Assessment of urinary exosome NHE3 as a biomarker of acute kidney injury

The diagnosis of acute kidney injury (AKI) traditionally depends on the serum creatinine (SCr) and urine output, which lack sufficient sensitivity and specificity. Urinary exosome as a biomarker has unique advantages. To assessed whether urinary exosome Na+/H+ exchanger isoform 3 (NHE3) protein as a nonivasive diagnostic biomarker of AKI, we constructed 4 AKI rat models: cisplatin (7.5mg/kg) injection intraperitoneally (IP), furosemide (20mg/kg, IP) with low NaCl (0.03%) diet, low NaCl (0.03%) diet with candesartan (1mg/kg, IP) and bilaterally ischemia and reperfusion(I/R) injury for 40 minutes. Besides, we assessed 6 sepsis associated AKI patients and 6 healthy volunteers. Urinary exosomes were extracted by ultra-centrifugation and NHE3 protein abundance was tested by immunoblotting in all AKI rats and human subjects. The isolated cup-shape particles with an average diameter of 70nm and enrichment in CD63 were identified as exosomes. NHE3 abundance was 6 times higher in exosome than in original urine. In cisplatin induced AKI rats, urinary exosome NHE3 was increased at day 2, 1 day earlier than the increases of serum creatinine creatinine (SCr) and blood urea nitrogen (BUN). In additional rats, urinary exosome NHE3 decreased along with the decline of SCr after EPO pretreatment. In volume depletion AKI induced by furosemide injection with low NaCl diet, urinary exosome NHE3 expression was higher than control. In low NaCl diet with candesartan related AKI, urinary exosome NHE3 was elevated at day 5, 2 days earlier than SCr. In I/R injury AKI, urinary exosome NHE3 was also raised compared with control. In humans, the urinary exosome NHE3 level was also elevated in sepsis associated AKI patients in comparison with the healthy volunteers. Then urinary exosome NHE3 protein may be used as a noninvasive diagnostic biomarker of AKI. Impact StatementThe high non-recognition acute kidney injury (AKI) is due to lacking of alarming symptoms or specific early biomarkers. Urinary exosome as a biomarker has unique advantages. In our study, we detected urinary exosome NHE3 protein abundance in 4 different cause of AKI rat model. Urinary exosome NHE3 was increased in all 4 AKI, and even elevated earlier than SCr in some cases. Another novel point was we established a new AKI model of low NaCl diet with candesartan injection, which was common in patients with hypertension or proteinuria clinically. The detailed methods and mechanisms of this new AKI model were presented in another article being submitted. Third, we are not limited to animals, but also selected sepsis associated AKI patients to study. The conclusion that urinary exosome NHE3 may be used as a diagnosis biomarker of AKI has important clinical application value.