Anion-conducting channelrhodopsins are light-gated ion channels that open in response to light and let negatively charged ions (such as chloride) enter a cell. All channelrhodopsins use retinal as light-sensitive pigment, but they differ in their ion selectivity. Anion-conducting channelrhodopsins are used as tools to manipulate brain activity in mice, fruit flies and other model organisms (Optogenetics). Neurons expressing anion-conducting channelrhodopsins are silenced when illuminated with light, an effect that has been used to investigate information processing in the brain. For example, suppressing dendritic calcium spikes in specific neurons with light reduced the ability of mice to perceive a light touch to a whisker. [2] Studying how the behavior of an animal changes when specific neurons are silenced allows scientists to determine the role of these neurons in the complex circuits controlling behavior.
The first anion-conducting channelrhodopsins were engineered from the cation-conducting light-gated channel Channelrhodopsin-2 by removing negatively charged amino acids from the channel pore (Fig. 1). [3] As the main anion of extracellular fluid is chloride (Cl−), anion-conducting channelrhodopsins are also known as “chloride-conducting channelrhodopsins” (ChloCs). Naturally occurring anion-conducting channelrhodopsins (ACRs) were subsequently identified in cryptophyte algae. [4] [5] [6] The crystal structure of the natural GtACR1 has recently been solved, paving the way for further protein engineering. [7] [8]
name | species of origin | absorption | reference | properties, applications |
---|---|---|---|---|
slowChloC | Chlamydomonas reinhardtii | blue | Wietek et al. 2014 [3] | first generation, mixed conductance |
iC1C2 | Chlamydomonas reinhardtii | blue | Berndt et al. 2014 [10] | first generation, mixed conductance |
iChloC | Chlamydomonas reinhardtii | blue | Wietek et al. 2015 [1] | inhibition of perception in mice [2] |
iC++ | Chlamydomonas reinhardtii | blue | Berndt et al. 2016 [11] | inhibition of sleep in mice [12] |
GtACR1 | Guillardia theta | green | Govorunova et al. 2015 [4] | inhibition of behavior in Drosophila [13] [14] inhibition of rat heart muscle cells [15] holographic spike suppression in mouse cortex [16] |
GtACR1(C102A) | Guillardia theta | green on red off | Govorunova et al. 2018 [6] | bistable |
GtACR1(R83Q/N239Q) FLASH | Guillardia theta | green on | Kato et al. 2018 [7] | very fast closing, large currents inhibition of swimming in C. elegans, inhibition of spiking in mouse [7] |
GtACR2 | Guillardia theta | blue | Govorunova et al. 2015 [4] | inhibition of behavior in Drosophila [13] inhibition of fear extinction in mice [17] |
PsACR1 | Proteomonas sulcata | green | Wietek et al. 2016, [18] Govorunova et al. 2016 [19] | large currents |
ZipACR | Proteomonas sulcata | green | Govorunova et al. 2017 [5] | very fast |
RapACR | Rhodomonas salina | green | Govorunova et al. 2018 [6] | very fast, large currents |
SwiChR++ | Chlamydomonas reinhardtii | blue on red off | Berndt et al. 2016 [11] | bistable |
Phobos CA | Chlamydomonas reinhardtii | blue on red off | Wietek et al. 2017 [20] | bistable |
Aurora | Chlamydomonas reinhardtii | orange-red | Wietek et al. 2017 [20] | stop locomotion of Drosophila larvae |
MerMAIDs | unknown | green | Oppermann et al. 2019 [21] | rapidly inactivating |
Anion-conducting channelrhodopsins (ACRs) have been used as optogenetic tools to inhibit neuronal activation. When expressed in nerve cells, ACRs act as light-gated chloride channels. Their effect on the activity of the neuron is comparable to GABAA receptors, ligand-gated chloride channels found in inhibitory synapses: As the chloride concentration in mature neurons is very low, illumination results in an inward flux of negatively charged ions, clamping the neuron at the chloride reversal potential (- 65 mV). Under these conditions, excitatory synaptic inputs are not able to efficiently depolarize the neuron. This effect is known as shunting inhibition (as opposed to inhibition by hyperpolarization). Illuminating the dendrite prevents the generation of dendritic calcium spikes while illumination of the entire neuron blocks action potential initiation in response to sensory stimulation. [2] [1] Axon terminals, however, have a higher chloride concentration and are therefore excited by ACRs. [22] To inhibit neurons with wide-field illumination, it has proven useful to restrict ACRs to the somatic compartment (ST variants). [17] [16]
Due to their high light sensitivity, ACRs can be activated with dim light which does not interfere with visual stimulation, even in very small animals like the fruit fly Drosophila . [14] When combined with a red-light sensitive cation-conducting channelrhodopsin, ACRs allow for bidirectional control of neurons: Silencing with blue light, activation with red light ('Bipoles'). [23]
Neuron Review (2017): Silencing neurons: Tools, Applications, and Experimental Constraints [24]
Research highlight: A better way to turn off neurons [25]
Perspective: Expanding the optogenetics toolkit [26]
Related: Halorhodopsin, a light-driven chloride pump
Channelrhodopsins are a subfamily of retinylidene proteins (rhodopsins) that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis: movement in response to light. Expressed in cells of other organisms, they enable light to control electrical excitability, intracellular acidity, calcium influx, and other cellular processes. Channelrhodopsin-1 (ChR1) and Channelrhodopsin-2 (ChR2) from the model organism Chlamydomonas reinhardtii are the first discovered channelrhodopsins. Variants that are sensitive to different colors of light or selective for specific ions have been cloned from other species of algae and protists.
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Retinylidene proteins, or rhodopsins in a broad sense, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include all forms of opsin and rhodopsin. While rhodopsin in the narrow sense refers to a dim-light visual pigment found in vertebrates, usually on rod cells, rhodopsin in the broad sense refers to any molecule consisting of an opsin and a retinal chromophore in the ground state. When activated by light, the chromophore is isomerized, at which point the molecule as a whole is no longer rhodopsin, but a related molecule such as metarhodopsin. However, it remains a retinylidene protein. The chromophore then separates from the opsin, at which point the bare opsin is a retinylidene protein. Thus, the molecule remains a retinylidene protein throughout the phototransduction cycle.
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Gero Andreas Miesenböck is an Austrian scientist. He is currently Waynflete Professor of Physiology and Director of the Centre for Neural Circuits and Behaviour (CNCB) at the University of Oxford and a fellow of Magdalen College, Oxford.
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Edward S. Boyden is an American neuroscientist and entrepreneur at MIT. He is the Y. Eva Tan Professor in Neurotechnology, and a full member of the McGovern Institute for Brain Research. He is recognized for his work on optogenetics and expansion microscopy. Boyden joined the MIT faculty in 2007, and continues to develop new optogenetic tools as well as other technologies for the manipulation and analysis of brain structure and activity. He received the 2015 Breakthrough Prize in Life Sciences.
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Peter Hegemann is a Hertie Senior Research Chair for Neurosciences and a professor of Experimental Biophysics at the Department of Biology, Faculty of Life Sciences, Humboldt University of Berlin, Germany. He is known for his discovery of channelrhodopsin, a type of ion channels regulated by light, thereby serving as a light sensor. This created the field of optogenetics, a technique that controls the activities of specific neurons by applying light. He has received numerous accolades, including the Rumford Prize, the Shaw Prize in Life Science and Medicine, and the Albert Lasker Award for Basic Medical Research.
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