An Enhanced Red Bioluminescent Indicator for Responsive Detection of Physiological Calcium Dynamics in Cells and Mice

Calcium (Ca²⁺) is a crucial metal ion and signaling messenger. While bioluminescent indicators for Ca²⁺ have emerged as powerful imaging tools, their performance has been suboptimal. In this study, we developed an enhanced bioluminescent red indicator for Ca²⁺ (eBRIC) by using a physiological Ca²⁺ concentration range during library screening. Compared with its predecessors, this new sensor demonstrates substantially improved Ca²⁺ responsiveness in protein-based assays, cultured cell lines, and primary neurons. We further demonstrated the utility of eBRIC for the in vivo recording of Ca²⁺ dynamics in the brains of live mice, using both a microscope setup and a luminescent imaging dark box. Notably, by combining eBRIC with our recently developed water-soluble luciferin, we achieved minimally invasive, video-rate imaging of Ca²⁺ activity in a defined brain region of awake mice. In a footshock-induced basolateral amygdala activation paradigm, eBRIC elicited approximately double the response compared to the previous BRIC indicator. The improved responsiveness offered by eBRIC underscores its potential as a powerful tool for investigating Ca²⁺ dynamics in living systems.