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How does optogenetics work?

How does optogenetics work?



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I am aware of the post here 'Optogenetics - How do microbial opsins work?' however it is a bit too technical for me. I am struggling to understand how the neurons can be genetically engineered to produce these light-sensitive molecules in optogenetics, and how genetically engineering the neurones is even a usable method considering it seems you would have to genetically engineer the neurons one by one. So I suppose my question is: in the trials run of optogenetics so far (with mice), how where the neurons made to respond to the light stimulus?

Also, I was wondering whether the principle behind optogenetics- controlling calls with light- could be used for other treatments e.g. to kill cancer cells?

My biology knowledge is limited to A-Level (final year of high school), however I really want to understand this topic thoroughly, so detailed but not too technical answers would be much appreciated! Thank you!

EDIT: I just wanted to add two more quick questions…

  1. Are the light-sensitive molecules (which I now have read are channelrhodopsins) present all along the axolemma, or just the membrane of the soma of the neuron? (I hope I used those terms correctly!)

  2. Wikipedia says that these channelrhodopsins are non-specific (with some exceptions: The L132C mutation (CatCh) increases the permeability for calcium and generates very large currents.[17] Mutating E90 to the positively charged amino acid arginine turns channelrhodopsin from an unspecific cation channel into an anion channel. This chloride-conducting channelrhodopsin (ChloC) inhibits neuronal spiking when illuminated with blue light.[18]). How then can these channels work, if any cation can diffuse any way through the channel?


Short answer
Optogenetics can be used to make cells responsive to light. One striking use of this technology is to restore vision in the blind by making the surviving neurons in the retina light-sensitive. This can be done by transfecting the retina with genetically engineered viruses that induce the expression of bacterial channelrhodopsin proteins in retinal neurons.


Background

I will explain optogenetics by focusing on one of the most striking applications: to make the blind see. In humans light is sensed by photoreceptors in the retina (see further reading #2). Rods infer grey-scale vision (i.e., night vision), while cones provide color vision when there is enough light available. Rods and cones contain light-sensitive proteins: the photopigments. The pigment in rod photoreceptors is called rhodopsin, in cones they are referred to as cone pigments, which are used for color vision. When light hits the rods and cones, the photopigments catch the light and activate a cascade of second messenger reactions, eventually causing hyperpolarization of the membrane. This hyperpolarization in turn activates secondary neurons in the retina, which eventually carry the receptor signal to the brain, where it is processed and used to generate a visual percept.

Light sensing across single-celled and multicellelar animals is mediated by two large families of proteins: the opsins. Opsin genes are divided into two distinct superfamilies: microbial opsins (type I) and animal opsins (type II). The rohodpsin and cone pigments found in humans belong to type II. Among other differences, type I opsins are bound to ion channels, while type II is not. Type II opsins therefore need a battery of intracellular second messenger molecules to operate. In contrast, the type I opsins, being channel proteins, directly cause intracellular changes by changing the membrane potential upon activation due to ion flow. Type I opsins, being channel proteins are referred to as channelrhodopsins.

In retinal degenerative diseases such as retinitis pigmentosa (RP) the photoreceptors degenerate and patients eventually loose their vision. The secondary neurons in the retina, however, partly survive. As explained above, these secondary neurons are responsible for processing the photoreceptor signal and sending it to the optic nerve. Hence, although RP patients miss the photoreceptors, the rest of the visual pathway is ready to rock! Hence, somehow these neurons should be activated. There are a battery of approaches to do just this. Retinal implants activate the neurons directly by electrical activation (see further reading #3).

Optogenetics (Fenno et al., 2013) is another approach where the secondary surviving neurons are made photosensitive, thereby bypassing the photoreceptors by turning the secondary neurons into photoreceptors. The neurons are made photosensitive by making them express channelrhodospins. This is done by injecting a viral vector into the eye such that the virus infects the retinal cells. Instead of a viable virus, however, it is tinkered aound with and the only thing the virus does is incorporate the channelrhodopsin into the genome of the neurons, which start to express it.

Channelrhodopsins are cation channels, and therefore, upon light activation, they allow positively charged ions (notably Na+, but also Ca+) into the cell. This depolarizes the neuron and can induce action potential firing (see further reading #1). Action potentials is the neuron way of transmitting signals to the brain. Hence, by activating the secondary neurons through channelrhodopsin transformation, they start sending light-induced neural activity to the brain. The brain will handle this input as if evoked by photoreceptors and hence will interpret it as light-evoked visual stimuli.

