Optogenetics

Optogenetics (from Greek optikós, meaning 'seen, visible') is a biological technique that involves the use of light to control cells in living tissue, typically neurons, that have been genetically modified to express light-sensitive ion channels. It is a neuromodulation method that uses a combination of techniques from optics and genetics to control and monitor the activities of individual neurons in living tissue—even within freely-moving animals—and to precisely measure these manipulation effects in real-time. The key reagents used in optogenetics are light-sensitive proteins. Neuronal control is achieved using optogenetic actuators like channelrhodopsin, halorhodopsin, and archaerhodopsin, while optical recording of neuronal activities can be made with the help of optogenetic sensors for calcium (GCaMP), vesicular release (synapto-pHluorin), neurotransmitter (GluSnFRs), or membrane voltage (arc lightning, ASAP1). Control (or recording) of activity is restricted to genetically defined neurons and performed in a spatiotemporal-specific manner by light. Optogenetics (from Greek optikós, meaning 'seen, visible') is a biological technique that involves the use of light to control cells in living tissue, typically neurons, that have been genetically modified to express light-sensitive ion channels. It is a neuromodulation method that uses a combination of techniques from optics and genetics to control and monitor the activities of individual neurons in living tissue—even within freely-moving animals—and to precisely measure these manipulation effects in real-time. The key reagents used in optogenetics are light-sensitive proteins. Neuronal control is achieved using optogenetic actuators like channelrhodopsin, halorhodopsin, and archaerhodopsin, while optical recording of neuronal activities can be made with the help of optogenetic sensors for calcium (GCaMP), vesicular release (synapto-pHluorin), neurotransmitter (GluSnFRs), or membrane voltage (arc lightning, ASAP1). Control (or recording) of activity is restricted to genetically defined neurons and performed in a spatiotemporal-specific manner by light. In 2010, optogenetics was chosen as the 'Method of the Year' across all fields of science and engineering by the interdisciplinary research journal Nature Methods. At the same time, optogenetics was highlighted in the article on 'Breakthroughs of the Decade' in the academic research journal Science. These journals also referenced recent public-access general-interest video Method of the year video and textual SciAm summaries of optogenetics. The 'far-fetched' possibility of using light for selectively controlling precise neural activity (action potential) patterns within subtypes of cells in the brain was thought of by Francis Crick in his Kuffler Lectures at the University of California in San Diego in 1999. An earlier use of light to activate neurons was carried out by Richard Fork, who demonstrated laser activation of neurons within intact tissue, although not in a genetically-targeted manner. The earliest genetically targeted method that used light to control rhodopsin-sensitized neurons was reported in January 2002, by Boris Zemelman and Gero Miesenböck, who employed Drosophila rhodopsin cultured mammalian neurons. In 2003, Zemelman and Miesenböck developed a second method for light-dependent activation of neurons in which single inotropic channels TRPV1, TRPM8 and P2X2 were gated by photocaged ligands in response to light. Beginning in 2004, the Kramer and Isacoff groups developed organic photoswitches or 'reversibly caged' compounds in collaboration with the Trauner group that could interact with genetically introduced ion channels. TRPV1 methodology, albeit without the illumination trigger, was subsequently used by several laboratories to alter feeding, locomotion and behavioral resilience in laboratory animals. However, light-based approaches for altering neuronal activity were not applied outside the original laboratories, likely because the easier to employ channelrhodopsin was cloned soon thereafter. Peter Hegemann, studying the light response of green algae at the University of Regensburg, had discovered photocurrents that were too fast to be explained by the classic g-protein-coupled animal rhodopsins. Teaming up with the electrophysiologist Georg Nagel at the Max Planck Institute in Frankfurt, they could demonstrate that a single gene from the alga Chlamydomonas produced large photocurents when expressed in the oocyte of a frog. To identify expressing cells, they replaced the cytoplasmic tail of the algal protein with the fluorescent protein YFP, generating the first generally applicable optogenetic tool. Zhuo-Hua Pan of Wayne State University, researching on restore sight to blindness, thought about using channelrhodopsin when it came out in late 2003. By February 2004, he was trying channelrhodopsin out in ganglion cells—the neurons in our eyes that connect directly to the brain—that he had cultured in a dish. Indeed, the transfected neurons became electrically active in response to light. In 2005, Zhuo-Hua Pan reported successful in-vivo transfection of channelrhodopsin in retinal ganglion cells of mice, and electrical responses to photostimulation in retinal slice culture In April 2005, Susana Lima and Miesenböck reported the first use of genetically-targeted P2X2 photostimulation to control the behaviour of an animal. They showed that photostimulation of genetically circumscribed groups of neurons, such as those of the dopaminergic system, elicited characteristic behavioural changes in fruit flies. In August 2005, Karl Deisseroth's laboratory in the Bioengineering Department at Stanford including graduate students Ed Boyden and Feng Zhang published the first demonstration of a single-component optogenetic system in cultured mammalian neurons, using the channelrhodopsin-2(H134R)-eYFP construct from Nagel and Hegemann. In October 2005, Lynn Landmesser and Stefan Herlitze also published the use of channelrohodpsin-2 to control neuronal activity in cultured hippocampal neurons and chicken spinal cord circuits in intact developing embryos. In addition, they introduced for the first time vertebrate rhodopsin, a light-activated G protein coupled receptor, as a tool to inhibit neuronal activity via the recruitment of intracellular signaling pathways also in hippocampal neurons and the intact developing chicken embryo. The groups of Gottschalk and Nagel were first to use channelrhodopsin-2 for controlling neuronal activity in an intact animal, showing that motor patterns in the roundworm Caenorhabditis elegans could be evoked by light stimulation of genetically selected neural circuits (published in December 2005). In mice, controlled expression of optogenetic tools is often achieved with cell-type-specific Cre/loxP methods developed for neuroscience by Joe Z. Tsien back in the 1990s to activate or inhibit specific brain regions and cell-types in vivo.