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      Ultra-sensitive fluorescent proteins for imaging neuronal activity

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          Summary

          Fluorescent calcium sensors are widely used to image neural activity. Using structure-based mutagenesis and neuron-based screening, we developed a family of ultra-sensitive 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 micrometers long). GCaMP6 sensors thus provide new windows into the organization and dynamics of neural circuits over multiple spatial and temporal scales.

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          Most cited references50

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          Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin.

          Important Ca2+ signals in the cytosol and organelles are often extremely localized and hard to measure. To overcome this problem we have constructed new fluorescent indicators for Ca2+ that are genetically encoded without cofactors and are targetable to specific intracellular locations. We have dubbed these fluorescent indicators 'cameleons'. They consist of tandem fusions of a blue- or cyan-emitting mutant of the green fluorescent protein (GFP), calmodulin, the calmodulin-binding peptide M13, and an enhanced green- or yellow-emitting GFP. Binding of Ca2+ makes calmodulin wrap around the M13 domain, increasing the fluorescence resonance energy transfer (FRET) between the flanking GFPs. Calmodulin mutations can tune the Ca2+ affinities to measure free Ca2+ concentrations in the range 10(-8) to 10(-2) M. We have visualized free Ca2+ dynamics in the cytosol, nucleus and endoplasmic reticulum of single HeLa cells transfected with complementary DNAs encoding chimaeras bearing appropriate localization signals. Ca2+ concentration in the endoplasmic reticulum of individual cells ranged from 60 to 400 microM at rest, and 1 to 50 microM after Ca2+ mobilization. FRET is also an indicator of the reversible intermolecular association of cyan-GFP-labelled calmodulin with yellow-GFP-labelled M13. Thus FRET between GFP mutants can monitor localized Ca2+ signals and protein heterodimerization in individual live cells.
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            Optimization of a GCaMP calcium indicator for neural activity imaging.

            Genetically encoded calcium indicators (GECIs) are powerful tools for systems neuroscience. Recent efforts in protein engineering have significantly increased the performance of GECIs. The state-of-the art single-wavelength GECI, GCaMP3, has been deployed in a number of model organisms and can reliably detect three or more action potentials in short bursts in several systems in vivo. Through protein structure determination, targeted mutagenesis, high-throughput screening, and a battery of in vitro assays, we have increased the dynamic range of GCaMP3 by severalfold, creating a family of "GCaMP5" sensors. We tested GCaMP5s in several systems: cultured neurons and astrocytes, mouse retina, and in vivo in Caenorhabditis chemosensory neurons, Drosophila larval neuromuscular junction and adult antennal lobe, zebrafish retina and tectum, and mouse visual cortex. Signal-to-noise ratio was improved by at least 2- to 3-fold. In the visual cortex, two GCaMP5 variants detected twice as many visual stimulus-responsive cells as GCaMP3. By combining in vivo imaging with electrophysiology we show that GCaMP5 fluorescence provides a more reliable measure of neuronal activity than its predecessor GCaMP3. GCaMP5 allows more sensitive detection of neural activity in vivo and may find widespread applications for cellular imaging in general.
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              A high signal-to-noise Ca(2+) probe composed of a single green fluorescent protein.

              Recently, several groups have developed green fluorescent protein (GFP)-based Ca(2+) probes. When applied in cells, however, these probes are difficult to use because of a low signal-to-noise ratio. Here we report the development of a high-affinity Ca(2+) probe composed of a single GFP (named G-CaMP). G-CaMP showed an apparent K(d) for Ca(2+) of 235 nM. Association kinetics of Ca(2+) binding were faster at higher Ca(2+) concentrations, with time constants decreasing from 230 ms at 0.2 microM Ca(2+) to 2.5 ms at 1 microM Ca(2+). Dissociation kinetics (tau approximately 200 ms) are independent of Ca(2+) concentrations. In HEK-293 cells and mouse myotubes expressing G-CaMP, large fluorescent changes were observed in response to application of drugs or electrical stimulations. G-CaMP will be a useful tool for visualizing intracellular Ca2+ in living cells. Mutational analysis, together with previous structural information, suggests the residues that may alter the fluorescence of GFP.
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                Author and article information

                Journal
                0410462
                6011
                Nature
                Nature
                Nature
                0028-0836
                1476-4687
                24 July 2013
                18 July 2013
                18 January 2014
                : 499
                : 7458
                : 295-300
                Affiliations
                [1 ]Janelia Farm Research Campus, Howard Hughes Medical Institute, 19700 Helix Drive, Ashburn, Virginia 20147, USA
                [2 ]Champalimaud Neuroscience Programme, Champalimaud Centre for the Unknown, Avenida Brasília, Doca de Pedrouços, 1400-038, Lisbon, Portugal
                [3 ]Department of Neurobiology, University of California Los Angeles, Los Angeles, CA 90095, USA
                Author notes
                Correspondence should be addressed to M.B.O. ( michael.orger@ 123456neuro.fchampalimaud.org ) for zebrafish, V.J. ( jayaramanv@ 123456janelia.hhmi.org ) for flies, L.L.L. ( loogerl@ 123456janelia.hhmi.org ) for GCaMP protein structure information, K.S. ( svobodak@ 123456janelia.hhmi.org ) for mice, and D.S.K. ( kimd@ 123456janelia.hhmi.org ) for neuronal culture screen information and constructs
                [†]

                Present address: Marine Biological Laboratory, Program in Sensory Physiology and Behavior, 7 MBL Street, Woods Hole, Massachusetts 02543, USA

                Article
                HHMIMS489004
                10.1038/nature12354
                3777791
                23868258
                5fe24738-f97c-4821-a8cb-d000542296a0

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                Funding
                Funded by: Howard Hughes Medical Institute :
                Award ID: || HHMI_
                Funded by: Howard Hughes Medical Institute :
                Award ID: || HHMI_
                Funded by: Howard Hughes Medical Institute :
                Award ID: || HHMI_
                Funded by: Howard Hughes Medical Institute :
                Award ID: || HHMI_
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