UCSF

Water transport across epithelial monolayers is of central importance in mammalian fluid homeostasis, and epithelial aquaporin (AQP) water channels are potential drug targets. Current methods to measure transepithelial water permeability based on indicator dilution have limited accuracy and can require hours for a single measurement. We report here a microfluidics platform for rapid and accurate measurement of water transport across a conventionally cultured epithelial monolayer on a porous filter requiring only a single image obtained using a standard laboratory fluorescence microscope. The undersurface of a porous polyester filter containing cultured epithelial cells on top is contacted with a perfused microfluidic channel of 100 μm width, 20 μm height and 10 cm length with folded geometry, with in-plane size of 3.2 × 3.2 mm² for visualization with a 2× objective lens. Osmotic water permeability is measured from the steady-state concentration profile along the length of the channel of a membrane-impermeant fluorescent dye in the perfusate, in which an osmotic gradient is imposed by an anisosmolar solution overlying the epithelial monolayer; diffusional water permeability is measured using a D₂O/H₂O-sensing fluorescent dye in the perfusate with a D₂O-containing isosmolar solution overlying the cell layer. Permeability values are deduced from single fluorescence images. The method, named fluid transport on a chip (FT-on-Chip), was applied to measure transepithelial osmotic and diffusional water permeability in control and AQP4-expressing epithelial cell monolayers. FT-on-Chip allows for rapid, accurate and repeated measurements of transepithelial water permeability, and is generalizable to transport measurements of ions and solutes using suitable indicator dyes.