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      Primary Cilia Are Not Calcium-Responsive Mechanosensors

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          Abstract

          Primary cilia are solitary, generally non-motile, hair-like protrusions that extend from the surface of cells between cell divisions. Their antenna-like structure leads naturally to the assumption that they sense the surrounding environment, the most common hypothesis being sensation of mechanical force through calcium-permeable ion channels within the cilium 1 . This Ca 2+- Responsive MechanoSensor (CaRMS) hypothesis for primary cilia has been invoked to explain a large range of biological responses, from control of left-right axis determination in embryonic development to adult progression of polycystic kidney disease and some cancers 2, 3 . Here, we report the complete lack of mechanically induced calcium increases in primary cilia, in tissues upon which this hypothesis has been based. First, we developed a transgenic mouse, Arl13b-mCherry-GECO1.2, expressing a ratiometric genetically encoded calcium indicator (GECI) in all primary cilia. We then measured responses to flow in primary cilia of cultured kidney epithelial cells, kidney thick ascending tubules, crown cells of the embryonic node, kinocilia of inner ear hair cells, and several cell lines. Cilia-specific Ca 2+ influxes were not observed in physiological or even highly supraphysiological levels of fluid flow. We conclude that mechanosensation, if it originates in primary cilia, is not via calcium signaling.

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          Most cited references 52

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          Ultra-sensitive fluorescent proteins for imaging neuronal activity

          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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            Imaging neural activity in worms, flies and mice with improved GCaMP calcium indicators

            Genetically encoded calcium indicators (GECIs) can be used to image activity in defined neuronal populations. However, current GECIs produce inferior signals compared to synthetic indicators and recording electrodes, precluding detection of low firing rates. We developed a single-wavelength GECI based on GCaMP2 (GCaMP3), with increased baseline fluorescence (3x), dynamic range (3x), and higher affinity for calcium (1.3x). GCaMP3 fluorescence changes triggered by single action potentials were detected in pyramidal cell dendrites, with signal-to-noise ratio and photostability significantly better than GCaMP2, D3cpVenus, and TN-XXL. In Caenorhabditis elegans chemosensory neurons and the Drosophila melanogaster antennal lobe, sensory stimulation-evoked fluorescence responses were significantly enhanced with the new indicator (4–6x). In somatosensory and motor cortical neurons in the intact mouse, GCaMP3 detected calcium transients with amplitudes linearly dependent on action potential number. Long-term imaging in the motor cortex of behaving mice revealed large fluorescence changes in imaged neurons over months.
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              An expanded palette of genetically encoded Ca²⁺ indicators.

              Engineered fluorescent protein (FP) chimeras that modulate their fluorescence in response to changes in calcium ion (Ca(2+)) concentration are powerful tools for visualizing intracellular signaling activity. However, despite a decade of availability, the palette of single FP-based Ca(2+) indicators has remained limited to a single green hue. We have expanded this palette by developing blue, improved green, and red intensiometric indicators, as well as an emission ratiometric indicator with an 11,000% ratio change. This series enables improved single-color Ca(2+) imaging in neurons and transgenic Caenorhabditis elegans. In HeLa cells, Ca(2+) was imaged in three subcellular compartments, and, in conjunction with a cyan FP-yellow FP-based indicator, Ca(2+) and adenosine 5'-triphosphate were simultaneously imaged. This palette of indicators paints the way to a colorful new era of Ca(2+) imaging.
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                Author and article information

                Journal
                0410462
                6011
                Nature
                Nature
                Nature
                0028-0836
                1476-4687
                20 February 2016
                23 March 2016
                31 March 2016
                23 September 2016
                : 531
                : 7596
                : 656-660
                Affiliations
                [1 ]Department of Cardiology, Howard Hughes Medical Institute, Boston Children’s Hospital, Boston, MA, USA
                [2 ]Department of Neurobiology, Howard Hughes Medical Institute, Harvard Medical School, Boston, MA, USA
                [3 ]Image and Data Analysis Core (IDAC), Harvard Medical School, Boston, MA, USA
                Author notes
                [# ]Corresponding authors: David E. Clapham: dclapham@ 123456enders.tch.harvard.edu , David P. Corey: dcorey@ 123456hms.harvard.edu
                [*]

                Co-first authors

                Article
                NIHMS761034
                10.1038/nature17426
                4851444
                27007841

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