Urinary exosomes: a reservoir for biomarker discovery and potential mediators of intrarenal signalling.

Over the last decade, there has been increasing research interest in urinary exosomes and their relationship with kidney physiology and disease. Protocols for isolating urinary exosomes have been refined and the exosomal proteome has been extensively catalogued and reported to contain proteins from the kidney's glomerulus and all sections of the nephron. In animal and human biomarker discovery studies, this proteome changes to reflect the underlying pathophysiology of certain kidney diseases. In addition to proteins, exosomes from urine have been demonstrated to contain RNA species, another new reservoir for biomarker discovery. Exosomes have the capacity to shuttle their cargo between kidney cells and change the recipient cell's proteome and function, and may represent a mechanism for cell-to-cell signalling along the nephron. Significant challenges remain; methods for urinary exosome collection need optimisation if "real-life" clinical utility is to be achieved, consensus is needed regarding normalisation of changes in exosomal protein and RNA, larger scale exosome biomarker validation studies remain to be performed, and whether exosomes signal between cells in vivo remains an intriguing, but untested, hypothesis.