Exosomes are vesicles that are released from the kidney into urine. They contain protein and RNA from the glomerulus and all sections of the nephron and represent a reservoir for biomarker discovery. Current methods for identifying and quantifying urinary exosomes are time-consuming and only semi-quantitative. Nanoparticle tracking analysis (NTA) counts and sizes particles by measuring their Brownian motion in solution.
In this study researchers at Edinburgh University, United Kingdom applied NTA to human urine and identified particles with a range of sizes. Using antibodies against the exosomal proteins CD24 and aquaporin 2 (AQP2), conjugated to a fluorophore, they could identify a sub-population of CD24 and AQP2 positive particles of characteristic exosomal size. Extensive pre-NTA processing of urine was not necessary. However, the intra-assay variability in the measurement of exosome concentration was significantly reduced when an ultra-centrifugation step preceded NTA. Without any sample processing, NTA tracked exosomal AQP2 upregulation induced by desmopressin stimulation of kidney collecting duct cells. NTA was also able to track changes in exosomal AQP2 concentration that followed desmopressin treatment of mice and a patient with central diabetes insipidus. When urine was stored at room temperature, 4 C or frozen, nanoparticle concentration was reduced; freezing at -80 C with the addition of protease inhibitors produced the least reduction. In conclusion, with appropriate sample storage, NTA has potential as a tool for characterising and quantifying extra-cellular vesicles in human urine.
- Oosthuyzen W, Sime NE, Ivy JR, Turtle EJ, Street JM, Pound J, Bath LE, Webb DJ, Gregory CD, Bailey MA, Dear JW. (2013) Quantification of human urinary exosomes by nanoparticle tracking analysis. J Physiol [Epub ahead of print]. [abstract]