Sodium-Selective Channelrhodopsins

Abstract

Channelrhodopsins (ChRs) are light-gated ion channels originally discovered in algae and are commonly used in neuroscience for controlling the electrical activity of neurons with high precision. Initially-discovered ChRs were non-selective cation channels, allowing the flow of multiple ions, such as Na⁺, K⁺, H⁺, and Ca²⁺, leading to membrane depolarization and triggering action potentials in neurons. As the field of optogenetics has evolved, ChRs with more specific ion selectivity were discovered or engineered, offering more precise optogenetic manipulation. This review highlights the natural occurrence and engineered variants of sodium-selective channelrhodopsins (NaChRs), emphasizing their importance in optogenetic applications. These tools offer enhanced specificity in Na⁺ ion conduction, reducing unwanted effects from other ions, and generating strong depolarizing currents. Some of the NaChRs showed nearly no desensitization upon light illumination. These characteristics make them particularly useful for experiments requiring robust depolarization or direct Na⁺ ion manipulation. The review further discusses the molecular structure of these channels, recent advances in their development, and potential applications, including a proposed drug delivery system using NaChR-expressing red blood cells that could be triggered to release therapeutic agents upon light activation. This review concludes with a forward-looking perspective on expanding the use of NaChRs in both basic research and clinical settings.

Keywords: ChR2; channelrhodopsin variants; channelrhodopsins; optogenetics; red blood cells; sodium-selectivity.

Figure 1

(A) Volvox colony with a few thousand small Chlamydomonas-like somatic cells and 16 reproductive gonidia. The inset shows a few enlarged somatic cells with their red eyes. The cells at the surface of the colony with their flagella are shown schematically. (B) Volvox ChR in the eye spot is a light-gated cation channel, which is composed of a seven-transmembrane helix domain with the all-trans-retinal chromophore and a C-terminal tail of unknown function. Carotenoid-rich vesicles reflect the light and create a front-to-back contrast at the location of the ChR-photoreceptor. Absorption of light by Volvox ChR induces the opening of the channel and, thus, changes the membrane potential. This figure is reproduced with permission from Ernst et al. 2008 [13].

Figure 2

Application example of Channelrhodopsin 2 (ChR2). Isolated cardiac myocytes from a mouse with cardiac expression of ChR2 show contraction after being electrically paced (left) and light stimulated (right). The colored graphs present the cell length of two individual cells as indicated in the leftmost lower image. As seen in the extent of the induced cell shortening, optical stimulation was more effective compared to electrical field stimulation. The figure has been adapted with permission from Kaestner et al. 2018 [15].

Figure 3

(A) Overall structure presentation of the Channelrhodopsin 2 (ChR2) dimer. Cysteine bridges are shown in purple. The retinal is shown in light blue. (B,C) General structure presentation of the ChR2 protomer. (B) Four cavities and three gates form the channel pore. (C) Extended hydrogen-bond network. The “DC gate” is shown in the red dashed ellipse. The black arrows and gray horizontal lines show the putative ion pathway and position of hydrophobic/hydrophilic boundaries, respectively. The figure has been adapted with permission from Volkov et al. 2017 [19].