The Development of FRET-Based IP3 Biosensors and Their Use for Monitoring IP3 Dynamics during Ca2+ Oscillations and Ca2+ Waves in Non-Excitable Cells

High resolution imaging of intracellular calcium (Ca2+) concentrations ([Ca2+];) has revealed that Ca2+ signals show diverse responses, such as Ca2+ oscillations and Ca2+ waves ; these signals are exploited to control diverse cellular processes. Ca2+ responses in non-excitable cells are primarily regulated by inositol 1,4,5-trisphosphate (IP3). To elucidate the mechanisms of these diverse Ca2+ responses, we have developed a series of fluorescent IP3 biosensors, the LIBRAv series, using cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) in combination with fluorescence resonance energy transfer (FRET) technologies. These fluorescent IP3 biosensors are specific to IP3 and permit the monitoring of IP3 dynamics in single living cells. Studies conducted with IP3 biosensors during agonist-induced Ca2+ oscillations revealed cell type-specific differences in IP3 dynamics, non-fluctuating rises in cytosolic IP3 concentrations ([IP3]) and repetitive IP3 spikes. These results provide important experimental information for resolving long-standing questions regarding the mechanisms responsible for Ca2+ oscillations. IP3 biosensors also demonstrated an intracellular gradient of IP3 and its wave-like expansion during mechanical stimulation-induced Ca2+ waves. As such, continued improvements in IP3 biosensors and the development of additional novel fluorescent biosensors are highly likely to provide a better understanding of the regulatory mechanisms of various forms of Ca2+ signals and clarification of their physiologic roles.