Light-gated ion channels are a family ofion channels regulated byelectromagnetic radiation. Other gating mechanisms for ion channels includevoltage-gated ion channels,ligand-gated ion channels,mechanosensitive ion channels, andtemperature-gated ion channels. Most light-gated ion channels have been synthesized in the laboratory for study, although two naturally occurring examples,channelrhodopsin andanion-conducting channelrhodopsin, are currently known.[1][2]Photoreceptor proteins, which act in a similar manner to light-gated ion channels, are generally classified instead asG protein-coupled receptors.
Light-gated ion channels function in a similar manner to other gated ion channels. Suchtransmembrane proteins form pores throughlipid bilayers to facilitate the passage ofions. These ions move from one side of the membrane to another under the influence of anelectrochemical gradient. When exposed to a stimulus, aconformational change occurs in the transmembrane region of the protein to open or close the ion channel. In the specific case of light-gated ion channels, the transmembrane proteins are usually coupled with a smaller molecule that acts as aphotoswitch, wherebyphotons bind to the switching molecule, to then alter the conformation of the proteins, so that the pore changes from a closed state to an open state, or vice versa, thereby increasing or decreasing ion conductance.Retinal is a good example of a molecular photoswitch and is found in the naturally occurring channelrhodopsins.[3][4]
Once channelrhosopsin had been identified and characterized, the channel's ion selectivity was modified in order to controlmembrane potential throughoptogenetic control. Directed mutations of the channel changed the charges lining the pore, resulting in a pore which instead excludedcations in favor ofanions.[5]
Other types of gated ion channels,ligand-gated andvoltage-gated, have been synthesized with a light-gated component in an attempt to better understand their nature and properties. By the addition of a light-gated section, the kinetics and mechanisms of operation can be studied in depth. For example, the addition of a light-gated component allows for the introduction of many highly similar ligands to be introduced to the binding site of a ligand-gated ion channel to assist in the determination of the mechanism.
Such ion channels have been modified by binding a photoswitch to confer photosensitivity on the ion channel. This is done through careful selection of a tether which can lengthen or shorten throughphotoisomerization. One side of the tether is bound to the ion channel protein and the other end of the tether is bound to a blocking group, which has a high binding affinity for an exposed portion of the pore. When the tether is lengthened, it allows the blocking section to bind to the pore and prevent ionic current. When the tether is shortened, it disrupts this obstruction and opens the pore. Kinetic studies have demonstrated that fine temporal and spatial control can be achieved in this manner.[6][7]
Azobenzene is a common choice for the functional portion of a tether for synthetically-developed light-gated ion channels because of its well documented length change as eithercis ortrans isomers, as well as the excitationwavelength needed to induce photoisomerization. Azobenzene converts to its longertrans-isomer at a wavelength of λ=500 nm and to itscis-isomer at λ=380 nm.[6]
In 1980, the first ion channel to be adapted for study with a light-gated mechanism was thenicotinic acetylcholine receptor.[8] This receptor was well-known at the time, and so was aptly suited to adaptation, and allowed for a study of the kinetics as not allowed before.
The expression of light-gated ion channels in a specific cell type throughpromoter control allows for the regulation of cell potential by eitherdepolarizing the membrane to 0 mV for cation-permeant channelrhodopsin or by holding the voltage at –67 mV for anion-conducting channelrhodopsin.[9] Depolarization can conduct acurrent in the range of 5 fA per channel and occurs on the timescale ofaction potentials and neurotransmitterexocytosis.[10][4] They have an advantage over other types of ion channel regulation in that they provide non-invasive, reversible membrane potential changes with fine temporal and spatial control granted by induction throughlaser stimuli.[3][6] They reliably stimulate single action potentials with rapid depolarization and can be utilizedin vivo because they do not require high intensity illumination to maintain function, unlike other techniques like light-activatedproton pumps andphotoactivatable probes.[5][10]
Examples of light-gated ion channels occur in both natural and synthetic environments. These include: