Podocyte effacement closely links to suPAR levels at time of posttransplantation focal segmental glomerulosclerosis occurrence and improves with therapy

Pathogenesis of Focal Segmental Glomerulosclerosis and Related Disorders

Focal segmental glomerulosclerosis (FSGS) is the morphologic manifestation of a spectrum of kidney diseases that primarily impact podocytes, cells that create the filtration barrier of the glomerulus. As its name implies, only parts of the kidney and glomeruli are affected, and only a portion of the affected glomerulus may be sclerosed. Although the diagnosis is based primarily on microscopic features, patient stratification relies on clinical data such as proteinuria and etiological criteria. FSGS affects both children and adults and has an elevated risk of progression to end-stage renal disease. The prevalence of FSGS is rising among various populations, and the efficacy of various therapies is limited. Therefore, understanding the pathophysiology of FSGS and developing targeted therapies to address the complex needs of FSGS patients are topics of great interest that are currently being studied across various clinical trials. We discuss the etiology of FSGS, describe the major contributing pathophysiological pathways, and outline emerging therapeutic strategies along with their pitfalls.

The Relationship Between Soluble Urokinase-Type Plasminogen Activator Receptor (suPAR) Levels and Treatment Response in Patients With Glomerulonephritis: A Single-Center Experience

Aim Soluble urokinase-type plasminogen activator receptor (suPAR) is an important protein that is reported to increase in a broad range of inflammatory processes. We aimed to determine whether suPAR is a significant biomarker in glomerulonephritis for distinguishing patients with treatment response from patients without treatment response in our study. Materials and methods This was a prospective study in which 117 patients with biopsy-proven glomerulonephritis and 54 healthy individuals without a known chronic disease (control group) were enrolled. A total of 117 patients were divided into two groups: "treatment responsive" and "treatment nonresponsive." Blood samples were collected from the patients upon their outpatient clinic visits, and the demographical and lab parameters were compared between the groups. Results For the patient group consisting of 117 individuals, 56.4% were male, the mean age was 49.6 years, and the mean follow-up duration was 32.2 months. The most commonly diagnosed glomerular disease was focal segmental glomerulosclerosis (FSGS), followed by IgA nephropathy and membranoproliferative glomerulonephritis, respectively. While suPAR levels were significantly higher in the patient group (166.06 ± 127.66 vs. 119.67 ± 70.53 pg/ml, p = 0.001) (suPAR level ± standard deviation), no significant relationship was found between suPAR levels, treatment response status, and disease severity. Besides, there was no significant relationship between suPAR levels and proteinuria levels, BMI of the patients, and the type of immunosuppressive agent used in the treatment and BMI. Conclusion Our study showed that suPAR levels could distinguish a patient with glomerulonephritis from a healthy individual, whereas it has no value in predicting the disease progression and treatment responsiveness.

The podocyte: glomerular sentinel at the crossroads of innate and adaptive immunity

Focal segmental glomerulosclerosis (FSGS) is a common glomerular disorder that manifests clinically with the nephrotic syndrome and has a propensity to recur following kidney transplantation. The pathophysiology and therapies available to treat FSGS currently remain elusive. Since the podocyte appears to be the target of apparent circulating factor(s) that lead to recurrence of proteinuria following kidney transplantation, this article is focused on the podocyte. In the context of kidney transplantation, the performance of pre- and post-reperfusion biopsies, and the establishment of in vitro podocyte liquid biopsies/assays allow for the development of clinically relevant studies of podocyte biology. This has given insight into new pathways, involving novel targets in innate and adaptive immunity, such as SMPDL3b, cGAS-STING, and B7-1. Elegant experimental studies suggest that the successful clinical use of rituximab and abatacept, two immunomodulating agents, in our case series, may be due to direct effects on the podocyte, in addition to, or perhaps distinct from their immunosuppressive functions. Thus, tissue biomarker-directed therapy may provide a rational approach to validate the mechanism of disease and allow for the development of new therapeutics for FSGS. This report highlights recent progress in the field and emphasizes the importance of kidney transplantation and recurrent FSGS (rFSGS) as a platform for the study of primary FSGS.

Increased soluble urokinase plasminogen activator levels modulate monocyte function to promote atherosclerosis

People with kidney disease are disproportionately affected by atherosclerosis for unclear reasons. Soluble urokinase plasminogen activator receptor (suPAR) is an immune-derived mediator of kidney disease, levels of which are strongly associated with cardiovascular outcomes. We assessed suPAR's pathogenic involvement in atherosclerosis using epidemiologic, genetic, and experimental approaches. We found serum suPAR levels to be predictive of coronary artery calcification and cardiovascular events in 5,406 participants without known coronary disease. In a genome-wide association meta-analysis including over 25,000 individuals, we identified a missense variant in the plasminogen activator, urokinase receptor (PLAUR) gene (rs4760), confirmed experimentally to lead to higher suPAR levels. Mendelian randomization analysis in the UK Biobank using rs4760 indicated a causal association between genetically predicted suPAR levels and atherosclerotic phenotypes. In an experimental model of atherosclerosis, proprotein convertase subtilisin/kexin-9 (Pcsk9) transfection in mice overexpressing suPAR (suPARTg) led to substantially increased atherosclerotic plaques with necrotic cores and macrophage infiltration compared with those in WT mice, despite similar cholesterol levels. Prior to induction of atherosclerosis, aortas of suPARTg mice excreted higher levels of CCL2 and had higher monocyte counts compared with WT aortas. Aortic and circulating suPARTg monocytes exhibited a proinflammatory profile and enhanced chemotaxis. These findings characterize suPAR as a pathogenic factor for atherosclerosis acting at least partially through modulation of monocyte function.

Knowledge Domain and Emerging Trends in Podocyte Injury Research From 1994 to 2021: A Bibliometric and Visualized Analysis

Background: Podocyte injury has a direct causal relationship with proteinuria and glomerulosclerosis and, on a chronic level, can lead to irreversible disease progression. Podocyte injury plays a critically decisive role in the development of proteinuric kidney disease. In recent years, the research on podocyte injury has developed rapidly all over the world. However, no report has summarized the field of podocyte injury as a whole to date. Using bibliometric analysis, this study aimed to evaluate the current state of worldwide podocyte injury research in the last 30 years and identify important achievements, primary research fields, and emerging trends.

