Abstract
After the release of bilateral ureteral obstruction (BUO), postobstructive diuresis from an impaired urine concentration mechanism is associated with reduced aquaporin 2 (AQP2) abundance in the inner medullary collecting duct (IMCD). However, the underlying molecular mechanism of this AQP2 reduction is incompletely understood. To elucidate the mechanisms responsible for this phenomenon, we studied molecular changes in IMCDs isolated from rats with 4-h BUO or sham operation at the early onset of AQP2 downregulation using mass spectrometry-based proteomic analysis. Two-hundred fifteen proteins had significant changes in abundances, with 65% of them downregulated in the IMCD of 4-h BUO rats compared with sham rats. Bioinformatic analysis revealed that significantly changed proteins were associated with functional Gene Ontology terms, including "cell-cell adhesion," "cell-cell adherens junction," "mitochondrial inner membrane," "endoplasmic reticulum chaperone complex," and the KEGG pathway of glycolysis/gluconeogenesis. Targeted liquid chromatography-tandem mass spectrometry or immunoblot analysis confirmed the changes in 19 proteins representative of each predominant cluster, including AQP2. Electron microscopy demonstrated disrupted tight junctions, disorganized adherens junctions, swollen mitochondria, enlargement of the endoplasmic reticulum lumen, and numerous autophagosomes/lysosomes in the IMCD of rats with 4-h BUO. AQP2 and seven proteins chosen as representative of the significantly altered clusters had a significant increase in immunofluorescence-based colocalization with autophagosomes/lysosomes. Immunogold electron microscopy confirmed colocalization of AQP2 with the autophagosome marker microtubule-associated protein 1A/1B-light chain 3 and the lysosomal marker cathepsin D in IMCD cells of rats with 4-h BUO. We conclude that enhanced autophagic degradation of AQP2 and other critical proteins, as well as endoplasmic reticulum stress in the IMCD, are initiated shortly after BUO.