Epithelial cells in the proximal tubule of the kidney reclaim and metabolize protein from the glomerular filtrate. Proteinuria, an overabundance of protein in the urine, affects tubular cell function and is a major factor in the progression of chronic kidney disease. By developing experimental systems to study tubular protein handling in a setting that simulates some of the environmental conditions of the kidney tubule in vivo, we can better understand how microenviromental conditions affect cellular protein handling to determine if these conditions are relevant in disease. To this end, we used two in vitro microfluidic models to evaluate albumin handling by renal proximal tubule cells. For the first system, cells were grown in a microfluidic channel and perfused with physiological levels of shear stress to evaluate the role of mechanical stress on protein uptake. In the second system, a porous membrane was used to separate an apical and basolateral compartment to evaluate the fate of protein following cellular metabolism. Opossum kidney (OK) epithelial cells were exposed to fluorescently labeled albumin, and cellular uptake was determined by measuring the fluorescence of cell lysates. Confocal fluorescence microscopy was used to compare uptake in cells grown under flow and static conditions. Albumin processed by the cells was examined by size exclusion chromatography (SEC) and SDS-PAGE. Results showed that cellular uptake and/or degradation was significantly increased in cells exposed to flow compared to static conditions. This was confirmed by confocal microscopy. Size exclusion chromatography and SDS-PAGE showed that albumin was broken down into small molecular weight fragments and excreted by the cells. No trace of intact albumin was detectable by either SEC or SDS-PAGE. These results indicate that fluid shear stress are important factors mediating cellular protein handling and microfluidic in vitro models provide a novel tool to investigate these processes.

We have developed a multilayer microfluidic bioreactor to evaluate polarized epithelial cell behavior under controlled fluid shear stress. The system consists of two microfluidic chambers separated by a synthetic porous membrane which provides a substrate for cell grown and allows apical to basolateral solute transport. The system has been used to evaluate a number of shear stress‐dependent cellular behaviors in kidney epithelial cells. We found that primary human renal epithelial cells undergo cytoskeletal rearrangement when exposed to shear stress. Endocytosis, metabolism and secretion of albumin were also evaluated using the bioreactor. Fluid shear stress increased the cellular capacity for apical uptake of albumin in immortalized proximal tubule epithelial cells. Following internalization, albumin was metabolized into small molecular weight fragments and excreted into the apical and basolateral compartments. This microfluidic bioreactor is a useful tool for evaluating kidney epithelial cell behavior in response to fluid shear stress and may have broader applications for studying a variety of shear stress sensitive cell types. This work was supported by the Wildwood Foundation and a T32 training grant from the National Institutes of Health (T32‐DK‐007470–25).