Methods: Publications related to podocyte injury were retrieved from Web of Science Core Collection. HistCite, VOSviewer, CiteSpace, and the Bibliometrix Package were used for bibliometric analysis and visualization, including the analysis of the overall distribution of annual outputs, leading countries, active institutions and authors, core journals, co-cited references, and keywords. Total global citation score and total local citation score were used to assess the quality and impact of publications.

Results: A total of 2,669 publications related to podocyte injury were identified. Publications related to podocyte injury tended to increase continuously. A total of 10,328 authors from 2,171 institutions in 69 countries published studies related to podocyte injury. China (39.46%) was the most prolific country, and the number of citations of studies in the United States (cited 36,896 times) ranked first. Moin A Saleem, John Cijiang He, and Zhihong Liu were the top three contributing authors, and Journal of the American Society of Nephrology and Kidney International were the most popular journals in the field. “Diabetic nephropathy” is the primary focus area of podocyte injury research, and “autophagy,” “microRNA,” and “inflammation” were the top keywords of emerging research hotspots, and traditional Chinese medicine monomer may be a neglected research gap.

Conclusion: Our research found that global publications on podocyte injury have increased dramatically. Diabetic nephropathy is the main research field of podocyte injury, whereas autophagy, microRNA, and inflammation are the top topics getting current attention from scholars and which may become the next focus in podocyte injury research.

Antibody-Mediated Rejection and Recurrent Primary Disease: Two Main Obstacles in Abdominal Kidney, Liver, and Pancreas Transplants

The advances in acute phase care have firmly established the practice of organ transplantation in the last several decades. Then, the next issues that loom large in the field of transplantation include antibody-mediated rejection (ABMR) and recurrent primary disease. Acute ABMR is a daunting hurdle in the performance of organ transplantation. The recent progress in desensitization and preoperative monitoring of donor-specific antibodies enables us to increase positive outcomes. However, chronic active ABMR is one of the most significant problems we currently face. On the other hand, recurrent primary disease is problematic for many recipients. Notably, some recipients, unfortunately, lost their vital organs due to this recurrence. Although some progress has been achieved in these two areas, many other factors remain largely obscure. In this review, these two topics will be discussed in light of recent discoveries.

The role of the immune system in idiopathic nephrotic syndrome

Idiopathic nephrotic syndrome (INS) in children is characterized by massive proteinuria and hypoalbuminemia and usually responds well to steroids. However, relapses are frequent, which can require multi-drug therapy with deleterious long-term side effects. In the last decades, different hypotheses on molecular mechanisms underlying INS have been proposed and several lines of evidences strongly indicate a crucial role of the immune system in the pathogenesis of non-genetic INS. INS is traditionally considered a T-cell-mediated disorder triggered by a circulating factor, which causes the impairment of the glomerular filtration barrier and subsequent proteinuria. Additionally, the imbalance between Th17/Tregs as well as Th2/Th1 has been implicated in the pathomechanism of INS. Interestingly, B-cells have gained attention, since rituximab, an anti-CD20 antibody demonstrated a good therapeutic response in the treatment of INS. Finally, recent findings indicate that even podocytes can act as antigen-presenting cells under inflammatory stimuli and play a direct role in activating cellular pathways that cause proteinuria. Even though our knowledge on the underlying mechanisms of INS is still incomplete, it became clear that instead of a traditionally implicated cell subset or one particular molecule as a causative factor for INS, a multi-step control system including soluble factors, immune cells, and podocytes is necessary to prevent the occurrence of INS. This present review aims to provide an overview of the current knowledge on this topic, since advances in our understanding of the immunopathogenesis of INS may help drive new tailored therapeutic approaches forward.

Recurrent Glomerular Disease after Kidney Transplantation: Diagnostic and Management Dilemmas

Recurrent glomerular disease after kidney transplant remains an important cause of allograft failure. Many of the different entities post-transplant still suffer from incomplete knowledge on pathophysiology, and therefore lack targeted and effective therapies. In this review, we focus on specific clinical dilemmas encountered by physicians in managing recurrent glomerular disease by highlighting new insights into the understanding and treatment of post-transplant focal segmental glomerulosclerosis, membranous nephropathy, atypical hemolytic uremic syndrome, C3 glomerulopathy, amyloid light-chain (AL) amyloidosis, and IgA nephropathy.

suPAR, a Circulating Kidney Disease Factor

Urokinase plasminogen activator receptor (uPAR) is a multifaceted, GPI-anchored three-domain protein. Release of the receptor results in variable levels of soluble uPAR (suPAR) in the blood circulation. suPAR levels have been linked to many disease states. In this mini-review, we discuss suPAR as a key circulating molecule mediating kidney disease with a particular focus on differently spliced isoforms.

Immune-mediated entities of (primary) focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) represents a glomerular scar formation downstream of various different mechanisms leading to podocytopathy and podocyte loss. Recently, significant advances were made in understanding genetic factors, podocyte intrinsic mechanisms, and adaptive mechanisms causing FSGS. However, while most cases of nephrotic FSGS are being treated with immunosuppressants, the underlying immune dysregulation, involved immune cells, and soluble factors are only incompletely understood. Thus, we here summarize the current knowledge of proposed immune effector cells, secreted soluble factors, and podocyte response in immune-mediated (primary) FSGS.

Soluble urokinase-type plasminogen activator receptor (suPAR) is a risk indicator for eGFR loss in kidney transplant recipients

The prognostic significance of suPAR in various kidney diseases has recently been demonstrated. Its role in transplantation-specific outcomes is still largely unknown. Therefore, we prospectively investigated the prognostic relevance of suPAR in patients before and one year after kidney transplantation (KTx). We included 100 patients who had received a kidney transplantation between 2013 and 2015. The plasma concentration of suPAR was measured by ELISA assay. In recipients of living donations (LD), pre-transplant suPAR levels were significantly lower than those of recipients of deceased donations (DD). After KTx, suPAR levels significantly declined in LD and DD recipients, without a detectable difference between both groups any more. Higher suPAR levels in recipients one year after KTx were associated with a more severe eGFR loss in the following three years in multivariable cox-regression (n = 82, p = 0.021). suPAR-levels above 6212 pg/ml one year after KTx are associated with eGFR loss > 30%, which occurred almost twice as fast as in patients with suPAR ≤ 6212 pg/ml (p < 0.001). Hence, suPAR level at one year mark might be a risk indicator of increased eGFR loss.

