Proteinuria observed post-uninephrectomy stems from partial suppression of protein endocytosis along the proximal tubule

Abstract

Living kidney donors are at higher risk of developing proteinuria, yet few studies have addressed the etiology. To investigate whether post-uninephrectomy (UNX) proteinuria was due to increased filtration and/or reduced endocytosis/reabsorption along the proximal tubule (PT), we compared renal function, morphology, and PT endocytic machinery in male Sprague Dawley rats before and after UNX.At 12 wk post-UNX, the weight of the remnant kidney rose by 50%, PT diameters by 30%, and single nephron GFR by 50% (based on 26% fall in creatinine clearance and 50% decrease in the number of nephrons). Urinary albumin excretion increased 10-fold post-UNX. Since the filtered load of proteins decreases in parallel with GFR post-UNX, increased albumin excretion likely reflects decreased endocytosis along the PT, a notion supported by a 50% reduction in abundance of albumin and plasminogen in renal cortex homogenates from remnant versus explant kidneys. We tested the hypothesis that the protein endocytic machinery was reduced following UNX. Immunoblot assays of renal cortex revealed decreased abundance of megalin and Dab2, two key mediators of protein endocytosis along the PT, and no change in cubilin abundance. Immunohistochemistry confirmed a 50% fall in megalin along the PT apical membrane.Mathematical modeling predicted that albumin uptake per nephron increases post-UNX, owing to higher single nephron GFR and PT hypertrophy, but is attenuated by the decrease in megalin abundance; moreover, modeling predicts that if the endocytic machinery were not partly suppressed, overall albumin excretion would remain at pre-UNX levels.In addition to albumin, urinary plasminogen and angiotensinogen were similarly increased post-UNX, while uromodulin (synthesized and secreted distally) was decreased post-UNX. Urinary biomarkers of renal injury, RBP4 and KIM-1, were increased post-UNX likely reflecting the impact of the increased endocytosis of albumin which can carry bound toxic lipids into the PT.Taken together, our findings suggest that the blunting of albumin endocytosis post-UNX, apparent as increased albuminuria, may serve to protect PT cells from albumin-induced cytotoxicity. DK083785; Keck School of Medicine Dean’s Pilot Funding This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.