With regard to cancer: It has been shown that expression of channelrhodopsin and illumination of whole-mice resulted in diminished subcutaneous cancer cell counts (yang et al., 2013).

With regard to your first added question: dependent on the exact peptide regulatory sequences added to the protein, or by using viral vectors specifically targeting axons, expression of channelrhodopsins can occur wherever one likes.

Your second added question: Cation channels activate neurons, anion channels inhibit them. This means you can play around with different variants to control neurons at will.

References
Fenno et al. Annu Rev Neurosci 2011;34:389-412
Yang et al. Cell death and disease 2013;4:e893

Further reading
1. Action potentials and neurons: how-do-the-brain-and-nerves-create-electrical-pulses
2. Photoreceptors: parallels-between-pixelized-image-and-the-human-retina
3. Retina and retinal implants: does-the-retina-encode-visual-information-like-a-bitmap-or-an-svg


Addition to the previous answer.

How to express channelrhodopsins in neurons?

Other than the method involving direct viral injection, as described in the previous answer, these proteins can be expressed in specific cells by making transgenics (by using promoters that will express the downstream gene in only specific cells).

How does it work?

When a neuron is made light sensitive by ectopic expression of channel-rhodopsins, then they can be made to "fire" (generate action potential) by simple illumination. Fibre optics are used to illuminate specific neurons in the brain. See the image below:

Are the light-sensitive molecules (which I now have read are channelrhodopsins) present all along the axolemma…

They would be expressed everywhere but their expression on the axons can be limited because of myelination. It is possible to make certain proteins express in the dendritic region. This requires localization signals called zipcodes (this process is still not understood very well).

How then can these channels work, if any cation can diffuse any way through the channel?

Chr2 can conduct Na+, Ca2+ and K+. Its conductivity for sodium is higher than that for potassium. When the channels open the higher inward flow of sodium would trigger action potential (AP). Moreover Calcium also helps in strengthening the AP. Though the channel can conduct Li+, the ion is not present in the extracellular fluid. Usually the nonspecific channels are not used.

Also, I was wondering whether the principle behind optogenetics- controlling calls with light- could be used for other treatments e.g. to kill cancer cells?

Light (as lasers) is used for killing cancer cells but it does not involve optogenetic techniques. You would need to genetically modify the cancer cell first.


T oday optogenetics is a widely accepted technology for probing the inner workings of the brain, but a decade ago it was the source of some anxiety for then assistant professor of bioengineering Karl Deisseroth.

Deisseroth had sunk most of the funds he'd been given to start his lab at Stanford into a crazy idea – that with a little help from proteins found in pond scum he could turn neurons on and off in living animals, using light. If it didn't work he'd be out of funds with no published research, and likely looking for a new job.

Luckily, it worked, and has just earned Deisseroth, now the D. H. Chen Professor of bioengineering and of psychiatry and behavioral science, the 2014 Keio Medical Science Prize. Thousands of labs around the world are now using optogenetics to understand and develop treatments for diseases of the brain and mental health conditions and to better understand the complex wiring of our brains.

Deisseroth described the first step of his success in a seminal paper in 2005, but it was many years and many more academic papers before he could breathe easy. "There was a period of several years when not everyone who tried optogenetics got it working," Deisseroth said. "There were some people who were skeptical about how useful it would be, and rightly so because there were a number of problems we still had to solve."

Scientists worldwide have now used optogenetics to probe addiction, depression, Parkinson's disease, autism, pain, stroke and myriad other conditions.

"Optogenetics has revolutionized neuroscience," says Rob Malenka, a professor of psychiatry and behavioral sciences. Malenka is Deisseroth's former postdoctoral advisor and is now a frequent collaborator. "It has allowed neuroscientists to manipulate neural activity in a rigorous and sophisticated way and in a manner that was unimaginable 15-20 years ago."

Deisseroth adds, "I thought it would work but wasn't sure it would quite reach this point."

Karl had the insight to realize how important this was going to be. The idea had been floating around but he recognized the importance and made it work.