Autoimmunity in Focal Segmental Glomerulosclerosis: A Long-Standing Yet Elusive Association

Focal segmental glomerulosclerosis (FSGS) is a histological term that describes a pathologic renal entity affecting both adults and children, with a wide array of possible underlying etiologies. Podocyte damage with scarring, the hallmark of this condition, leads to altered permeability of the glomerular barrier, which may result in massive proteinuria and relentless renal function deterioration. A definite cause of focal segmental glomerulosclerosis can be confirmed in a minority of cases, while most forms have been traditionally labeled as primary or idiopathic. Despite this definition, increasing evidence indicates that primary forms are a heterogenous group rather than a single disease entity: several circulating factors that may affect glomerular permeability have been proposed as potential culprits, and both humoral and cellular immunity have been implicated in the pathogenesis of the disease. Consistently, immunosuppressive drugs are considered as the cornerstone of treatment for primary focal segmental glomerulosclerosis, but response to these agents and long-term outcomes are highly variable. In this review we provide a summary of historical and recent advances on the pathogenesis of primary focal segmental glomerulosclerosis, focusing on implications for its differential diagnosis and treatment.

Angiotensin II receptor 1 antibodies associate with post-transplant focal segmental glomerulosclerosis and proteinuria

BACKGROUND

Angiotensin II type 1 receptors (AT1Rs) are expressed on podocytes, endothelial and other cells, and play an essential role in the maintenance of podocyte function and vascular homeostasis. The presence of AT1R antibodies (AT1R-Abs) leads to activation of these receptors resulting in podocyte injury and endothelial cell dysfunction. We assessed the correlation between AT1R-Abs and the risk of post-transplant FSGS.

METHODS

This is a retrospective study, which included all kidney transplant recipients with positive AT1R-Abs (≥ 9 units/ml), who were transplanted and followed at our center between 2006 and 2016. We assessed the development of biopsy proven FSGS and proteinuria by urine protein to creatinine ratio of ≥1 g/g and reviewed short and long term outcomes.

RESULTS

We identified 100 patients with positive AT1R-Abs at the time of kidney transplant biopsy or proteinuria. 49% recipients (FSGS group) had biopsy-proven FSGS and/or proteinuria and 51% did not (non-FSGS group). Pre-transplant hypertension was present in 89% of the FSGS group compared to 72% in the non-FSGS group, p = 0.027. Of the FSGS group, 43% were on angiotensin converting enzyme inhibitors or angiotensin receptor blockers prior to transplantation, compared to 25.5% in the non-FSGS group, p = 0.06. Primary idiopathic FSGS was the cause of ESRD in 20% of the FSGS group, compared to 6% in the non-FSGS group, p = 0.03. The allograft loss was significantly higher in the FSGS group 63% compared to 39% in non-FSGS. Odds ratio and 95% confidence interval were 2.66 (1.18-5.99), p = 0.017.

CONCLUSIONS

Our data suggest a potential association between AT1R-Abs and post-transplant FSGS leading to worse allograft outcome. Therefore, AT1R-Abs may be considered biomarkers for post-transplant FSGS.

Idiopathic nephrotic syndrome and serum permeability factors: a molecular jigsaw puzzle

Nephrotic syndrome is traditionally defined using the triad of edema, hypoalbuminemia, and proteinuria, but this syndrome is very heterogeneous and difficult to clarify. Its idiopathic form (INS) is probably the most harmful and essentially comprises two entities: minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS). We will consider some hypotheses regarding the mechanisms underlying INS: (i) the presence of several glomerular permeability factors in the sera of patients that alter the morphology and function of podocytes leading to proteinuria, (ii) the putative role of immune cells. Thanks to recent data, our understanding of these disorders is evolving towards a more multifactorial origin. In this context, circulating factors may be associated according to sequential kinetic mechanisms or micro-environmental changes that need to be determined. In addition, the resulting proteinuria may trigger more proteinuria enhancing the glomerular destabilization.

Clinical value of soluble urokinase type plasminogen activator receptors in chronic kidney disease

Chronic kidney disease (CKD) will progress to end stage without treatment, the decline off renal function may not linear. A sensitive marker such as soluble urokinase-type plasminogen activator receptors (suPARs) may allow potential intervention and treatment in earlier stages of CKD.

OBJECTIVES

This study was designed to measure plasma (suPAR) in patients with CKD with different stages and to find its correlation with the disease severity.

METHODS

This study was conducted on 114 subjects, 84 were patients with different stages and different causes of CKD, and 30 healthy subjects as controls. Blood urea, serum creatinine, serum high-sensitive C-reactive protein, estimated glomerular filtration rate, and 24 hours proteinuria were measured, renal biopsy was done for all patients, and plasma (suPAR) was measured using enzyme-linked immunosorbent assay.

RESULTS

suPAR plasma levels were significantly higher in patients with CKD (7.9 ± 3.82 ng/mL) than controls (1.76 ± 0.77 ng/mL, P < .001). suPAR correlated with the disease severity. In stage 1 to 2 group, it was 3.7 ± 1.5 ng/mL, in stage 3 to 4, it was 10.10 ± 1.22 ng/mL, and in stage 5 group, it was 12.34 ± 0.88 ng/mL; the difference between the 3 groups was highly significant (P < .001). A cutoff point 2.5 ng/mL of suPAR was found between controls and stage 1 group. According to the cause of CKD, although patients with obstructive cause and those with focal glomerulosclerosis had the higher levels 9.11 ± 3.32 ng/mL and 8.73 ± 3.19 ng/mL, respectively, but there was no significant difference between patients with CKD according to the cause of the CKD.

CONCLUSION

Plasma (suPAR) increased in patients with CKD and correlated with disease severity.

ACTH Gel in Resistant Focal Segmental Glomerulosclerosis After Kidney Transplantation

BACKGROUND

Treatment of focal segmental glomerular sclerosis (FSGS) after kidney transplantation is challenging with unpredictable outcomes. The objective was to investigate the use of adrenocorticotropic hormone (ACTH) analogue gel in kidney transplant recipients with de novo or recurrent FSGS resistant to therapeutic plasma exchange (TPE) and/or rituximab.

