Impact of sex, flow, and glomerular permeability on endocytic uptake along the kidney proximal tubule - ABSTRACT/SUMMARY Proteinuria occurs when the kidney proximal tubule (PT) is unable to fully reclaim albumin and other proteins that escape the glomerular filtration barrier. It is linked to increased risk and progression of chronic kidney disease (CKD). Proteinuria is associated with higher risk for mortality and CKD progresses more rapidly in men compared to women. Understanding how urinary excretion of albumin and other proteins changes with disease and is tied to PT function is critical to unravel the link between proteinuria and CKD. This requires a greater understanding of where and how endocytic uptake is regulated along the PT and how the PT responds to normal and pathologic variations in glomerular function. Filtered proteins are internalized by the PT via receptor-mediated endocytosis facilitated by the multiligand receptors megalin and cubilin which are differentially expressed along the functionally distinct S1, S2, and S3 subsegments that comprise the PT. Uptake along the PT is modulated by receptor expression, cellular trafficking kinetics, tubule size, and the rate of fluid filtration and reabsorption, all of which may vary significantly between females and males and in kidney disease. For example, female rodents appear to have lower single nephron GFR (SNGFR), smaller PT volume, and lower PT reabsorption of Na+ and water. This project aims to investigate the determinants of PT endocytic capacity, focusing on how sex and alterations in glomerular filtration rate (GFR) and permeability impact PT endocytic uptake. The first aim will develop a multiscale model of protein uptake along the PT and characterize baseline sex-based differences in glomerular number, volume and diameter of PT subsegments, and expression and subcellular distribution of megalin and cubilin in along the PT. The second aim will assess how acute variations in fluid shear stress affect endocytic uptake in cell culture and in vivo. The third aim will utilize an Alport syndrome mouse model to determine how increased protein filtration impacts megalin and cubilin expression and the urinary excretion of both albumin and low molecular weight protein cystatin C. By integrating multiple quantitative imaging modalities in animals along with in vitro studies, this research will enhance our ability to correlate the urinary excretion of proteins with PT and/or glomerular dysfunction and to predict disease outcomes. The proposed research and training activities under the advisement of a mentorship team with diverse expertise will provide Dr. Shipman with opportunities to develop an extensive quantitative imaging skillset, proficiency with mouse kidney disease models, and strong collaborations throughout the renal physiology community. The collaborative environment and resources at the University of Pittsburgh will provide critical support, networking, and training. The skills developed along with the research findings resulting from this project will allow Dr. Shipman to secure their own NIH R01 funding to develop more comprehensive mathematical models of transport along the kidney nephron in normal and disease states.
