Ultrasensitive Fluorescent Proteins for Imaging Neuronal Activity

Fluorescent calcium sensors are widely used to image neural activity. Using structure-based mutagenesis and neuron-based screening, we developed a family of ultrasensitive protein calcium sensors (GCaMP6) that outperformed other sensors in cultured neurons and in zebrafish, flies and mice in vivo. In layer 2/3 pyramidal neurons of the mouse visual cortex, GCaMP6 reliably detected single action potentials in neuronal somata and orientation-tuned synaptic calcium transients in individual dendritic spines. The orientation tuning of structurally persistent spines was largely stable over timescales of weeks. Orientation tuning averaged across spine populations predicted the tuning of their parent cell. Although the somata of GABAergic neurons showed little orientation tuning, their dendrites included highly tuned dendritic segments (5–40-µm long). GCaMP6 sensors thus provide new windows into the organization and dynamics of neural circuits over multiple spatial and temporal scales. Sensitive protein sensors of calcium have been created; these new tools are shown to report neural activity in cultured neurons, flies and zebrafish and can detect single action potentials and synaptic activation in the mouse visual cortex in vivo. Genetically encoded calcium sensors have brought neuronal recording to the tiny brains of invertebrates, but the methodology has lagged behind classical electrophysiology in vertebrates. Now Douglas Kim and colleagues have used selective mutagenesis to engineer a new ultrasensitive probe, GCaMP6, demonstrating improved spatial and temporal resolution in vivo, from flies to zebrafish. In addition, in mouse visual cortex GCaMP6 can reliably detect single action potentials and single-spine orientation tuning. GCaMP6 sensors can be used to image large groups of neurons as well as tiny synaptic compartments over multiple imaging sessions separated by months, offering a flexible new tool for brain research and calcium signalling studies.