METHODS

We performed a retrospective review of cases of de novo or recurrent resistant FSGS at 2 large US transplant centers between April 2012 and December 2016. Proteinuria was measured by urine protein to creatinine ratio.

RESULTS

We identified 20 cases of posttransplant recurrent and de novo FSGS resistant to conventional therapy with TPE and rituximab. Mean ± SD age was 49 ± 15.5 years, 14 (70%) were male, 13 (65%) were whites, and 8 (38%) had previous kidney transplants. Median (interquartile range) of recurrent and de novo FSGS was 3 (0.75-7.5) months posttransplant. The majority of patients, 15 (75%), received TPE as a treatment at the time of diagnosis and 10 (50%) received rituximab, which was started before the use of ACTH gel. There was a significant improvement of urine protein to creatinine ratio from a mean ± SD of 8.6 ± 7.6 g/g before ACTH gel to 3.3 ± 2.3 g/g after the use of ACTH gel (P = 0.004). Ten (50%) patients achieved complete or partial remission.

CONCLUSIONS

Although, the response varied among the recipients, ACTH gel might be an effective therapy for posttransplant resistant FSGS cases that fail to respond to TPE and rituximab.

Increased Serum Soluble Urokinase-Type Plasminogen Activator Receptor (suPAR) Levels in FSGS: A Meta-Analysis

Introduction

The soluble urokinase-type plasminogen activator receptor (suPAR) has been found to be elevated in primary focal segmental glomerulosclerosis (pFSGS). However, its usefulness as a biomarker for FSGS remains controversial. We conducted a meta-analysis aiming at investigating the significance of suPAR in diagnosing pFSGS.

Methods

Electronic databases (PubMed and EMBASE) were searched to identify studies comparing suPAR levels in FSGS patients and controls, from the earliest available date to May 1, 2018. A random-effects model with standardized mean difference (SMD) was used for meta-analyses. Risk of bias was assessed using the Newcastle-Ottawa quality assessment scale.

Results

A total of 187 articles were screened, and the final analysis included 13 articles. In comparison to healthy controls, serum suPAR levels were significantly increased in pFSGS patients (SMD, 1.07, 95% confidence interval (CI) 0.65 to 1.48; participants = 814; studies = 9, I ² = 85%). Higher suPAR levels were also found in patients with pFSGS compared to those with minimal change disease (SMD 0.53, 95% CI 0.22 to 0.84). Of note, such a difference was not found in pediatric groups (SMD 0.42, 95% CI -0.13 to 0.96) while it was more evidently noted in adult patients (SMD 1.32, 95% CI 0.90 to 1.74). Serum suPAR levels did not differ between pFSGS patients in remission compared to those in active proteinuric state (SMD 0.29, 95% CI -0.30 to 0.88). Comparison with membranous nephropathy and IgA nephropathy showed no significant difference.

Conclusions

Our meta-analysis demonstrated that, in comparison to both healthy controls and controls with minimal change disease, suPAR levels were significantly higher in adult patients with pFSGS. suPAR levels did not differ between pFSGS patients during the initial period of diagnosis and those in remission.

Monitoring suPAR levels in post-kidney transplant focal segmental glomerulosclerosis treated with therapeutic plasma exchange and rituximab

BACKGROUND

Therapeutic plasma exchange (TPE) is an important therapy for recurrent focal segmental glomerulosclerosis (rFSGS) post kidney transplant. suPAR has been causally implicated in rFSGS, and shown to be a unique biomarker for the occurrence and progression of chronic kidney disease. This study was targeted to evaluate the application of monitoring suPAR in TPE treated rFSGS.

METHODS

A retrospective (n = 19) and a prospective (n = 15) cohort of post transplant FSGS patients treated with TPE and rituximab were enrolled. We measured serum suPAR levels before and after the combined therapies, and assessed the role of suPAR changes on proteinuria reduction and podocyte β3- integrin activity.

RESULTS

Treatment with TPE and rituximab resulted in significant decrease in proteinuria and suPAR levels. Among the variables including baseline suPAR, serum creatinine, proteinuria, eGFR, age at diagnosis, age at transplantation, transplantation numbers, time to recurrence, and TPE course numbers, only the reduction in suPAR levels and baseline proteinuria significantly correlated with the changes in proteinuria after treatment, with the former performed better in predicting proteinuria alteration. Additionally, the mean podocyte β3 integrin activity significantly decreased after TPE and rituximab treatment (1.10 ± 0.08) as compared to before treatment (1.34 ± 0.08), p < 0.05. Only the reduction in suPAR predicted the response to therapies with an odds ratio of 1.43, 95% CI (1.02, 2.00), p < 0.05.

CONCLUSIONS

Serum suPAR levels reduced significantly after TPE and rituximab treatment in post transplant FSGS patients. The reduction in suPAR levels may be utilized to assess the changes in proteinuria and monitor the response to the therapies. Larger, multi-centered prospective studies monitoring serum suPAR levels in TPE managed post transplant FSGS are warranted.

Rituximab and Therapeutic Plasma Exchange in Recurrent Focal Segmental Glomerulosclerosis Postkidney Transplantation

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) is a common cause of end-stage renal disease with a high rate of recurrence after kidney transplantation. Several factors, such as white race, rapid progression, and previous allograft failure due to recurrence, were found to be risks of recurrence. Data are limited on the benefits of rituximab and/or therapeutic plasma exchange (TPE) in preventing recurrence. In this study, we sought to assess the efficacy of rituximab and TPE for the prevention and treatment of recurrent FSGS after kidney transplantation.

METHODS

We enrolled 66 patients with FSGS in this prospective observational study and followed their outcomes. Patients with high risk for recurrence received preventative therapy with TPE and/or rituximab.

RESULTS

Twenty-three (62%) of the 37 patients who received preventative therapy developed recurrence compared with 14 (51%) recurrences of the 27 patients who did not receive any therapy (P = 0.21). There was a trend for less relapse when rituximab was used as a therapy for recurrent FSGS (6/22 vs 9/18, P = 0.066). We used a clinical score of 5 values to assess the prediction of FSGS recurrence. A score of 3 or more had a predictive receiver operating characteristic curve of 0.72. Treatment with TPE and/or rituximab resulted in better allograft survival than historical studies. Allograft failure because of recurrent FSGS occurred in only 6 (9%) patients.

