Advancing Investigations With the Microfluidic Kidney Pump

The title of the paper “Kidney Epithelial Cells are Active Mechano-biological Fluid Pumps,” introduces a phenomenon that revolutionizes understanding of kidney epithelial function and that is previously undescribed in nephrology textbooks1. In the paper, a microfluidic device was used to reproduce the pumping function of kidney epithelial cells. The paper identified that healthy kidney cells pump fluid from apical (luminal) to basal (interstitial) sides until reaching a stall pressure of approximately 150 Pa (ΔP*). The next step following the discoveries of this paper is to understand why kidney epithelial cells stop pumping at this pressure—how they sense it. In this essay, the steps taken to investigate this query are described, implementing the microfluidic device from the paper by Choudhury et al. Based on the exploration performed up to this point, three major observations of kidney epithelia under basal pressure can be made: ΔP* decreases trans-epithelial fluid flux, ΔP* causes actin ring formation, and ΔP* causes spatio-temporal calcium fluctuations. To tie these three observations in addition to the observations of the paper by Choudhury et al. and conclusions in the literature together, we hypothesize that ΔP* causes invaginations in the basolateral cell membrane followed by an influx of calcium ions, leading to a Rho-A flare. This flare precipitates the polymerization of actin and myosin contractility, which causes the invagination to be pushed back to the original position. We predict that the decrease in transepithelial fluid flux is caused by increased endocytosis of Na+/K+-ATPase (NKA) that occurs in conjunction with the pushing out of the membrane invagination. We are now in the process of testing this hypothesis, doing so through the application of five different compounds that have different effects on the structure and function of kidney epithelium under pressure according to their respective mechanisms of action. These results provide insights into the mechanism of basolateral pressure sensing in kidney epithelial cells and generate avenues for further investigation to be pursued in the coming years. We also describe the initial exploration of other epithelia in the microfluidic device, noting that we have been able to establish viable monolayers with these cell types and have begun investigating the ability of these cells to function as fluid pumps