Anion-conducting channelrhodopsins are light-gated

ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ...

s that open in response to light and let negatively charged ions (such as chloride) enter a cell. All

channelrhodopsins 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 ...

use

retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use re ...

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 A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...

(

Optogenetics Optogenetics is a biological technique to control the activity of neurons or other cell types with light. This is achieved by expression of light-sensitive ion channels, pumps or enzymes specifically in the target cells. On the level of indiv ...

). 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. 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 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 i ...

by removing negatively charged amino acids from the channel pore (Fig. 1). As the main anion of

extracellular fluid In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body weight; women and the obese typically have a lowe ...

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. The crystal structure of the natural GtACR1 has recently been solved, paving the way for further protein engineering.

Variants

Applications

Anion-conducting channelrhodopsins (ACRs) have been used as

optogenetic Optogenetics is a biological technique to control the activity of neurons or other cell types with light. This is achieved by expression of light-sensitive ion channels, pumps or enzymes specifically in the target cells. On the level of indivi ...

tools to inhibit neuronal activation. When expressed in nerve cells, ACRs act as light-gated

chloride channel Chloride channels are a superfamily of poorly understood ion channels specific for chloride. These channels may conduct many different ions, but are named for chloride because its concentration ''in vivo'' is much higher than other anions. Several ...

s. Their effect on the activity of the neuron is comparable to GABA_A receptors, ligand-gated chloride channels found in

inhibitory synapses An inhibitory postsynaptic potential (IPSP) is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential.Purves et al. Neuroscience. 4th ed. Sunderland (MA): Sinauer Associates, Incorporated; 2008. ...

: 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 In a biological membrane, the reversal potential is the membrane potential at which the direction of ionic current reverses. At the reversal potential, there is no net flow of ions from one side of the membrane to the other. For channels that are pe ...

(- 65 mV). Under these conditions, excitatory synaptic inputs are not able to efficiently depolarize the neuron. This effect is known as

shunting inhibition Shunting inhibition, also known as divisive inhibition, is a form of postsynaptic potential inhibition that can be represented mathematically as reducing the excitatory potential by division, rather than linear subtraction. The term "shunting" is ...

(as opposed to inhibition by hyperpolarization). Illuminating the

dendrite Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the ...

prevents the generation of dendritic calcium spikes while illumination of the entire neuron blocks

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

initiation in response to sensory stimulation.

Axon terminal Axon terminals (also called synaptic boutons, terminal boutons, or end-feet) are distal terminations of the telodendria (branches) of an axon. An axon, also called a nerve fiber, is a long, slender projection of a nerve cell, or neuron, that cond ...

s, however, have a higher chloride concentration and are therefore excited by ACRs. To inhibit neurons with wide-field illumination, it has proven useful to restrict ACRs to the somatic compartment (ST variants). 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 ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many s ...

''. 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').

References

{{Optogenetics Ion channels

Variants

Applications

Further reading

References