Optogenetics
Encyclopedia
Optogenetics is the combination of genetic and optical methods to control specific events in targeted cells of living tissue, even within freely moving mammal
s and other animals, with the temporal precision (millisecond
-timescale) needed to keep pace with functioning intact biological systems.
In 2010, optogenetics was chosen as the Method of the Year (MOTY) across all fields of science and engineering by the interdisciplinary research journal Nature Methods (MOTY primer, MOTY editorial, MOTY commentary). At the same time, optogenetics was highlighted in the article on “Breakthroughs of the Decade” in the scientific research journal Science BotD; these journals also referenced recent public-access general-interest video MOTY video and textual SciAm summaries of optogenetics.
, who employed Drosophila multiple-protein cascades initiated by G protein-coupled rhodopsin photoreceptors for controlling neural activity in cultured neurons. Miesenbock later employed a combination chemical-and-G-protein coupled receptor method to modulate the behavior of fruit flies with light, and the Kramer and Isacoff groups likewise employed synthesized organic photoswitches or “caged” compounds that could interact with genetically-introduced ion channels (Zemelman 2002, Zemelman 2003, Banghart 2004, Lima 2005).
In 2005, the first of many studies using mammals, instead of invertebrates, was initiated by Karl Deisseroth's group at Stanford University
(2005)). They brought the first single-component optogenetic system to neurobiology (beginning with channelrhodopsin, a single-component light-activated cation channel from unicellular algae
), which allowed millisecond-scale temporal control in mammals, required only one gene to be expressed in order to work, and responded to visible-spectrum light with a chromophore
retinal
that was already present and supplied to the channelrhodopsin (ChR) by the vertebrate tissues. The surprising experimental utility of this single-component “microbial opsin” approach was quickly proven with many additional microbial opsin classes and in a variety of animal models ranging from behaving mammals to classical model organisms such as flies, worms, and zebrafish. The “optogenetic” terminology was coined in 2006 (Deisseroth 2006), and since 2005 hundreds of laboratories around the world have employed microbial opsins to study complex biological systems (references below).
patterns in defined neurons). Indeed, to probe the neural code, optogenetics by definition must operate on the millisecond timescale to allow addition or deletion of precise activity patterns within specific cells in the brains of intact animals, including mammals. By comparison, the temporal precision of traditional genetic manipulations (employed to probe the causal role of specific genes within cells, via “loss-of-function” or “gain of function” changes in these genes) is rather slow, from hours or days to months. It is important to also have fast readouts in optogenetics that can keep pace with the optical control. This can be done with electrical recordings ("optrodes") or with reporter proteins that are biosensor
s, where scientists have fused fluorescent proteins to detector proteins. An example of this is voltage-sensitive fluorescent protein (VSFP2).
The hallmark of optogenetics therefore is introduction of fast light-activated channels and enzymes that allow temporally precise manipulation of electrical and biochemical events while maintaining cell-type resolution through the use of specific targeting mechanisms. Among the microbial opsins which can be used to investigate the function of neural systems are the channelrhodopsin
s (ChR2, ChR1, VChR1, and SFOs) to excite neurons (gain of function). For loss of function, halorhodopsin
(NpHR), enhanced halorhodopsins (eNpHR2.0 and eNpHR3.0), archaerhodopsin (Arch), Leptosphaeria maculans
fungal opsins (Mac), and enhanced bacteriorhodopsin (eBR) have been employed to inhibit neurons, including in freely-moving mammals (Witten 2010).
Moreover, optogenetic control of well-defined biochemical events within behaving mammals is now also possible. Building on prior work fusing vertebrate opsins to specific G-protein coupled receptors (Kim 2005), a family of chimeric
single-component optogenetic tools was created that allowed researchers to manipulate within behaving mammals the concentration of defined intracellular messengers such as cAMP and IP3 in targeted cells (Airan 2009). Other biochemical approaches to optogenetics (crucially, with tools that displayed low activity in the dark) followed soon thereafter, when optical control over small GTPases and adenylyl cyclases was achieved in cultured cells using novel strategies from several different laboratories (Levskaya 2009, Wu 2009, Yazawa 2009, Stierl 2011, Ryu 2010). This emerging repertoire of optogenetic probes now allows cell-type-specific and temporally precise control of multiple axes of cellular function within intact animals.
Optogenetics also necessarily includes 1) the development of genetic targeting strategies such as cell-specific promoters or other customized conditionally-active viruses, to deliver the light-sensitive probes to specific populations of neurons in the brain of living animals (e.g. worms, fruit flies, mice, rats, and monkeys), and 2) hardware (e.g. integrated fiberoptic and solid-state light sources) to allow specific cell types, even deep within the brain, to be controlled in freely behaving animals. Most commonly, the latter is now achieved using the fiberoptic-coupled diode technology introduced in 2007 (Aravanis et al., 2007, Adamantidis et al., 2007, Gradinaru et al., 2007). To stimulate superficial brain areas such as the cerebral cortex, optical fibers or LED
s can be directly mounted to the skull of the animal. More deeply implanted optical fibers have been used to deliver light to deeper brain areas. In invertebrates such as worms and fruit flies some amount of Retinal isomerase
all-trans-retinal (ATR) is supplemented with food. A key advantage of microbial opsins as noted above is that they are fully functional without the addition of exogenous co-factors in vertebrates.
