Abstract
Human urinary extracellular vesicles (uEVs) contain proteins from all nephron segments. An assumption for years has been that uEVs might provide a non-invasive liquid biopsy that reflect physiological regulation of transporter protein expression in human. We hypothesized that protein abundance in human kidney tissue and uEV are directly related and tested this in paired collections of nephrectomy tissue and urine sample from 12 patients. Kidney tissue was fractioned into total kidney protein, crude membrane (plasma membrane and large intracellular vesicles) and intracellular vesicle enriched fractions, as well as sections for immunolabelling. uEVs were isolated from spot urine samples. Antibodies were used to quantify 6 segment-specific proteins (proximal tubular expressed Na/Phosphate cotransporter NaPi-2a, thick ascending limb expressed Tamm-Horsfall protein and renal-outer-medullary K+channel ROMK, distal convoluted tubular expressed NaCl cotransporter NCC, intercalated cell expressed proton-pump subunit ATP6V1G3 and principal cell expressed aquaporin 2 (AQP2)) and 3 uEV markers (exosomal CD63, microvesicle marker VAMP3 and β-actin) in each fractions. By western blotting and immunofluorescence labelling, we found significant positive correlations between abundance of CD63, NCC, AQP2 and ATP6V1G3, respectively, within the different kidney-derived fractions. We detected all 9 proteins in uEVs, but their level did not correlate with kidney tissue protein abundance. The uEV protein levels showed higher inter-patient variability than the kidney-derived fractions, indicating that factors, besides kidney protein abundance, contribute to the uEV protein level. Our data suggest that, in a random sample of nephrectomy patients, uEV protein level is not a predictor of kidney protein abundance.
https://www.physiology.org/doi/abs/10.1152/ajprenal.00242.2019