Fast GCaMPs for improved tracking of neuronal activity

The use of genetically encodable calcium indicator proteins to monitor neuronal activity is hampered by slow response times and a narrow Ca 2+ -sensitive range. Here we identify three performance-limiting features of GCaMP3, a popular genetically encodable calcium indicator protein. First, we find that affinity is regulated by the calmodulin domain’s Ca 2+ -chelating residues. Second, we find that off-responses to Ca 2+ are rate-limited by dissociation of the RS20 domain from calmodulin’s hydrophobic pocket. Third, we find that on-responses are limited by fast binding to the N-lobe at high Ca 2+ and by slow binding to the C-lobe at lower Ca 2+ . We develop Fast-GCaMPs, which have up to 20-fold accelerated off-responses and show that they have a 200-fold range of K D , allowing coexpression of multiple variants to span an expanded range of Ca 2+ concentrations. Finally, we show that Fast-GCaMPs track natural song in Drosophila auditory neurons and generate rapid responses in mammalian neurons, supporting the utility of our approach. Genetically encoded calcium indicators are commonly used to study cellular activity, but their usefulness is limited by their response kinetics. Here the authors generate indicators with faster responses to calcium events in both Drosophila melanogaster and mammalian neurons.