CONCLUSIONS

Preventative therapies do not decrease the recurrence rate of recurrent FSGS. However, prompt treatment of recurrence with these therapies may result in improved outcomes.

Protecting Podocytes: A Key Target for Therapy of Focal Segmental Glomerulosclerosis

BACKGROUND

Focal segmental glomerulosclerosis (FSGS) is a histologic pattern of injury demonstrated by renal biopsy that can arise from a diverse range of causes and mechanisms. It has an estimated incidence of 7 per 1 million and is the most common primary glomerular disorder leading to end-stage renal disease in the United States. This review focuses on damage to the podocyte and the consequences of this injury in patients with FSGS, the genetics of FSGS, and approaches to treatment with a focus on the effects on podocytes.

SUMMARY

The podocyte is central to the glomerular filtration barrier and is particularly vulnerable because of its highly differentiated post-mitotic phenotype. The progressive structural changes involved in the pathology of FSGS include podocyte foot process effacement, death of podocytes and exposure of the glomerular basement membrane, filtration of nonspecific plasma proteins, expansion of capillaries, misdirected filtration at points of synechiae, and mesangial matrix proliferation. Although damage to and death of podocytes can result from single-gene disorders, evidence also suggests a role for soluble factors, such as soluble urokinase-type plasminogen activator receptor, cardiotrophin-like cytokine-1, and anti-CD40 antibodies, that promote FSGS recurrence post transplant. Several classes of medications, including corticosteroids, calcineurin inhibitors, endothelin receptor antagonists, adrenocorticotropic hormone, and rituximab, have been shown to be effective for the treatment of FSGS and have been demonstrated to have significant protective effects on podocytes. Key Messages: Greater understanding of podocyte biology is essential to the identification of new treatment targets and medications for the management of patients with FSGS.

suPAR and chronic kidney disease-a podocyte story

The soluble urokinase-type plasminogen activator receptor (suPAR) is a circulating signaling molecule derived from immature myeloid cells. Elevated levels of suPAR have been linked to the pathogenesis of the kidney disease focal and segmental glomerulosclerosis. Here, suPAR acts on podocytes by activating αvβ3 integrins. Large observational studies showed that suPAR also predicts chronic kidney disease incidence and progression by predating the disease by several years prior to any other known marker of renal dysfunction. suPAR is rapidly developing into a prime target for pharmacotherapy as its neutralization is forecasted to be feasible and safe.

Lysyl oxidase‑like 2 is expressed in kidney tissue and is associated with the progression of tubulointerstitial fibrosis

Tubulointerstitial fibrosis is a common end point of chronic kidney diseases, and preventing its progression is key to avoiding renal failure. Transforming growth factor-β (TGF-β) and associated molecules promote tubulointerstitial fibrosis; however, effective therapies targeting these molecules have yet to be developed. Lysyl oxidase-like 2 (LOXL2), which is involved in invasive growth and metastasis of malignant neoplasms, has recently been reported to serve a key role in hepatic and pulmonary fibrosis. However, little is currently known regarding LOXL2 expression in the kidney and its involvement in tubulointerstitial fibrosis. The present study evaluated LOXL2 expression in human and mouse kidney tissues, as well as in cultured renal cells. LOXL2 protein expression was detected in glomerular capillary loops and tubular epithelial cells in human and mouse kidneys. Glomerular LOXL2 was localized to the cytoplasm of podocytes, as determined by double immunofluorescence microscopy using a podocyte marker (synaptopodin). This result was supported by western blot analysis, which demonstrated that LOXL2 protein expression is present in cultured human podocytes and HK-2 human proximal tubular cells. In addition, the mRNA and protein expression levels of LOXL2 were higher in a mouse model of tubulointerstitial fibrosis compared with in control mice. In addition, immunohistochemistry results demonstrated that LOXL2 is present in the fibrous interstitium and infiltrating mononuclear cells in a mouse model of tubulointerstitial fibrosis. The present study demonstrated that LOXL2 is expressed in compartments of renal tissue, where it appears to contribute to the progression of tubulointerstitial fibrosis.

Acute Kidney Injury in Transplant Setting: Differential Diagnosis and Impact on Health and Health Care

Acute kidney injury (AKI) is common in kidney transplant recipients. In addition to the usual causes of AKI in native kidneys, certain features and risk factors are unique to kidney allografts. In this article, we will present an overview of the common transplant-specific AKI etiologies that include increased susceptibility to hemodynamic-mediated AKI, acute rejection, medication-induced AKI, recurrence of native kidney disease, infections, urinary tract obstruction, vascular thrombosis and post-transplant lymphoproliferative disorder. AKI is independently associated with allograft loss and patient mortality. It is, therefore, prudent for transplant centers to address it as a major quality measure.

Removal of focal segmental glomerulosclerosis (FSGS) factor suPAR using CytoSorb

Treatment of primary focal segmental glomerulosclerosis (FSGS) and its recurrence after kidney transplantation associated with rapid deterioration of kidney function remains to be challenging despite advances in immunosuppressive therapy. The presence of circulating factors has been postulated to be a pivotal player in the pathogenesis of FSGS, although suPAR and CLCF-1 have been identified as the most promising causative factors. The potential therapeutic effect of suPAR elimination in an FSGS patient using CytoSorb, a hemoadsorption device that gained attention in the cytokine elimination in septic patients, was studied. Efficiency of total plasma exchange to remove suPAR was determined. CytoSorb hemoadsorption caused a 27.33% reduction of the suPAR level in a single treatment, whereas total plasma exchange showed a suPAR level reduction of 25.12% (n = 3; 95% confidence interval, 0.2777-0.8090; P < 0.01), which may indicate therapeutic potential in the treatment of primary FSGS and its recurrence in a kidney transplant.

