE in sclerotic glomeruli, while comparably nephrotic patients with membranous nephropathy had an increased glomerular expression of apoE.

CONCLUSIONS

ApoE is dysregulated in NS with a marked increment in serum, which is a part of the complex lipid metabolism. Down-regulation of glomerular apoE instead is a peculiarity of FSGS and may contribute to the pathogenesis of the disease. The normal distribution of apoE genotypes in nephrotic patients with FSGS excludes a pathogenetic role of genetic variants.

Depletion of clusterin in renal diseases causing nephrotic syndrome

BACKGROUND

Clusterin is a lipoprotein that has anti-complement effects in membranous nephropathy (MN). In focal segmental glomerulosclerosis (FSGS), it inhibits permeability plasma factor activity and could influence proteinuria. Moreover, with aging, knockout mice for clusterin develop a progressive glomerulopathy with sclerosis.

METHODS

Since little is known about clusterin metabolism in humans, we determined clusterin levels and composition in the sera and urine of 23 patients with MN, 25 with FSGS and 23 with steroid-responsive nephrotic syndrome (NS). Renal localization was evaluated by immunofluorescence and morphometry.

RESULTS

Serum clusterin was markedly reduced in active MN, in FSGS and in children with NS compared to controls; after stable remission of proteinuria, nearly normal levels were restored. Among various biochemical variables, serum clusterin was inversely correlated with hypercholesterolemia. Urinary clusterin, representing a 0.01 fraction of serum, was higher in the urine from normal subjects and FSGS patients in remission with proteinuric MN, FSGS and idiopathic NS; clusterin was inversely correlated with proteinuria. In all cases, urinary and serum clusterin was composed of the same 80 kD isoforms. Finally, a decrease in focal segmental or global clusterin staining was found in FSGS glomeruli, especially in areas of sclerosis. Instead, in MN an overall increment of staining was observed that ranged from mild/focal to very intense/diffuse.

CONCLUSIONS

The overall pool of clusterin is reduced in glomerular diseases causing nephrotic syndrome, with hypercholesterolemia appearing as the unifying feature. Depletion of clusterin should negatively affect the clinical outcome in nephrotic patients and efforts should be aimed at normalizing clusterin overall pool.

The renal risks of smoking: an update

PURPOSE OF REVIEW

Smoking increases the renal risk both in diabetic and in nondiabetic renal disease. The purpose of the present review is to summarize the current state of knowledge about this important remediable renal risk factor.

RECENT FINDINGS

The deleterious effect of smoking on renal function extends beyond patients with primary or secondary renal disease and patients with a renal transplant, because recent studies document a relation between smoking and loss of filtration rate, even in cardiovascular high-risk populations without primary renal disease such as the elderly, the patient with severe essential hypertension, or the patient with widespread atherosclerosis. Furthermore, recent studies show that in nondiabetic patients without primary renal disease, albuminuria, a potential surrogate marker of glomerular damage, is correlated with smoking. The mechanisms underlying the adverse renal effects of smoking are still incompletely understood. Beyond its effect on progression of renal failure, smoking is also an important cardiovascular risk factor in the patient with renal failure or the patient with a renal transplant.

SUMMARY

Smoking is one of the most important remediable renal risk factors. The exact mechanisms of smoking-induced renal damage remain to be determined. For all the above reasons cessation of smoking should be recommended to renal patients - a recommendation which is infrequently given and even less frequently followed.

Proteinuria after injection of human focal segmental glomerulosclerosis factor

BACKGROUND

Recurrent focal segmental glomerulosclerosis (FSGS) is heralded by proteinuria that may remit after treatment with plasmapheresis or immunoadsorption. Study of recurrent FSGS has been hampered by lack of an animal model that exhibits a pattern of proteinuria that mimics human disease. We have obtained a component of FSGS patient plasma (FSGS factor) that increases glomerular albumin permeability (P(alb)) in vitro and causes transient proteinuria in vivo.

METHODS

Plasma fractions containing FSGS factor and comparable plasma fractions from normal donors were injected into normal male Sprague-Dawley rats. Urinary protein, albumin, and creatinine were measured at various time points. Additionally, plasma samples from test animals were collected after injection and tested for FS activity defined by increased P(alb). Finally, glomeruli were isolated from animals after injection and P(alb) of these glomeruli tested.

RESULTS

Proteinuria and albuminuria were increased by 24 hr after injection with FSGS factor, and returned to baseline by 48 hr after injection. Injection with the same fraction of normal plasma had no effect on urinary protein. FSGS factor increased urinary protein in a dose-dependent manner. Serum collected from rats 15 or 60 min after injection with FSGS factor increased P(alb) of glomeruli in vitro, whereas serum collected 3 or more hours after injection had no effect. Glomeruli isolated from rats receiving injections with FSGS factor had increased in vitro P(alb) compared with glomeruli from rats injected with a fraction from normal plasma.

CONCLUSIONS

We have demonstrated that a single injection of FSGS factor increases P(alb) and, causes transient albuminuria and proteinuria in rats. FS activity in the plasma of recipient rats is also transient. This is the first detailed description of the time course and dose-dependence of proteinuria caused by FSGS factor in an animal model.

Induction of metallothionein-I protects glomeruli from superoxide-mediated increase in albumin permeability

Metallothioneins (MT) are low-molecular-weight, heat-stable, cysteine-rich proteins with four isoforms. MT-I and MT-II are ubiquitous and are induced by oxidative, physical, and chemical stress. MT-I is an efficient scavenger of superoxide (*O2) and hydroxyl ion (OH(-)). We have demonstrated that *O2 and hypohalous acid can cause an increase in glomerular albumin permeability (P(alb)) in vitro. The purpose of this study was to document the protective effect of MT gene product on the *O2-mediated increase in P(alb). Glomeruli from Sprague-Dawley rats in 4% BSA medium were incubated for 4 hr at 37 degrees C in duplicate tubes. Each set contained glomeruli alone or with 5 microM Cd(++), 0.3 mM Spermine-NONOate (NO donor), 0.3 mM Sulfo-NONOate (nitrous oxide donor), 0.6 mM SNP (nonspecific NO donor) and SNP + carboxy-PTIO (10 mg/ml). After incubation, one set of tubes was used to isolate total RNA for the measurement of the mRNA levels of MT-I by reverse transcriptase polymerase chain reaction (RT-PCR). Duplicate tubes were incubated for an additional 10 min with 10 nM of *O2, and P(alb) was measured using video microscopy. RT-PCR of total RNA from Cd(++) and Spermine-NONOate treated glomeruli revealed a 2-fold induction of MT-I expression at the mRNA level. *O2 caused a significant increase in P(alb) (0.8 +/- 0.06 vs. control 0.0 +/- 0.12, P < 0.05) and induction of MT-I in glomeruli by Cd(++) or by Spermine-NONOate blocked this effect (0.21 +/- 0.12 and 0.24 +/- 0.19, respectively, P < 0.05 vs. *O2). In contrast, Sulfo-NONOate and SNP did not induce mRNA for MT-I in glomeruli and did not provide protection against *O2-mediated increase in P(alb.) We conclude that MT-I gene products may play an important role in protecting the glomerular filtration barrier from the injury induced by reactive oxygen species in immune and/or nonimmune renal diseases.