Albuminuria is not associated with elevated urinary vesicle concentration but can confound nanoparticle tracking analysis.

AIM: Extracellular vesicles, such as exosomes, are present in urine with reports of roles in intercellular signalling and diagnostic utility. However, the extent to which the concentration and characteristics of urinary vesicles are altered in albuminuric renal disease has not been well characterized. In this study, we examined the number and characteristics of extracellular vesicles in albuminuric urine. METHODS: Vesicles were isolated from the urine of 32 patients with varying levels of albuminuria using ultracentrifugation and density gradient purification and were examined using nanoparticle tracking analysis, immunoblotting and transmission electron microscopy. The size profile of particles in these urine preparations was compared with albumin-containing solutions. RESULTS: Overall, there were no substantial differences in the number, or characteristics, of vesicles released into proteinuric urine. Analysis of albumin-containing solutions showed particles of exosome-like size, suggesting that such particles can mimic exosomes in standard nanoparticle tracking analysis. Albumin and IgG depletion of proteinuric urine resulted in a substantial reduction in the concentration of particles detected by nanoparticle tracking analysis. CONCLUSION: There was no increase in urinary vesicle concentration in patients with albuminuria. Furthermore, these results demonstrate the need for cautious interpretation of nanoparticle tracking analysis of vesicle concentration in biological fluids containing protein and for sophisticated preparative methods in vesicle purification from urine.

Exosomes and the kidney: blaming the messenger.

Exosomes are membrane-bound vesicles of endosomal origin, present in a wide range of biological fluids, including blood and urine. They range between 30 and 100 nm in diameter, and consist of a limiting lipid bilayer, transmembrane proteins and a hydrophilic core containing proteins, mRNAs and microRNAs (miRNA). Exosomes can act as extracellular vehicles by which cells communicate, through the delivery of their functional cargo to recipient cells, with many important biological, physiological and pathological implications. The exosome release pathway contributes towards protein secretion, antigen presentation, pathogen transfer and cancer progression. Exosomes and exosome-mediated signalling have been implicated in disease processes such as atherosclerosis, calcification and kidney diseases. Circulating levels of exosomes and extracellular vesicles can be influenced by the progression of renal disease. Advances in methods for purification and analysis of exosomes are leading to potential diagnostic and therapeutic avenues for kidney diseases. This review will focus on biophysical properties and biogenesis of exosomes, their pathophysiological roles and their potential as biomarkers and therapeutics in kidney diseases.

Review: The role of microRNAs in kidney disease.

MicroRNAs (miRNAs) are short non-coding RNAs that modulate physiological and pathological processes by inhibiting target gene expression via blockade of protein translation or by inducing mRNA degradation. These miRNAs potentially regulate the expression of thousands of proteins. As a result, miRNAs have emerged rapidly as a major new area of biomedical research with relevance to kidney disease. MiRNA expression has been shown to differ between the kidney and other organs as well as between different kidney regions. Furthermore, miRNAs have been found to be functionally important in models of podocyte development, diabetic nephropathy and polycystic kidney disease. Of particular interest, podocyte-specific deletion of Dicer, a key enzyme in the biogenesis of miRNA, results in proteinuria and severe renal impairment in mice. One miRNA (miR-192) can also act as an effector of transforming growth factor-ß activity in the high-glucose environment of diabetic nephropathy. Differential expression of miRNAs has been reported in kidney allograft rejection. It is anticipated that future studies involving miRNAs will generate new insights into the complex pathophysiology underlying various kidney diseases, generate diagnostic biomarkers and might be of value as therapeutic targets for progressive kidney diseases. The purpose of this review is to highlight key miRNA developments in kidney diseases and how this might influence the diagnosis and management of patients with kidney disease in the future.