Guidelines on the Use of Therapeutic Apheresis in Clinical Practice-Evidence-Based Approach from the Writing Committee of the American Society for Apheresis: The Seventh Special Issue

The American Society for Apheresis (ASFA) Journal of Clinical Apheresis (JCA) Special Issue Writing Committee is charged with reviewing, updating, and categorizing indications for the evidence-based use of therapeutic apheresis in human disease. Since the 2007 JCA Special Issue (Fourth Edition), the Committee has incorporated systematic review and evidence-based approaches in the grading and categorization of apheresis indications. This Seventh Edition of the JCA Special Issue continues to maintain this methodology and rigor to make recommendations on the use of apheresis in a wide variety of diseases/conditions. The JCA Seventh Edition, like its predecessor, has consistently applied the category and grading system definitions in the fact sheets. The general layout and concept of a fact sheet that was used since the fourth edition has largely been maintained in this edition. Each fact sheet succinctly summarizes the evidence for the use of therapeutic apheresis in a specific disease entity. The Seventh Edition discusses 87 fact sheets (14 new fact sheets since the Sixth Edition) for therapeutic apheresis diseases and medical conditions, with 179 indications, which are separately graded and categorized within the listed fact sheets. Several diseases that are Category IV which have been described in detail in previous editions and do not have significant new evidence since the last publication are summarized in a separate table. The Seventh Edition of the JCA Special Issue serves as a key resource that guides the utilization of therapeutic apheresis in the treatment of human disease. J. Clin. Apheresis 31:149-162, 2016. © 2016 Wiley Periodicals, Inc.

Current status of pediatric renal transplant pathology

Histopathology is still an indispensable tool for the diagnosis of kidney transplant dysfunction in adult and pediatric patients. This review presents consolidated knowledge, recent developments and future prospects on the biopsy procedure, the diagnostic work-up, classification schemes, the histopathology of rejection, including antibody-mediated forms, ABO-incompatible transplants, protocol biopsies, recurrent and de novo disease, post-transplant lymphoproliferative disorder, infectious complications and drug-induced toxicity. It is acknowledged that frequently the correct diagnosis can only be reached in consensus with clinical, serological, immunogenetical, bacteriological and virological findings. This review shall enhance the understanding of the pediatric nephrologist for the thought processes of nephropathologists with the aim to facilitate teamwork between these specialist groups for the benefit of the patient.

Pathogenesis of Focal Segmental Glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is characterized by focal and segmental obliteration of glomerular capillary tufts with increased matrix. FSGS is classified as collapsing, tip, cellular, perihilar and not otherwise specified variants according to the location and character of the sclerotic lesion. Primary or idiopathic FSGS is considered to be related to podocyte injury, and the pathogenesis of podocyte injury has been actively investigated. Several circulating factors affecting podocyte permeability barrier have been proposed, but not proven to cause FSGS. FSGS may also be caused by genetic alterations. These genes are mainly those regulating slit diaphragm structure, actin cytoskeleton of podocytes, and foot process structure. The mode of inheritance and age of onset are different according to the gene involved. Recently, the role of parietal epithelial cells (PECs) has been highlighted. Podocytes and PECs have common mesenchymal progenitors, therefore, PECs could be a source of podocyte repopulation after podocyte injury. Activated PECs migrate along adhesion to the glomerular tuft and may also contribute to the progression of sclerosis. Markers of activated PECs, including CD44, could be used to distinguish FSGS from minimal change disease. The pathogenesis of FSGS is very complex; however, understanding basic mechanisms of podocyte injury is important not only for basic research, but also for daily diagnostic pathology practice.

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.

Treatment of Focal Segmental Glomerulosclerosis Recurrence in the Renal Allograft: A Report of Two Cases

Focal segmental glomerulosclerosis (FSGS) causes glomerular lesions that can progress to end-stage renal disease. It is suspected to be caused by a circulating factor that is amenable to plasmapheresis removal and exhibits a risk for recurrence in the renal allograft. We present two patients with FSGS recurrence in their allograft kidneys diagnosed by biopsy after significant proteinuria developed in the posttransplant setting. Treatment with therapeutic plasma exchange induced long-term remission in both patients. Spot urine protein:creatinine ratios were monitored and treatment was continued until a target of <0.5 was achieved. In patient number two, a second peak in proteinuria and azotemia was ultimately attributable to ureteral stenosis and these values normalized following repair. In conclusion, therapeutic plasma exchange is an effective treatment for FSGS recurring following renal transplant.

Primary and Recurrent Focal Segmental Glomerulosclerosis Closely Link to Serum Soluble Urokinase-type Plasminogen Activator Receptor Levels

BACKGROUND

Serum soluble urokinase-type plasminogen activator receptor (suPAR) is implicated in the pathogenesis of native and recurrent focal segmental glomerulosclerosis (FSGS). It is elevated in two-thirds of subjects with primary FSGS, but not in people with other glomerular diseases that can differentiate FSGS and other glomerular diseases.

METHODS

We measured the serum soluble urokinase receptor levels and determined their association with clinical and pathologic data in 86 patients with primary FSGS, 5 repeat renal biopsy FSGS, and 6 recurrent FSGS post-transplantation. Healthy controls and patients with minimal change disease and membranous nephropathy were used as controls. The suPAR levels were measured by commercial enzyme-linked immunosorbent assay kits.

RESULTS

Patients with primary FSGS (median: 4232, interquartile range 1299-9714 pg/mL) had significantly higher levels of suPAR than those of patients with minimal change disease (median: 2784 pg/mL), membranous nephropathy (median: 3478 pg/mL), and healthy individuals (median: 1994 pg/mL). There was no significant difference in suPAR levels between the 65 patients with minimal change disease and 85 patients with membranous nephropathy. The suPAR levels increased in the 5 repeated renal biopsy FSGS and 6 recurrent FSGS post-transplantation.

CONCLUSIONS

The suPAR levels were significantly but positively correlated with FSGS, not only primary FSGS but also recurrent FSGS post-transplantation, but negatively correlated with other glomerular diseases. Thus, suPAR levels can differentiate primary FSGS and other glomerular diseases.

Recurrent Primary Focal Segmental Glomerulosclerosis Managed With Intensified Plasma Exchange and Concomitant Monitoring of Soluble Urokinase-Type Plasminogen Activator Receptor-Mediated Podocyte β3-integrin Activation

BACKGROUND

Primary focal segmental glomerulosclerosis (FSGS) often causes nephrotic proteinuria and frequently results in end-stage renal disease and recurrence after kidney transplantation. Recent studies describe soluble urokinase-type plasminogen activator receptor (suPAR) as a circulating factor implicated in FSGS.

