Background: Aldosterone and the MR (mineralocorticoid receptor) are important therapeutic targets for hypertension and cardiovascular diseases. However, biomarkers of tissue MR signaling are not fully established. Extracellular vesicles released from eukaryotic cells can provide information on tissue signaling. Using samples from patients with primary aldosteronism (PA), we explored the potential of urinary extracellular vesicles (uEVs) as a noninvasive indicator of MR signaling to guide treatment.
Methods: We analyzed proteins contained in PA uEVs by liquid chromatography tandem mass spectrometry. We narrowed down candidate biomarkers by referring to an existing database of urinary exosomes. The results were validated through Western blot analysis involving 63 patients with PA and 11 healthy volunteers.
Results: We identified a total of 1940 proteins in PA uEVs. Comparative analysis with the existing database narrowed down the pathways enriched in PA uEVs, which were related to diabetic complications, Rac1 signaling, and aldosterone-regulated sodium reabsorption. A closer look at the identified proteins revealed ENaCγ (epithelial Na+ channel γ) peptides near the proteolytic cleavage sites, and Western blot analysis confirmed the predominant presence of cleaved ENaCγ, a marker of aldosterone signaling in renal tubules. In PA uEVs, cleaved ENaCγ showed a 4.8-fold increase compared with healthy volunteers and was significantly correlated with the aldosterone-to-renin ratio, aldosterone levels, and fractional excretion of K+. Targeted treatment in PA reduced the abundance of cleaved ENaCγ, suggesting a causal role for MR in its induction.
Conclusions: This study provides a list of proteins contained in PA uEVs and suggests that ENaCγ in uEVs is a promising biomarker for renal MR signaling.
Keywords: biomarkers; eukaryotic cells; exosomes; peptides; sodium.