The development and application of optogenetics

Abstract

Genetically encoded, single-component optogenetic tools have made a significant impact on neuroscience, enabling specific modulation of selected cells within complex neural tissues. As the optogenetic toolbox contents grow and diversify, the opportunities for neuroscience continue to grow. In this review, we outline the development of currently available single-component optogenetic tools and summarize the application of various optogenetic tools in diverse model organisms.

Figure 1

Optogenetic tool families. Channelrhodopsins conduct cations and depolarize neurons upon illumination (left). Halorhodopsins conduct chloride ions into the cytoplasm upon yellow light illumination (center). OptoXRs are rhodopsin-GPCR (G protein–coupled receptor) chimeras that respond to green (500 nm) light with activation of the biological functions dictated by the intracellular loops used in the hybrid (right).

Figure 2

Spectral and kinetic diversification of optogenetic tools. Deactivation time constants (τ_off) and approximate peak activation/inactivation wavelengths are shown for blue light–activated opsins, blue ChETAs (including E123T/T159C) (Berndt et al. 2011, Mattis et al. 2011), red-shifted opsins, red-shifted ChETAs, bistable (SFO) opsins, and modulatory/inhibitory tools in a compact look-up table. ChR2 is listed among the bistable group for scale purposes only. Where precise values are not available, decay kinetics were measured (courtesy of J. Mattis, personal communication) or estimated from published traces; all values were recorded at room temperature (except for optoXRs measured at 37°C), with substantial acceleration in kinetics (∼50%) expected for all at 37°C.

Figure 3

Low-leak Cre-dependent expression using the doublefloxed inverted open-reading-frame (DIO) strategy. The combination of a transgenic mouse expressing Cre recombinase in specific neuronal subtypes and the injection of a virally encoded DIO opsin (a) results in the physical inversion of the open reading frame (ORF) in only that population (b,c), which may be transient and revert back to the original state or undergo further recombination to be permanently anchored in the sense direction, resulting in functional expression of the opsin (c). The DIO strategy may be contrasted with the lox-stop-lox (“floxed STOP”) strategy (d,e). In the absence of Cre recombinase, lox-stop-lox (d) allows for some level of expression leak as assayed by both enhanced yellow fluorescent protein (eYFP) expression and fluorescence-activated cell sorting (FACS) analysis. Because the ORF of an opsin in DIO configuration encodes nonsense (e), there is no functional expression in the absence of Cre recombinase. Adapted with permission from Sohal et al. (2009) and F. Zhang and K. Deisseroth.