METHODS

This single-center study included 12 adult patients with histologically proven primary FSGS (n = 2) or recurrent FSGS after transplantation (n = 10). The effect of plasma exchange (PE) on clinical outcome, suPAR levels, and in vitro podocyte β3-integrin activation was investigated over a median of 11 (6-18) sessions of PE.

RESULTS

The course of treatment was monitored in a total of 70 sessions of PE, which partly eliminated suPAR, with a mean reduction of 37 ± 12% of serum concentration per session. However, a substantial rebound was observed between sessions, with suPAR levels reaching 99 ± 22% of the pretreatment levels after a median of 4 days. Podocyte β3-integrin activation dropped significantly after PE but rebounded within 4 days concomitant with a rising suPAR level. In 11 of 12 patients, multimodal treatment (including extensive PE) reduced proteinuria significantly (from 5.3 [2.0-7.8] to 1.0 [0.4-1.6] g/d), indicating clinical efficacy of the therapy. One patient suffered allograft loss due to FSGS recurrence. A persisting response was independent of a lasting reduction in the level of total suPAR because there was no sustained significant change in suPAR levels before and after the course of intensified treatment (3814 ± 908 to 3595 ± 521 pg/mL; P = 0.496).

CONCLUSIONS

We conclude that multimodal therapy including extensive PE was associated with stabilization of recurrent FSGS and a temporary lowering of plasma suPAR as well as podocyte β3-integrin activation. Whether a sustained lowering of total suPAR results in further improved outcomes requires additional study.

Recurrent focal segmental glomerulosclerosis after kidney transplantation

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

A circulating antibody panel for pretransplant prediction of FSGS recurrence after kidney transplantation

Recurrence of focal segmental glomerulosclerosis (rFSGS) after kidney transplantation is a cause of accelerated graft loss. To evaluate pathogenic antibodies (Abs) in rFSGS, we processed 141 serum samples from 64 patients with and without primary rFSGS and 34 non-FSGS control patients transplanted at four hospitals. We screened about 9000 antigens in pretransplant sera and selected 10 Abs targeting glomerular antigens for enzyme-linked immunosorbent assay (ELISA) validation. A panel of seven Abs (CD40, PTPRO, CGB5, FAS, P2RY11, SNRPB2, and APOL2) could predict posttransplant FSGS recurrence with 92% accuracy. Pretransplant elevation of anti-CD40 Ab alone had the best correlation (78% accuracy) with rFSGS risk after transplantation. Epitope mapping of CD40 with customized peptide arrays and rFSGS sera demonstrated altered immunogenicity of the extracellular CD40 domain in rFSGS. Immunohistochemistry of CD40 demonstrated a differential expression in FSGS compared to non-FSGS controls. Anti-CD40 Abs purified from rFSGS patients were particularly pathogenic in human podocyte cultures. Injection of anti-CD40/rFSGS Ab enhanced suPAR (soluble urokinase receptor)-mediated proteinuria in wild-type mice, yet no sensitizing effect was noted in mice deficient in CD40 or in wild-type mice that received blocking Ab to CD40. In conclusion, a panel of seven Abs can help identify primary FSGS patients at high risk of recurrence before transplantation. Intrarenal CD40 (and possibly other specific glomerular antigens) is an important contributor to FSGS disease pathogenesis. Human trials of anti-CD40 therapies are warranted to evaluate their efficacy for preventing rFSGS and improving graft survival.

Novel therapies for FSGS: preclinical and clinical studies

Focal segmental glomerulosclerosis (FSGS) is a rare but important cause of end-stage kidney disease in children and adults. Current therapy, consisting of corticosteroids and calcineurin inhibitors, fails to achieve a sustained remission in most patients. Therefore, there is a pressing need to develop new treatments for this glomerulopathy. Traditional approaches have focused on agents that modulate the immune system. In this review, we summarize preclinical and clinical data with newer agents that may ameliorate FSGS. We focus on drugs that inhibit immune injury or inflammation, such as abatacept, rituximab, adalimumab, and stem cells. The potential of agents that block the glomerular action of circulating permeability factors such as soluble urokinase receptor is reviewed. Finally, because fibrosis represents the final common pathway of glomerular damage in FSGS, the experience with a wide range of antifibrotic agents is presented. Despite extensive research on the podocyte dysfunction in the pathogenesis of FSGS, there are few agents that directly target podocyte structure or viability. We conclude that FSGS is a heterogeneous disorder and that intensified translational research is vital to improve our understanding of distinct subtypes that have a defined prognosis and predictable response to targeted therapeutic interventions.

Individualized scheme of immunoadsorption for the recurrence of idiopathic focal segmental glomerulosclerosis in the graft: a single center experience

OBJECTIVES

To explore the role of immunoadsorption (IA) for the treatment of idiopathic focal segmental glomerulosclerosis (FSGS) recurrence in the renal allograft, if applied in a personalized manner.

METHODS

We studied patients with end-stage renal disease (ESRD) due to idiopathic FSGS, transplanted between 2001 and 2010. Patients with FSGS recurrence were treated with daily sessions of IA for the first week, followed by an every other day scheme and then individualized tapering until discontinuation. Complete remission was defined as a reduction of 24-h proteinuria to ≤ 0.5 g/day and partial remission as a reduction of 24-h proteinuria to 50% or more from baseline.

RESULTS

Of the 18 renal transplant recipients with ESRD due to idiopathic FSGS, 12 (66.7%) experienced disease recurrence in a mean time of 0.75 months post-transplantation (KTx), with a mean proteinuria of 8.9 g/day at the time of recurrence. The mean recipient age was 30.8 years; the mean donor age was 47.4 years, while living related donors provided the allograft in seven cases. Four of the patients received therapy with rituximab in addition to IA. During a mean time of follow-up of 48.3 months, seven patients (58.3%) achieved complete remission, and five (41.7%) partial remission. At the end of follow-up, eight patients (66.7%) had functioning grafts, being in sustained remission, in contrast to four patients (33.3%), who ended up in ESRD because of FSGS recurrence.