&mdashRob Malenka, professor of psychiatry and behavioral sciences


Optogenetics: A Virtual Reality System for Controlling Living Cells

Our brains communicate with electrical and chemical signaling, but scientists have discovered that light stimulation could hold potential keys to manipulating neuronal communication pathways that influence motor control, sensory perception, memory, neurochemical production and mood -- or cellular virtual reality, as a report from the Journal of Cell Biology describes it.

Who's Interested in Optogenetics?

With the roll out of the White House's $300 million BRAIN Initiative in 2013, interest in uncovering the secrets of the human brain has accelerated and now includes many government agencies, public/private partnerships and universities.

Dating back to at least 1971, optogenetic research has matured enough to gain the attention of organizations such as the NIH, DARPA and IARPA, who are exploring the role that light-sensitive cells could soon play in fields surrounding neurobiological, including physical and mental health, human-machine interfacing, and advancing artificial intelligence through reverse brain engineering.

How Does Optogenetics Work?

Current optogenetic experiments rely on extracting "opsins" (light-sensitive proteins) from plants which can be introduced to mammals by methods including injection and infection via adenovirus.

Once delivered into an organism, opsins can be expressed in eye, brain or skin cells, allowing their light-sensitivity to be remotely activated or silenced with timed pulses of light in different color wavelengths across the light spectrum that can target multiple bodily systems and cause a variety of biological effects.

Researchers have suggested however that introducing opsins into an organism may not be a long-term requirement as methods are sought for using optogenetics on mammalian cells that respond naturally to light, such as those in the human retina.

Current Capabilities and Interests

As part of the BRAIN Initiative, scientists have been working on neuronal barcoding and completing a detailed online brain atlas for researchers. This is hoped to eventually provide a detailed circuit diagram of every neuron and synapse in the brain, which would allow various neuronal patterns to be identified so they can be triggered for the desired effect.

If targeted precisely enough with the appropriate light, it's thought that optogenetics could be used by manipulating neural circuits involved with pain, fear, reward, wakefulness and social behaviors. In one Yale study, for example, mice were infected with a virus which made their neurons sensitive to blue light. Scientists then used that light pathway to activate predatory behavior.

". The researchers used a tiny optic fibre to shine a blue laser on the amygdala. This prompted the animals to tense their jaw and neck muscles. 'It's not just physiological, it's hunting, biting, releasing and eating. Those are motor sequences that require a lot of information. ' [said an MIT neuroscientist]"

In 2015, optogenetics was combined with CRISPR to develop a set of photoactivatable tools that enable the editing of an organism's genome through the external use of light. Said tools can control the location, timing and reversibility of the genome editing process, whether that be activating, repressing or modifying a gene.

Optogenetics is also mentioned as an integral feature of the DARPA-funded Neural Engineering System Design (NESD) program, a joint effort between six teams who are aiming to create an implantable neural interface over the next four years that is capable of high resolution brain-to-machine communication. Such advancements, for instance, could facilitate the development of mind-controlled prosthetics featuring touch sensation like the DARPA-backed 'Luke' arm (previously known as the 'Deka' arm).

An NESD-developed interface for monitoring and stimulating neurons alongside the 'Luke' arm.

In the past, DARPA has looked to optogenetic memory manipulation techniques for treating veterans with traumatic brain injury and/or PTSD through memory restoration or deletion.

More recently, during a November 2017 mental health conference with 30,000 attendees in Washington D.C., optogenetics was noted for the impact it's having on the ability to study the brain. According NPR science correspondent Jon Hamilton, the technology has allowed aspects of human mental health disorders to be reproduced in animals, aiding the mapping of neuronal circuits involved with issues such as depression.

Clinical Trial and Future Technologies

Companies interested in the application of optogenetic technologies have begun emerging over the last decade, particularly since the FDA approved the technology in 2015 for use in treating an eye disorder known as "retinitis pigmentosa."

The approval prompted a clinical trial and optogenetic developments have since been used to restore partial vision in patients who were described as being "profoundly blind." Chronic pain management, epilepsy and Parkinson's are among many health issues that researchers are experimenting with addressing through optogenetics.

The technology is also contributing to other areas of research such as "sonogenetics," which uses low-pressure ultrasound to activate ultrasonically sensitized neurons. This is another area of interest for DARPA, which has funded Columbia University's endeavor to stimulate neurons using ultrasound and believes it could eventually lead to a magnetic version of the technology called "magnetogenetics."

