Quantification of Protein Reabsorption Energetics in the Kidney Proximal Tubule

Abstract

Proteinuria can result from increased glomerular protein filtration, but insufficient or reduced protein reabsorption in the proximal tubule (PT) may also play a role. Protein is reabsorbed via receptor-mediated endocytosis, a process that is pH-dependent and requires energy in the form of ATP. Specifically, ATP is hydrolyzed by V-type ATPase, and the resulting energy is used to acidify the endocytic vesicle by pumping H+ ions into the endosome. This reduces the internal endosome pH and causes protein-receptor complex dissociation, which allows the protein to be further processed by the cell. Alterations in intracellular pH or ATP availability may impact the PT epithelial cells’ ability to form endosomes and reabsorb protein. To quantify the relationship between cellular pH, protein reabsorption, and ATP utilization, we developed a mathematical model that quantifies the energy demands of protein reabsorption in the kidney proximal tubule. The model assumes normal kidney function and focuses on albumin reabsorption for simplification. To determine the energy required to acidify a single endosome (ATP/endosome), we expanded upon a published mathematical model of general lysosome acidification that quantifies the H+ ions required to acidify a vesicle and the final pH reached, accounting for ClC-7 antiporters and proton leak, as a function of intracellular pH (Ishida et al. J Gen Phys 2013). The Nernst equation was used to determine the cumulative energy (ΔG) required to pump H+ ions into one endosome until a final stable internal pH is reached. To determine the rate of endosome formation required to reabsorb a normal filtered albumin load, we first calculated the albumin reabsorption rate per unit length of the proximal tubule as a function of albumin filtration rate, nephron number, and tubular dimensions. Next, using previous experimentally measured values for membrane internalization velocity and early endocytic radius, the rate of endosome formation required to reabsorb the filtered albumin load was calculated (total endosomes formed /sec). Total rate of ATP required to reabsorb a filtered load of albumin was then determined as the product of ATP/endosomes and total endosomes formed / second. At normal intracellular pH and normal filtered albumin load, it is estimated that 83,600 endosomes, with a radius of 0.34 µm, and 25,000 molecules of ATP are needed to reabsorb 740 µg of albumin per second for one proximal tubule. This is a very small fraction of total ATP utilized by the cell. To our knowledge, this model is the first to quantify the energetics of PT endosome formation and protein reabsorption. It provides a tool to quantitatively evaluate the impact of variations in factors such as intracellular pH, filtered protein load, and energy availability on the PT's ability to acidify endosomes and reabsorb protein. This may aid in future elucidation of pathological mechanisms of proteinuria.