Engineering next generation bioluminescent Ca2+ sensors through directed evolution

The use of genetically encoded fluorescent sensors for calcium ion (Ca²⁺) has revolutionized neuroscience research by allowing for the recording of dozens of neurons at the single-cell level in living animals. However, fluorescence imaging has some limitations such as the need for excitation light, which can result in high auto-fluorescent background and phototoxicity. In contrast, bioluminescent sensors using luciferase do not require excitation light, making them ideal for non-invasive deep tissue imaging in mammals. Our lab has previously developed a bioluminescent Ca²⁺ sensor CaMBI to image Ca²⁺ activity in the mouse liver (Oh, et al. Nat Chem Biol 2019), but its responsiveness to Ca²⁺ changes was suboptimal. To improve the performance of this sensor, we applied directed evolution to screen for genetic variants with increased responsiveness. Through several rounds of evolution, we identified variants with more than five times improved responsiveness in vitro. We characterized the improved sensors in culture cell lines and dissociated rat neurons and confirmed that they exhibited higher sensitivity to changes in intracellular Ca²⁺ levels compared their progenitor. These optimized Ca²⁺ sensors have the potential for non-invasive imaging of Ca²⁺ activity in vivo, particularly in the brain.