Tubular Renal Epithelial Cells Are Active Mechanobiological Water Pumps

Autosomal dominant polycystic kidney disease (ADPKD) is a common hereditary disorder caused by mutations in the genes PKD1 and PKD2. A major characteristic of the disease is development and progressive growth of numerous fluid-filled cysts in the renal tubules. Previous studies have shown that cysts are formed due to rapid cell proliferation and are sustained by hydrostatic pressure generated by the epithelial cells by pumping ions and water into the lumen. However, no work has been done to quantify the fluid pumping performance of normal and diseased kidney cells. Therefore, we have measured the fluid flux and hydrostatic pressure difference across a renal epithelium by using a novel Kidney-on-a-chip microfluidic device with a volumetric resolution ∼0.31 μL. The fluid flux across Madin Darby Canine Kidney II (MDCK-II) epithelium decreases with an increase in pressure difference with a maximum flux of 0.3 μL/min, which is consistent with the results of our theoretical model for active water pumping. In case of normal cells derived from human kidneys, the direction of fluid flux was apical-to-basal. However, the flow direction changed from basal-to-apical in case of cystic cells derived human ADPKD kidneys. The fluid flux in cystic cells increases with osmotic and drug perturbation from 0.5 μL/min (Control) to 1.5 μL/min (50% osmotic gradient) and 2.5 μL/min (arginine vasopressin (AVP)). Interestingly, in cystic cells, the stall pressure (ΔP) doesn’t change with perturbation and is measured to be around 200 Pa, indicating a different mechanism of active and passive ion transport. As per our knowledge, for the first time we have shown experimentally, the dynamic nature of fluid flux and stall pressure difference in normal and diseased cells.