Development of a genetically encoded sensor for endogenous CaMKII activity

CaMKII is a crucial oligomeric enzyme in neuronal and cardiac signaling, fertilization and immunity. Here, we report the construction of a novel, substrate-based, genetically-encoded sensor for CaMKII activity, FRESCA (FRET-based Sensor for CaMKII Activity). Currently, there is one biosensor for CaMKII activity, Camui, which contains CaMKII. FRESCA allows us to measure all endogenous CaMKII variants, while Camui can track a single variant. Since there are ~40 CaMKII variants, using FRESCA to measure aggregate activity allows a fresh perspective on CaMKII activity. We show, using live-cell imaging, FRESCA response is concurrent with Ca2+ rises in HEK293T cells and mouse eggs. In eggs, we stimulate oscillatory patterns of Ca2+ and observe the differential responses of FRESCA and Camui. Our results implicate an important role for the variable linker region in CaMKII, which tunes its activation. FRESCA will be a transformative tool for studies in neurons, cardiomyocytes and other CaMKII-containing cells.