Microfluidic Platform for Multimodal Analysis of Enzyme Secretion in Nanoliter Droplet Arrays

High-throughput screening of cell-secreted proteins is essential for various biotechnological applications. In this article, we show a microfluidic approach to perform the analysis of cell-secreted proteins in nanoliter droplet arrays by two complementary methods, fluorescence microscopy and mass spectrometry. We analyzed the secretion of the enzyme phytase, a phosphatase used as an animal feed additive, from a low number of yeast cells. Yeast cells were encapsulated in nanoliter volumes by droplet microfluidics and deposited on spatially defined spots on the surface of a glass slide mounted on the motorized stage of an inverted fluorescence microscope. During the following incubation for several hours to produce phytase, the droplets can be monitored by optical microscopy. After addition of a fluorogenic substrate at a defined time, the relative concentration of phytase was determined in every droplet. Moreover, we demonstrate the use of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to monitor the multistep conversion of the native substrate phytic acid by phytase secreted in 7 nL droplets containing 50-100 cells. Our method can be adapted to various other protocols. As the droplets are easily accessible, compounds such as assay reagents or matrix molecules can be added to all or to selected droplets only, or part of the droplet volume could be removed. Hence, this platform is a versatile tool for questions related to cell secretome analysis.