Light-activated gene expression directs segregation of co-cultured cells in vitro

Light-directed gene patterning methods have been described as a means to regulate gene expression in a spatially and temporally controlled manner. Several methods have been reported that use photocaged forms of small molecule effectors to control ligand-dependent transcription factors. Whereas these methods offer many advantages including high specificity and transient light-sensitivity, the free diffusion of the uncaged effector can limit both the magnitude and resolution of localized gene induction. Methods to date have been limited by the small fraction of irradiated cells that have expression levels significantly above uninduced background and have not been shown to affect a defined biological response. The tetracycline-dependent transactivator/transrepressor system, RetroTET-ART, combined with a photocaged form of doxycycline (NvOC-Dox) can be used to form photolithographic patterns of induced expression wherein up to 85% of the patterned cells show expression levels above uninduced regions. The efficiency and inducibility of the RetroTET-ART system allows one to quantitatively measure the limits of resolution and the relative induction levels mediated by a small molecule photocaged effector for the first time. Well-defined patterns of reporter genes were reproducibly formed within 6-36 h with feature sizes as small as 300 microm. After photo-patterning, NvOC-Dox can be rapidly removed, rendering cells photoinsensitive and allowing one to monitor GFP product formation in real time. Patterned co-expression of the cell surface ligand ephrin A5 on cell monolayers creates well-defined patterns that are sufficient to direct and segregate co-cultured cells via either attractive or repulsive signaling cues. The ability to direct the arrangement of cells on living cell monolayers through the action of light may serve as a model system for engineering artificial tissues.