AQP2 in human urine is predominantly localized to exosomes with preserved water channel activities

AQP2 water channel is critical for urinary concentration in the kidney. Interestingly, AQP2 is abundantly excreted in the urine as extracellular vesicles (EVs), which is known to be a useful biomarker for water-balance disorders although the character of AQP2-enriched EVs is poorly understood including water channel function.

Human urine EVs were obtained by a differential centrifugation method. AQP2-bearing EVs were isolated by immunoprecipitation with an AQP2-specific antibody, and the proteins in the EVs were analyzed by LC-MS/MS proteomic analysis. Osmotic water permeability (Pf) of the AQP2-rich EVs was measured by a stopped-flow method monitoring scattered light intensity in response to outwardly directed osmotic gradient.

Sequential centrifugation of human urine showed that AQP2 was present predominantly (80%) in low-density EVs (160,000 g), whereas negligible amount in high-density EVs (17,000 g). Proteomic analysis of the AQP2-bearing EVs identified 137 proteins, mostly in the endosome pathway, including the components of ESCRT (endosomal sorting complex required transporter)-I, II, III. Pf value of the 160,000 g EVs was 4.75 ± 0.38 × 10^-4 cm s^-1 (mean ± SE) with the activation energy of 3.51 kcal mol^-1 which was inhibited with 0.3 mM HgCl₂ by 63%, suggesting a channel-mediated water transport. Moreover, Pf value showed a significant correlation with the abundance of AQP2 protein in EVs.

Taken together, AQP2 is localized predominantly to urinary exosomes with preserved water channel activities.