Focal Segmental Glomerulosclerosis (FSGS) is a major cause of primary glomerular disease in adults. The resulting glomerular damage can lead to generalised oedema, protein leakage into urine, and in some cases kidney failure. Its underlying cause remains unknown, however soluble urokinase receptor (suPAR) has been implicated in its aetiology. Mixed results over the involvement have suPAR, led Ming-hui Zhao, Jing Huang and colleagues from Peking University, China, to investigate its significance in a cohort of primary FSGS patients, as published in their recent study in BMC Medicine. Here Zhao and Huang explain how suPAR relates to FSGS and how their results could influence clinical practice.
What is Primary Focal Segmental Glomerulosclerosis (FSGS)?
Primary Focal Segmental Glomerulosclerosis (FSGS) is defined as a clinico-pathological syndrome without known aetiology. The ubiquitous clinical feature of the syndrome is proteinuria, which may be nephrotic or non-nephrotic. The ubiquitous pathological feature is focal segmental glomerular consolidation or scarring, which means some glomeruli are affected (focal) and the affected glomeruli show partial capillary loop involvement (segmental). Several distinctive histopathological patterns can be classified; collapsing FSGS, tip lesion FSGS, cellular FSGS, perihilar FSGS, and FSGS not otherwise specified (NOS).
FSGS is a major pathological type of refractory nephrotic syndrome in both children and adults. The degree of proteinuria is a predictor of the long term clinical outcome. Patients who have proteinuria of more than 10 g of protein per day have very poor long term renal survival, with the majority of patients reaching end stage renal disease (ESRD) within three years. In order to decrease proteinuria, renin-angiotensin-aldosterone system (RAAS) blockers are administered to almost all patients, and glucocorticoids or immunosuppressive therapy are given to those with nephrotic syndrome.
However, patients with primary FSGS remain frustrating to treat, and the aetiology and pathogenesis of the disease has not been well elucidated. The damage and detachment of podocytes from the glomerular basement membrane is regarded as the key point for the initiation and progression of FSGS; searching for pathogenic factors acting on podocytes may provide promising contributions to future disease therapies.
What is suPAR and how does it relate to FSGS?
Urokinase receptor (uPAR) is a glycosylphosphatidylinositol (GPI)-anchored protein with three domains (DI, DII, and DIII). It is expressed on the membrane of several different cell types, including kidney podocytes, neutrophils, monocytes, macrophages, activated T-lymphocytes, and endothelial cells. It can also be released into the circulation as soluble uPAR (suPAR) after cleavage of the GPI anchor. uPAR is additionally susceptible to cleavage at the linker region between DI and DII, thus both the whole receptor and various segments of it are detectable in circulation and are all referred to as suPAR.
In addition to the regulation of proteolysis, suPAR initiates signal transduction in cooperation with other transmembrane proteins such as integrins, caveolin and G-protein-coupled receptors, which promotes cell proliferation, invasion, motility and survival. Wei and colleagues (Nat Med, 2011, 17:952–960) reported that suPAR could bind to and activate β3 integrin on podocytes and thus cause proteinuria and FSGS in a mouse model, and proposed that suPAR may be a pathogenic circulating permeability factor for FSGS.
Our study found urinary suPAR levels of patients with primary FSGS was significantly elevated and was associated with disease severity and treatment response. Cellular experiments revealed urinary suPAR could active β3 integrin on podocytes and promote wound-healing function in cultured human podocytes (unpublished data). This indicated that suPAR in patients with primary FSGS might be pathogenic to podocytes. Taken together these findings support suPAR as a helpful marker for diagnosis and a potentially causative factor in primary FSGS.
In your study you looked at urinary levels of suPAR. Is this better than measuring serum suPAR levels?
