Biosensor-expressing spheroid cultures for imaging of drug-induced effects in three dimensions

In the past, the majority of antitumor compound-screening approaches had been performed in two-dimensional (2D) cell cultures. Although easy to standardize, this method provides results of limited significance because cells are surrounded by an artificial microenvironment, are not exposed to hypoxia gradients, and lack cell-cell contacts. These nonphysiological conditions directly affect relevant parameters such as the resistance to anticancer drugs. Multicellular tumor spheroids more closely resemble the in vivo situation in avascularized tumors. To monitor cellular reactions within this three-dimensional model system, we stably transfected a spheroid-forming glioblastoma cell line with Grx1-roGFP2, a green fluorescent protein (GFP)-based glutathione-specific redox sensor that detects alterations in the glutathione redox potential. Functionality and temporal dynamics of the sensor were verified with redox-active substances in 2D cell culture. Based on structured illumination microscopy using nonphototoxic light doses, ratio imaging was then applied to monitor the response of the glutathione system to exogenous hydrogen peroxide in optical sections of a tumor spheroid. Our approach provides a proof of concept for biosensor-based imaging in 3D cell cultures.