The field of optogenetics has furthered the fundamental scientific understanding of how specific cell types contribute to the function of biological tissues such as neural circuits in vivo (see references from the scientific literature below). Moreover, on the clinical side, optogenetics-driven research has led to insights into Parkinson's disease
and other neurological and psychiatric disorders. Indeed, optogenetics papers in 2009 have also provided insight into neural codes relevant to autism
, Schizophrenia
, drug abuse
, anxiety, and depression (Cardin 2009, Gradinaru 2009, Sohal 2009, Tsai 2009, Witten 2010).
It has been pointed out that beyond its scientific impact, optogenetics also represents an important case study in the value of both ecological conservation (as many of the key tools of optogenetics arise from microbial organisms occupying specialized environmental niches), and in the importance of pure basic science (as these opsins were studied over decades for their own sake by biophysicists and microbiologists, without involving consideration of their potential value in delivering insights into neuroscience and neuropsychiatric disease).
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s and other animals, with the temporal precision (millisecond
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
-timescale) needed to keep pace with functioning intact biological systems.
In 2010, optogenetics was chosen as the Method of the Year (MOTY) across all fields of science and engineering by the interdisciplinary research journal Nature Methods (MOTY primer, MOTY editorial, MOTY commentary). At the same time, optogenetics was highlighted in the article on “Breakthroughs of the Decade” in the scientific research journal Science BotD; these journals also referenced recent public-access general-interest video MOTY video and textual SciAm summaries of optogenetics.
History
The theoretical utility of selectively controlling precise neural activity (action potential) patterns within subtypes of cells in the brain (for example, using light to control optically-sensitized neurons) had been articulated by Francis Crick in his Kuffler Lectures at the University of California in San Diego (Crick 1999). An early use of light to activate neurons was carried out by Richard Fork and later Rafael Yuste, who demonstrated laser activation of neurons within intact tissue, although not in a genetically-targeted manner. The earliest genetically targeted photostimulation method was demonstrated by Gero MiesenbockGero Miesenböck
Gero Miesenböck is Waynflete Professor of Physiology at the University of Oxford and a fellow of Magdalen College. A native of Austria, he received his M.D. from the University of Innsbruck and undertook postdoctoral training with James Rothman...
, who employed Drosophila multiple-protein cascades initiated by G protein-coupled rhodopsin photoreceptors for controlling neural activity in cultured neurons. Miesenbock later employed a combination chemical-and-G-protein coupled receptor method to modulate the behavior of fruit flies with light, and the Kramer and Isacoff groups likewise employed synthesized organic photoswitches or “caged” compounds that could interact with genetically-introduced ion channels (Zemelman 2002, Zemelman 2003, Banghart 2004, Lima 2005).
In 2005, the first of many studies using mammals, instead of invertebrates, was initiated by Karl Deisseroth's group at Stanford University
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
(2005)). They brought the first single-component optogenetic system to neurobiology (beginning with channelrhodopsin, a single-component light-activated cation channel from unicellular algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
), which allowed millisecond-scale temporal control in mammals, required only one gene to be expressed in order to work, and responded to visible-spectrum light with a chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...
retinal
Retinal
Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision...
that was already present and supplied to the channelrhodopsin (ChR) by the vertebrate tissues. The surprising experimental utility of this single-component “microbial opsin” approach was quickly proven with many additional microbial opsin classes and in a variety of animal models ranging from behaving mammals to classical model organisms such as flies, worms, and zebrafish. The “optogenetic” terminology was coined in 2006 (Deisseroth 2006), and since 2005 hundreds of laboratories around the world have employed microbial opsins to study complex biological systems (references below).
Description
Millisecond-scale temporal precision is central to optogenetics, which allows the experimenter to keep pace with fast biological information processing (for example, in probing the causal role of specific action potentialAction potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
patterns in defined neurons). Indeed, to probe the neural code, optogenetics by definition must operate on the millisecond timescale to allow addition or deletion of precise activity patterns within specific cells in the brains of intact animals, including mammals. By comparison, the temporal precision of traditional genetic manipulations (employed to probe the causal role of specific genes within cells, via “loss-of-function” or “gain of function” changes in these genes) is rather slow, from hours or days to months. It is important to also have fast readouts in optogenetics that can keep pace with the optical control. This can be done with electrical recordings ("optrodes") or with reporter proteins that are biosensor
Biosensor
A biosensor is an analytical device for the detection of an analyte that combines a biological component with a physicochemical detector component.It consists of 3 parts:* the sensitive biological element A biosensor is an analytical device for the detection of an analyte that combines a biological...
s, where scientists have fused fluorescent proteins to detector proteins. An example of this is voltage-sensitive fluorescent protein (VSFP2).
