Light-Activated Ion Pumps and Channels for Temporally Precise Optical Control of Activity in Genetically Targeted Neurons

Abstract

The ability to turn on and off specific cell types and neural pathways in the brain, in a temporally precise fashion, has begun to enable the ability to test the sufficiency and necessity of particular neural activity patterns, and particular neural circuits, in the generation of normal and abnormal neural computations and behaviors by the brain. Over the last 5 years, a number of naturally occurring light-activated ion pumps and light-activated ion channels have been shown, upon genetic expression in specific neuron classes, to enable the voltage (and internal ionic composition) of those neurons to be controlled by light in a temporally precise fashion, without the need for chemical cofactors. In this chapter, we review three major classes of such genetically encoded "optogenetic" microbial opsins-light-gated ion channels such as channelrhodopsins, light-driven chloride pumps such as halorhodopsins, and light-driven proton pumps such as archaerhodopsins-that are in widespread use for mediating optical activation and silencing of neurons in species from Caenorhabditis elegans to nonhuman primates. We discuss the properties of these moleculesincluding their membrane expression, conductances, photocycle properties, ion selectivity, and action spectra-as well as genetic strategies for delivering these genes to neurons in different species, and hardware for performing light delivery in a diversity of settings. In the future, these molecules not only will continue to enable cutting-edge science but may also support a new generation of optical prosthetics for treating brain disorders.

Genetically encoded molecular tools for light-driven silencing of targeted neurons

“…This light-gated nonspecific cation channel, when expressed in neurons, enable the neurons to be depolarized with blue light (31, 32). The channelrhodopsins, and the origins of optogenetic tools, are reviewed elsewhere in this volume (CITE LIN CHAPTER HERE), as well as in other earlier references (33, 34). Like halorhodopsins and light-driven proton pumps, channelrhodopsins are all-trans-retinal binding seven-transmembrane proteins, and so there are some similarities in structure and function across these different opsin classes.…”

“…However, at high expression levels, Halo forms intracellular aggregates (39, 40), an issue that can be ameliorated by the appending of trafficking sequences from the Kir2.1 channel (39, 40, 87), which reduces aggregation and increases photocurrent manyfold over that of the baseline molecule. Other signal sequences, such as a prolactin localization sequence, also increase halorhodopsin photocurrent (33); it is important to note, however, that different sequences may do different things in different cell types in different species.…”