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Urinary exosomes are a novel reservoir for biomarker discovery The urine contains microparticles that are a rich source of information regarding the health of cells lining the urinary tract. Microparticles in the urine contain a sub-population of tiny lipid vesicles termed exosomes. Human urine samples were obtained from healthy volunteers with ethical approval and following established procedures [1]. Using western blot and liquid chromatography coupled tandem mass spectrometry (LC-MS/MS) we identified the exosomal markers TSG101, Flotillin-1 and CD24 in the microparticle fraction. Using a sucrose gradient, the density of the particles containing these markers corresponded to the density previously reported for exosomes. In total, 88 proteins were identified by LC-MS/MS including proteins known to be expressed only by certain regions of the nephron. Using electron microscopy structures appearing to match previous reports for exosomes in size and shape have been identified. The stimuli of exosomal release in the kidney are currently unknown and so to address this issue an in vitro model of exosome release in a murine cortical collecting duct cell line [2] has been established. Exosome release appears enhanced, as measured by western blot for Flotillin-1, by 1μM ionomycin, a known promoter of exosomal release (n = 3). The effect of physiological, toxic and inflammatory mediators on exosomal release is currently being investigated. In summary, the urine contains exosomes that possess a proteome partly derived from the cells of the kidney. The established in vitro model will allow the factors mediating exosome release from the kidney to be investigated.
1. Pisitkun T, Shen R-F, Knepper MA. Identification and proteomic profiling of exosomes in human urine. Proceedings of the National Academy of Sciences 2004; 101: 13368-73.
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