The hallmark of optogenetics therefore is introduction of fast light-activated channels and enzymes that allow temporally precise manipulation of electrical and biochemical events while maintaining cell-type resolution through the use of specific targeting mechanisms. Among the microbial opsins which can be used to investigate the function of neural systems are the channelrhodopsin
Channelrhodopsin
Channelrhodopsins are a subfamily of opsin proteins that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis, i.e. movement in response to light. Expressed in cells of other organisms, they enable the use of light to control...
s (ChR2, ChR1, VChR1, and SFOs) to excite neurons (gain of function). For loss of function, halorhodopsin
Halorhodopsin
Halorhodopsin is a light-driven ion pump, specific for chloride ions, and found in phylogenetically ancient archaea, known as halobacteria...
(NpHR), enhanced halorhodopsins (eNpHR2.0 and eNpHR3.0), archaerhodopsin (Arch), Leptosphaeria maculans
Leptosphaeria maculans
Leptosphaeria maculans is a fungal pathogen that is the causal agent of blackleg disease on Brassica crops. The major yield loss is due to stem canker....
fungal opsins (Mac), and enhanced bacteriorhodopsin (eBR) have been employed to inhibit neurons, including in freely-moving mammals (Witten 2010).
Moreover, optogenetic control of well-defined biochemical events within behaving mammals is now also possible. Building on prior work fusing vertebrate opsins to specific G-protein coupled receptors (Kim 2005), a family of chimeric
Chimera (genetics)
A chimera or chimaera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic...
single-component optogenetic tools was created that allowed researchers to manipulate within behaving mammals the concentration of defined intracellular messengers such as cAMP and IP3 in targeted cells (Airan 2009). Other biochemical approaches to optogenetics (crucially, with tools that displayed low activity in the dark) followed soon thereafter, when optical control over small GTPases and adenylyl cyclases was achieved in cultured cells using novel strategies from several different laboratories (Levskaya 2009, Wu 2009, Yazawa 2009, Stierl 2011, Ryu 2010). This emerging repertoire of optogenetic probes now allows cell-type-specific and temporally precise control of multiple axes of cellular function within intact animals.
Optogenetics also necessarily includes 1) the development of genetic targeting strategies such as cell-specific promoters or other customized conditionally-active viruses, to deliver the light-sensitive probes to specific populations of neurons in the brain of living animals (e.g. worms, fruit flies, mice, rats, and monkeys), and 2) hardware (e.g. integrated fiberoptic and solid-state light sources) to allow specific cell types, even deep within the brain, to be controlled in freely behaving animals. Most commonly, the latter is now achieved using the fiberoptic-coupled diode technology introduced in 2007 (Aravanis et al., 2007, Adamantidis et al., 2007, Gradinaru et al., 2007). To stimulate superficial brain areas such as the cerebral cortex, optical fibers or LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
s can be directly mounted to the skull of the animal. More deeply implanted optical fibers have been used to deliver light to deeper brain areas. In invertebrates such as worms and fruit flies some amount of Retinal isomerase
Retinal isomerase
In enzymology, a retinal isomerase is an enzyme that catalyzes the chemical reactionHence, this enzyme has one substrate, all-trans-retinal, and one product, 11-cis-retinal....
all-trans-retinal (ATR) is supplemented with food. A key advantage of microbial opsins as noted above is that they are fully functional without the addition of exogenous co-factors in vertebrates.
The field of optogenetics has furthered the fundamental scientific understanding of how specific cell types contribute to the function of biological tissues such as neural circuits in vivo (see references from the scientific literature below). Moreover, on the clinical side, optogenetics-driven research has led to insights into Parkinson's disease
Parkinson's disease
Parkinson's disease is a degenerative disorder of the central nervous system...
and other neurological and psychiatric disorders. Indeed, optogenetics papers in 2009 have also provided insight into neural codes relevant to autism
Autism
Autism is a disorder of neural development characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their...
, Schizophrenia
Schizophrenia
Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...
, drug abuse
Drug abuse
Substance abuse, also known as drug abuse, refers to a maladaptive pattern of use of a substance that is not considered dependent. The term "drug abuse" does not exclude dependency, but is otherwise used in a similar manner in nonmedical contexts...
, anxiety, and depression (Cardin 2009, Gradinaru 2009, Sohal 2009, Tsai 2009, Witten 2010).
It has been pointed out that beyond its scientific impact, optogenetics also represents an important case study in the value of both ecological conservation (as many of the key tools of optogenetics arise from microbial organisms occupying specialized environmental niches), and in the importance of pure basic science (as these opsins were studied over decades for their own sake by biophysicists and microbiologists, without involving consideration of their potential value in delivering insights into neuroscience and neuropsychiatric disease).
External links
- Optogenetics Resource Center, maintained by the Deisseroth lab.
- Synthetic Neurobiology Group, MIT, the portal of the Boyden lab.
- Sohal lab portal
- Nurmikko lab portal
- Lee lab portal
- http://www.med.wayne.edu/anatomy/department/pan.htm, Lab of Dr. Zhuo-Hua Pan
- OpenOptogenetics.org, optogenetics wiki, maintained from the Champalimaud Institute.
- Optophysiology at the Tyler lab
- Human brain is photosensitive