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ref: -0 tags: GEVI review voltage sensor date: 03-18-2020 17:43 gmt revision:22 [21] [20] [19] [18] [17] [16] [head]

Various GEVIs invented and evolved:

Ace-FRET sensors

  • PMID-26586188 Ace-mNeonGreen, an opsin-FRET sensor, might still be better in terms of SNR, but it's green.
    • Negative ΔF/F\Delta F / F with depolarization.
    • Fast enough to resolve spikes.
    • Rational design; little or no screening.
    • Ace is about six times as fast as Mac, and mNeonGreen has a ~50% higher extinction coefficient than mCitrine and nearly threefold better photostability (12)

  • PMID-31685893 A High-speed, red fluorescent voltage sensor to detect neural activity
    • Fusion of Ace2N + short linker + mScarlet, a bright (if not the brightest; highest QY) monomeric red fluorescent protein.
    • Almost as good SNR as Ace2N-mNeonGreen.
    • Also a FRET sensor; negative delta F with depolarization.
    • Ace2N-mNeon is not sensitive under two-photon illumination; presumably this is true of all eFRET sensors?
    • Ace2N drives almost no photocurrent.
    • Sought to maximize SNR: dF/F_0 X sqrt(F_0); screened 'only' 18 linkers to see what worked the best. Yet - it's better than VARNAM.
    • ~ 14% dF/F per 100mV depolarization.

Arch and Mac rhodopsin sensors

  • PMID-22120467 Optical recording of action potentials in mammalian neurons using a microbial rhodopsin Arch 2011
    • Endogenous fluorescence of the retinal (+ environment) of microbial rhodopsin protein Archaerhodopsin 3 (Arch) from Halorubrum sodomense.
    • Proton pump without proton pumping capabilities also showed voltage dependence, but slower kinetics.
      • This required one mutation, D95N.
    • Requires fairly intense illumination, as the QY of the fluorophore is low (9 x 10-4). Still, photobleaching rate was relatively low.
    • Arch is mainly used for neuronal inhibition.

  • PMID-25222271 Archaerhodopsin Variants with Enhanced Voltage Sensitive Fluorescence in Mammalian and Caenorhabditis elegans Neurons Archer1 2014
    • Capable of voltage sensing under red light, and inhibition (via proton pumping) under green light.
    • Note The high laser power used to excite Arch (above) fluorescence causes significant autofluorescence in intact tissue and limits its accessibility for widespread use.
    • Archers have 3-5x the fluorescence of WT Arch -- so, QY of ~3.6e-3. Still very dim.
    • Archer1 dF/F_0 85%; Archer2 dF/F_0 60% @ 100mV depolarization (positive sense).
    • Screened the proton pump of Gloeobacter violaceus rhodopsin; found mutations were then transferred to Arch.
      • Maybe they were planning on using the Geobacter rhodopsin, but it didn't work for some reason, so they transferred to Arch..
    • TS and ER export domains for localization.

  • PMID-24755708 Imaging neural spiking in brain tissue using FRET-opsin protein voltage sensors MacQ-mOrange and MacQ-mCitrine.
    • L. maculans (Mac) rhodopsin (faster than Arch) + FP mCitrine, FRET sensor + ER/TS.
    • Four-fold faster kinetics and 2-4x brighter than ArcLight.
      • No directed evolution to optimize sensitivity or brightness. Just kept the linker short & trimmed residues based on crystal structure.
    • ~5% delta F/F, can resolve spikes up to 10Hz.
    • Spectroscopic studies of the proton pumping photocycle in bacteriorhodopsin and Archaerhodopsin (Arch) have revealed that proton translocation through the retinal Schiff base changes chromophore absorption [24-26]
    • Used rational design to abolish the proton current (D139N and D139Q aka MacQ) ; screens to adjust the voltage sensing kinetics.
    • Still has photocurrents.
    • Seems that slice / in vivo is consistently worse than cultured neurons... in purkinje neurons, dF/F 1.2%, even though in vitro response was ~ 15% to a 100mV depolarization.
    • Imaging intensity 30mw/mm^2. (3W/cm^2)

  • PMID-24952910 All-optical electrophysiology in mammalian neurons using engineered microbial rhodopsins QuasAr1 and QuasAr1 2014
    • Directed evolution approach to improve the brightness and speed of Arch D95N.
      • Improved the fluorescence QY by 19 and 10x. (1 and 2, respectively -- Quasar2 has higher sensitivity).
    • also developed a low-intensity channelrhodopsin, Cheriff, which can be activated by blue light (lambda max = 460 nm)dim enough to not affect QuasAr.
    • They call the two of them 'Optopatch 2'.
    • Incident light intensity 1kW / cm^2 (!)