CONCLUSIONS

IA was shown efficacious in a small series of patients with recurrent FSGS in the graft. Renal function remained stable in eight of the 12 patients with FSGS recurrence.

Circulating suPAR levels are affected by glomerular filtration rate and proteinuria in primary and secondary glomerulonephritis

BACKGROUND

Circulating levels of soluble urokinase-like plasminogen activator receptor (suPAR) have been associated with proteinuria and renal function in focal segmental glomerulosclerosis (FSGS). This study aimed to evaluate if circulating suPAR levels are independently associated with proteinuria in patients with non-FSGS glomerulonephritis.

METHODS

This is a cross-sectional analysis of suPAR levels on 42 patients with primary non-FSGS glomerulonephritis (group GN) and 140 patients with secondary glomerulonephritis within an autoimmune disease (group AID).

RESULTS

suPAR serum levels were significantly higher in AID patients (4,733 ± 3,073 pg/ml) than in healthy controls (1,908 ± 1,685 pg/ml; p < 0.001), whereas GN patients displayed intermediate levels (3,670 ± 2,435 pg/ml; p = 0.021). Multivariate analysis for elevated serum suPAR (>3,000 pg/ml) showed an independent association with estimated glomerular filtration rate (eGFR) <60 ml/min/1.73 m(2) [odds ratio (OR) = 4.19, 95% confidence interval (CI): 1.67-10.54, p = 0.002], proteinuria >0.5 g/day (OR = 2.97; 95% CI: 1.32-6.70; p = 0.009) and presence of secondary vs. primary GN (OR = 2.87, 95% CI: 1.25-6.23; p = 0.013). A general linear model confirmed that suPAR levels were significantly affected by proteinuria >0.50 g/day (coefficient +1,477 pg/ml), eGFR (-38 pg/ml per 1 ml/min/1.73 m(2) increase) and presence of secondary vs. primary GN (+1,368 pg/ml).

CONCLUSIONS

This study shows that elevated serum suPAR levels are associated with reduced eGFR and presence of proteinuria in both primary and secondary GN, suggesting that circulating suPAR may represent a common biomarker of renal involvement in a wide spectrum of GN.

Decreased glomerular filtration as the primary factor of elevated circulating suPAR levels in focal segmental glomerulosclerosis

BACKGROUND

Circulating factor(s) has been thought to be the underlying cause of focal segmental glomerulosclerosis (FSGS), and recent studies foster this idea by demonstrating increased soluble urokinase receptor (suPAR) levels in the serum of FSGS patients.

METHODS

To explore the possible contribution of suPAR in FSGS pathogenesis, we analyzed serum suPAR levels in 17 patients with FSGS and compared them with those in patients with steroid-sensitive nephrotic syndrome, chronic glomerulonephritis, or non-glomerular kidney diseases.

RESULTS

Serum suPAR levels in patients with FSGS were higher than those in patients with steroid-sensitive nephrotic syndrome or chronic glomerulonephritis, but not higher than those in patients with non-glomerular kidney diseases. suPAR levels negatively correlate with estimated glomerular filtration rate and were decreased after renal transplantation in patients with FSGS as well as in those with non-glomerular kidney diseases. Furthermore, 6 FSGS patients with post-transplant recurrence demonstrated that suPAR levels were not high during the recurrence.

CONCLUSIONS

Based on our results, elevated suPAR levels in FSGS patients were attributed mainly to decreased glomerular filtration. These data warrant further analysis for involvement of possible circulating factor(s) in FSGS pathogenesis.

Permeability factors in focal and segmental glomerulosclerosis

Focal and segmental glomerulosclerosis (FSGS) represents a group of glomerular disorders, identified on kidney biopsy, that progress in the histopathologic pattern of sclerosis in parts of some glomeruli. Damage to podocytes usually marks the beginning of the disease, most evident in primary FSGS. In addition to genetic predisposition, there are many acquired causes that disturb normal podocyte homeostasis and allow for the development of FSGS. The aim of this review was to summarize recent findings of the most relevant circulating permeability factors that may serve as biomarkers of active primary idiopathic FSGS and aid in the diagnosis and prediction of recurrent FSGS after kidney transplantation.

Focal segmental glomerulosclerosis recurrence in the renal allograft

Focal segmental glomerulosclerosis (FSGS) represents a common histologic pattern of glomerular injury associated with a multitude of disease mechanisms. The etiology of FSGS is often classified into primary (idiopathic) and secondary forms in response to genetic abnormalities, infections, toxins, and systemic disorders that lead to adaptive changes, glomerular hyperfiltration, and proteinuria. Our understanding of the pathogenic mechanisms responsible for FSGS was substantially enhanced in recent years because of major advances in the cell biology of the podocyte and parietal epithelial cell. Recurrence of FSGS occurs mainly in its primary form and is only rarely described in secondary forms. The re-enactment of pathologic mechanisms of FSGS as recurrent disease after kidney transplantation represents a biologic experiment that can provide unique insight. Nonetheless, recurrent FSGS remains a notable clinical problem that correlates with poorer renal allograft outcomes. This is the focus of this particular review, concentrating on the most recent developments.

Focal segmental glomerulosclerosis: towards a better understanding for the practicing nephrologist

Focal and segmental glomerulosclerosis (FSGS) is a common histopathological lesion that can represent a primary podocytopathy, or occur as an adaptive phenomenon consequent to nephron mass reduction, a scar from a healing vasculitic lesion, direct drug toxicity or viral infection among other secondary causes. Thus, the presence of an FSGS lesion in a renal biopsy does not confer a disease diagnosis, but rather represents the beginning of an exploratory process, hopefully leading ultimately to identification of a specific etiology and its appropriate treatment. We define primary FSGS as a 'primary' podocytopathy characterized clinically by the presence of nephrotic syndrome in a patient with an FSGS lesion on light microscopy and widespread foot process effacement on electron microscopy (EM). Secondary FSGS is commonly characterized by the absence of nephrotic syndrome and the presence of segmental foot process effacement on EM. Failure to accurately differentiate between the primary and secondary forms of FSGS has resulted in many patients undergoing unnecessary immunosuppressive treatment. Here, we review some key points that may assist the practicing nephrologist to distinguish between primary and secondary FSGS.