In our opinion, urinary suPAR levels may be better than measuring serum suPAR levels. We propose three possible reasons: 1) For urinary suPAR levels, 67.7 percent of primary FSGS patients showed elevated urinary suPAR levels over the cut-off value from normal healthy donors, while this was only true for only 54.1 percent of primary FSGS patients for plasma suPAR levels (shown in our previous study). 2) Urinary suPAR levels have been adjusted by urinary creatinine, and our results showed that there was no association between urinary suPAR levels and glomerular filtration function, so we think that urinary suPAR levels were less affected by glomerular filtration function as compared with serum suPAR. 3) suPAR filtrated freely through the glomerular basement membrane. Podocytes themselves could also express uPAR on their cell membranes, which may also release into urine. Thus urinary suPAR levels potentially represent the final level of both circulating suPAR and uPAR expressed by podocytes, and therefore may be of more significant clinical value.
What were your main findings?
We found that the urinary suPAR levels of patients with primary FSGS was significantly higher than that of patients with minimal change disease, membranous nephropathy, secondary FSGS and normal subjects. The urinary suPAR levels of patients with cellular FSGS was significantly higher than that of those with tip FSGS and not otherwise specified (NOS) FSGS. Urinary suPAR levels in patients with primary FSGS positively correlated with 24 hour urine protein levels and negatively correlated with plasma albumin levels. During follow up, the urinary suPAR levels of patients with complete remission decreased significantly. We therefore conclude that urinary suPAR was specifically elevated in patients with primary FSGS and was associated with disease severity.
In order to demonstrate the pathogenic role of urinary suPAR in patients with primary FSGS, we investigated the activation effect of urinary suPAR on its ligand (AP5 staining), β3 integrin, in cultured human differentiated podocytes. The AP5 signal was strongly induced along cell membranes when human differentiated podocytes were incubated with the urine of patients with FSGS at presentation, but not with disease and normal controls. More importantly, the signal could be reduced by a blocking antibody specific to uPAR.
Why is it important to distinguish FSGS from other kidney diseases?
It is very important to distinguish primary FSGS from other kidney diseases, especially from minimal change disease and secondary FSGS.
The clinical features of FSGS and minimal change disease could be exactly the same, while the therapeutic strategy, treatment response and renal outcomes are quite different. Patients with minimal change disease respond well to corticosteroids and will not reach end stage renal disease (ESRD), while a large portion of patients with primary FSGS need a long term treatment of high dose corticosteroids even with the combination of immunosuppressive drugs, and may progress to ESRD for those with no remission of nephrotic syndrome.
Differential diagnosis depends on histopathological features revealed from renal biopsies. However, the required glomeruli sections are not always available in clinical practice. We therefore tried to explore urinary suPAR detection as an aid for differential diagnosis and its pathogenic role action on podocytes.
Another disease, which needs to be differentiated from primary FSGS, is secondary FSGS. For secondary FSGS, treatment should be focused on the primary causes, while for primary FSGS, patients might receive corticosteroids and immunosuppressive drug treatments. The histopathological features could be quite similar between primary and secondary FSGS and the aetiology is not readily found at diagnosis. Consequently other methods are needed for differential diagnosis.
How will this work influence clinical practice?
The detection of urinary suPAR levels might provide a helpful method for the diagnosis of primary FSGS and its differential diagnosis from minimal change disease and secondary FSGS, alongside a combination of clinical manifestations and histopathological features discerned from renal biopsy sections. Urinary suPAR levels might also be helpful in predicting disease severity and therapeutic response.
What further research needs to be done?
Our study is a retrospective study and the sample size is limited, so a large prospective study is needed to validate the clinical usage and significance of suPAR in patients with primary FSGS. suPAR has three domains and is heavily glycosylated. The pathogenic domain or pathogenic form of glycosylation or phosphorylation of suPAR has not been fully elucidated and needs further study.
Urinary soluble urokinase receptor levels are elevated and pathogenic in patients with primary focal segmental glomerulosclerosis
BMC Medicine 2014, 12:81
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