  • PMID-29483642 A robotic multidimensional directed evolution approach applied to fluorescent voltage reporters. Archon1 2018
    • Started with QuasAr2 (above), which was evolved from Arch. Intrinsic fluorescence of retinal in rhodopsin.
    • Expressed in HEK293T cells; then FACS, robotic cell picking, whole genome amplification, PCR, cloning.
    • Also evolved miRFP, deep red fluorescent protein based on bacteriophytochrome.
    • delta F/F of 80 and 20% with a 100mV depolarization.
    • We investigated the contribution of specific point mutations to changes in localization, brightness, voltage sensitivity and kinetics and found the patterns that emerged to be complex (Supplementary Table 6), with a given mutation often improving one parameter but worsening another.
    • If the original QY of Arch was 9e-4, and Quasar2 improved this by 10, and Archon1 improved this by 2.3x, then the QY of Archon1 is 0.02. Given the molar extinction coefficient is ~ 50000 for retinal, this means the brightness of the fluorescent probe is low, 1. (good fluorescent proteins and synthetic dyes have a brightness of ~90).
    • Big paper, moderate improvement.
    • SomArchon1 and SomCheriff serve as the basis of Optopatch4, e.g. All-optical electrophysiology reveals excitation, inhibition, and neuromodulation in cortical layer 1
    • Slow photobleaching, consistent with other Arch based GEVIs.

VSD - FP sensors

  • PMID-28811673 Improving a genetically encoded voltage indicator by modifying the cytoplasmic charge composition Bongwoori 2017
    • ArcLight derivative.
    • Arginine (positive charge) scanning mutagenesis of the linker region improved the signal size of the GEVI, Bongwoori, yielding fluorescent signals as high as 20% ΔF/F during the firing of action potentials.
    • Used the mutagenesis to shift the threshold for fluorescence change more negative, ~ -30mV.
    • Like ArcLight, it's slow.
    • Strong baseline shift due to the acidification of the neuron during AP firing (!)

  • Attenuation of synaptic potentials in dentritic spines
    • Found that SNR / dF / F_0 is limited by intracellular localization of the sensor.
      • This is true even though ArcLight is supposed to be in a dark state in the lower pH of intracellular organelles.. a problem worth considering.
      • Makes negative-going GEVI's more practical, as those not in the membrane are dark.

  • Fast two-photon volumetric imaging of an improved voltage indicator reveals electrical activity in deeply located neurons in the awake brain ASAP3 2018
    • Opsin-based GEVIs have been used in vivo with 1p excitation to report electrical activity of superficial neurons, but their responsivity is attenuated for 2p excitation. (!)
    • Site-directed evolution in HEK cells.
    • Expressed linear PCR products directly in the HEK cells, with no assembly / ligation required! (Saves lots of time: normally need to amplify, assemble into a plasmid, transfect, culture, measure, purify the plasimd, digest, EP PCR, etc).
    • Screened in a motorized 384-well conductive plate, electroporation electrode sequentially activates each on an upright microscope.
    • 46% improvement over ASAP2 R414Q
    • Ace2N-4aa-mNeon is not responsive under 2p illum; nor is Archon1 or Quasar2/3
    • ULOVE = AOD based fast local scanning 2-p random access scope.

  • Bright and tunable far-red chemigenetic indicators
    • GgVSD (same as ASAP above) + cp HaloTag + Si-Rhodamine JF635
    • ~ 4% dF/F_0 during APs.
    • Found one mutation, R476G in the linker between cp Halotag and S4 of the VSD, which doubled the sensitivity of HASAP.
    • Also tested a ArcLight type structure, CiVSD fused to Halotag.
      • HarcLght had negative dF/F_0 and ~ 3% change in response to APs.
    • No voltage sensitivity when the synthetic dye was largely in the zwitterionic form, eg. tetramethylrodamine.