Optogenetic control of neural activity: The biophysics of microbial rhodopsins in neuroscience

Abstract

Optogenetics, the use of microbial rhodopsins to make the electrical activity of targeted neurons controllable by light, has swept through neuroscience, enabling thousands of scientists to study how specific neuron types contribute to behaviors and pathologies, and how they might serve as novel therapeutic targets. By activating a set of neurons, one can probe what functions they can initiate or sustain, and by silencing a set of neurons, one can probe the functions they are necessary for. We here review the biophysics of these molecules, asking why they became so useful in neuroscience for the study of brain circuitry. We review the history of the field, including early thinking, early experiments, applications of optogenetics, pre-optogenetics targeted neural control tools, and the history of discovering and characterizing microbial rhodopsins. We then review the biophysical attributes of rhodopsins that make them so useful to neuroscience – their classes and structure, their photocycles, their photocurrent magnitudes and kinetics, their action spectra, and their ion selectivity. Our hope is to convey to the reader how specific biophysical properties of these molecules made them especially useful to neuroscientists for a difficult problem – the control of high-speed electrical activity, with great precision and ease, in the brain.

Fluorescence of the Retinal Chromophore in Microbial and Animal Rhodopsins

“…Recent studies on the application of microbial rhodopsins as fluorescent genetically encoded indicators of cell membrane potential facilitated the development of bright rhodopsin variants. A directed evolution approach has allowed researchers to obtain rhodopsin mutants with significantly more intense fluorescence than observed in the wild-type rhodopsins at physiological pH, and engineered proteins have been successfully applied in a large number of biological and medical studies [14][15][16][17][18][19][20][21]. An increasing number of applications of bright rhodopsin variants have raised the interest in rhodopsin fluorescence properties [22][23][24].…”

Fluorescence of the retinal chromophore in microbial and animal rhodopsins

“…Recent studies on the application of microbial rhodopsins as fluorescence voltage indicators in optogenetics facilitated the development of bright rhodopsin variants. Directed evolution approach has allowed the researchers to obtain rhodopsin mutants with significantly more intense fluorescence than observed in the wild-type rhodopsins at physiological pH, and engineered proteins have been successfully applied in a large number of biological and medical studies [13][14][15][16][17][18][19][20]. Increasing number of applications of bright rhodopsin variants have raised the interest in rhodopsins fluorescence properties [21][22][23].…”