Urine is a valuable diagnostic medium and, with the discovery of urinary extracellular vesicles, is viewed as a dynamic bioactive fluid. Extracellular vesicles are lipid-enclosed structures that can be classified into three categories: exosomes, microvesicles (or ectosomes) and apoptotic bodies. This classification is based on the mechanisms by which membrane vesicles are formed: fusion of multivesicular bodies with the plasma membranes (exosomes), budding of vesicles directly from the plasma membrane (microvesicles) or those shed from dying cells (apoptotic bodies). During their formation, urinary extracellular vesicles incorporate various cell-specific components (proteins, lipids and nucleic acids) that can be transferred to target cells. The rigour needed for comparative studies has fueled the search for optimal approaches for their isolation, purification, and characterization. RNA, the newest extracellular vesicle component to be discovered, has received substantial attention as an extracellular vesicle therapeutic, and compelling evidence suggests that ex vivo manipulation of microRNA composition may have uses in the treatment of kidney disorders. The results of these studies are building the case that urinary extracellular vesicles act as mediators of renal pathophysiology. As the field of extracellular vesicle studies is burgeoning, this Review focuses on primary data obtained from studies of human urine rather than on data from studies of laboratory animals or cultured immortalized cells.

Urinary microvesicles, such as 40-100 nm exosomes and 100-1000 nm microparticles, contain many proteins that may serve as biomarkers of renal disease. Microvesicles have been isolated by ultracentrifugation or nanomembrane ultrafiltration from normal urine; however, little is known about the efficiency of these methods in isolating microvesicles from patients with nephrotic-range proteinuria. Here we compared three techniques to isolate microvesicles from nephrotic urine: nanomembrane ultrafiltration, ultracentrifugation, and ultracentrifugation followed by size-exclusion chromatography (UC-SEC). Highly abundant urinary proteins were still present in sufficient quantity after ultrafiltration or ultracentrifugation to blunt detection of less abundant microvesicular proteins by MALDI-TOF-TOF mass spectrometry. The microvesicular markers neprilysin, aquaporin-2, and podocalyxin were highly enriched following UC-SEC compared with preparations by ultrafiltration or ultracentrifugation alone. Electron microscopy of the UC-SEC fractions found microvesicles of varying size, compatible with the presence of both exosomes and microparticles. Thus, UC-SEC following ultracentrifugation to further enrich and purify microparticles facilitates the search for prognostic biomarkers that might be used to predict the clinical course of nephrotic syndrome.

In conclusion, we demonstrate a microfiltration isolation method that preserves the exosome structure, reduces contamination from higher abundant urinary proteins, and can be easily implemented into mass spectrometry analysis for biomarker discovery efforts or incorporation into routine clinical laboratory applications to yield higher sample throughput.

Minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS) are the key histological findings in patients with idiopathic nephrotic syndrome (INS). Although MCD and idiopathic FSGS are often considered to represent separate entities based on differences in their presenting characteristics, histology and outcomes, little evidence exists for this separation. We propose that MCD and idiopathic FSGS are different manifestations of the same progressive disease. The gradual development of FSGS in patients with non-remitting or relapsing INS has been well documented. Moreover, FSGS is the uniform result of substantial podocyte loss in animal models, and a common feature of virtually all progressive human glomerulopathies. As evidence suggests a common aetiology, the pathogenesis of MCD and idiopathic FSGS should be studied together. In clinical trials, idiopathic FSGS should be considered to represent an advanced stage of disease progression that is less likely to respond to treatment than the earlier stage of disease, which is usually defined as MCD.

Podocyte foot process effacement is characteristic of proteinuric renal diseases. In minimal change nephrotic syndrome (MCNS) foot processes are diffusely effaced whereas the extent of effacement varies in focal segmental glomerulosclerosis (FSGS). Here we measured foot process effacement in FSGS and compared it to that in MCNS and in normal kidneys. A clinical diagnosis was used to differentiate idiopathic FSGS from secondary FSGS. Median foot process width, determined morphometrically by electron microscopy, was 3236 nm in 17 patients with idiopathic FSGS, 1098 nm in 7 patients with secondary FSGS, and 1725 nm in 15 patients with MCNS, as compared to 562 nm in 12 control patients. Multivariate analysis showed that foot process width did not correlate with proteinuria or serum albumin levels but was significantly associated as an independent factor with the type of disease. Foot process width over 1500 nm differentiated idiopathic from secondary FSGS with high sensitivity and specificity. Our results show that quantitative analysis of foot processes may offer a potential tool to distinguish idiopathic from secondary FSGS.

The collapsing variant was rare in our population. Renal survival and remission rates were higher in patients with the tip variant. Use of the classification scheme for FSGS may be clinically useful.

Focal segmental glomerulosclerosis (FSGS) is a common cause of steroid-resistant nephrotic syndrome in children and adults. Although FSGS is considered a podocyte disease, the aetiology is diverse. In recent years, many inheritable genetic forms of FSGS have been described, caused by mutations in proteins that are important for podocyte function. In the present commentary, we review these genetic causes of FSGS and describe their prevalence in familial and sporadic FSGS. In routine clinical practice, the decision to perform the costly DNA analysis should be based on the assessment if the results affect the care of the individual patient with respect to the evaluation of extra-renal manifestations, treatment decisions, transplantation and genetic counselling.

The soluble urokinase receptor (suPAR) promotes proteinuria and induces focal segmental glomerulosclerosis (FSGS)-like lesions in mice. A serum suPAR concentration cutoff of 3000 pg/ml has been proposed as a clinical biomarker for patients with FSGS. Interestingly, several studies in patients with glomerulopathy found an inverse correlation between the estimated glomerular filtration rate (eGFR) and suPAR. As patients with FSGS present at different eGFRs, we studied the relationship between eGFR and suPAR in a cohort of 476 non-FSGS patients and 54 patients with biopsy-proven idiopathic FSGS. In the non-FSGS patients, eGFR was the strongest significant determinant of suPAR. The proposed cutoff for suPAR in FSGS patients was exceeded in 17%, 39%, and 88% in patients with eGFRs of more than 60, 45-60, and 30-45 ml/min per 1.73 m(2), respectively. In patients with eGFR of <30 ml/min per 1.73 m(2), suPAR exceeded the cutoff in 95% of patients. Levels of suPAR in patients with idiopathic FSGS overlapped with non-FSGS controls and for any given eGFR did not discriminate FSGS cases from non-FSGS controls. In the overall cohort, there was a negative association between idiopathic FSGS and suPAR, and idiopathic FSGS was not an independent predictor of FSGS concentration over 3000 pg/ml. Thus, this study does not support an absolute, eGFR-independent, suPAR concentration cutoff as a biomarker for underlying FSGS pathology and questions the validity of relative, eGFR-dependent suPAR cutoff values.