To investigate the therapeutic use of optogenetics, acoustics and electromagnetic fields, DARPA launched the ElectRX (Electrical Prescription) program in 2015, which is capable of stimulating, modulating and monitoring the body's peripheral nervous system. The research agency is also exploring how artificial intelligence could be used in closed-loop brain implants, such as the ability to detect patterns associated with mood disorders.

With enough progress, it's believed that optogenetics and its surrounding bodies of research may open the door to real-time brain mapping and biofeedback technologies, which could be used to treat all manner of ailments on the fly through closed-loop neuromodulation signals coming to and from an implanted device, ultimately eliminating the need for pharmaceuticals.


Controlling nerve cells with light opened new ways to study the brain

Optogenetics turns nerve cells into light-controlled puppets.

SEBASTIAN KAULITZKI/Science Photo Library/Getty Images, adapted by E. Otwell

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Some big scientific discoveries aren’t actually discovered. They are borrowed. That’s what happened when scientists enlisted proteins from an unlikely lender: green algae.

Cells of the algal species Chlamydomonas reinhardtii are decorated with proteins that can sense light. That ability, first noticed in 2002, quickly caught the attention of brain scientists. A light-sensing protein promised the power to control neurons — the brain’s nerve cells — by providing a way to turn them on and off, in exactly the right place and time.

Nerve cells genetically engineered to produce the algal proteins become light-controlled puppets. A flash of light could induce a quiet neuron to fire off signals or force an active neuron to fall silent.

“This molecule is the light sensor that we needed,” says vision neuroscientist Zhuo-Hua Pan, who had been searching for a way to control vision cells in mice’s retinas.

The method enabled by these loaner proteins is now called optogenetics, for its combination of light (opto) and genes. In less than two decades, optogenetics has led to big insights into how memories are stored, what creates perceptions and what goes wrong in the brain during depression and addiction.

To celebrate our upcoming 100th anniversary, we’re launching a series that highlights some of the biggest advances in science over the last century. For more on the past, present and future of neuroscience, visit Century of Science: Our brains, our futures.

Using light to drive the activity of certain nerve cells, scientists have toyed with mouse hallucinations: Mice have seen lines that aren’t there and have remembered a room they had never been inside. Scientists have used optogenetics to make mice fight, mate and eat, and even given blind mice sight. In a big first, optogenetics recently restored aspects of a blind man’s vision.

An early clue to the potential of optogenetics came around 1 a.m. on August 4, 2004. Neuroscientist Ed Boyden was in a lab at Stanford, checking on a dish of neurons that possessed a gene for one of the algal light sensors, called channelrhodopsin-2. Boyden was going to flash blue light on the cells and see if they fired signals. To his amazement, the very first cell he checked responded to the light with a burst of action, Boyden wrote in a 2011 account. The possibilities raised by that little spark of activity, described in a 2005 technical report by Boyden, Karl Deisseroth of Stanford University and colleagues, quickly became realities.

In Pan’s lab, light-responsive proteins restored vision in mice with damaged retinas, a finding that has now led to a clinical trial in people. Optogenetics’ promise wasn’t a given in those early days, as scientists were first learning how to use these proteins in neurons. “At that time, no one anticipated that this optogenetic work would have such a huge impact,” Pan says.

Since those early discoveries, the algae’s light sensors have been adopted for use in numerous brain research arenas. Neuroscientist Talia Lerner of Northwestern University in Chicago, for example, uses optogenetics to study connections between cells in the mouse brain. The method allows her to tease apart the relationships between cells that produce and respond to dopamine, a chemical messenger involved in movement and reward. Those cellular links, illuminated by optogenetics, might help reveal details about motivation and learning. “My research really wouldn’t be possible in its current form without optogenetics,” she says.

Optogenetics is also indispensable for Jeanne Paz of the Gladstone Institutes in San Francisco. She and her colleagues have been hunting for the cells that can stop seizures from spreading across the brain. By giving her a way to control distinct groups of neurons, optogenetics is crucial to her search. “We really could not ask these questions with any other tool,” Paz says.

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Her optogenetics-aided search led Paz to a brain structure called the thalamus, a way station for many neural networks in the brain. “I remember the goose bumps I experienced the first time I shined the light into the thalamus and it stopped the seizure,” she says.

So far, optogenetics research has taken place mostly in mice. But insights into more complex brains, including those of primates, may soon be found, says Yasmine El-Shamayleh of Columbia University. In 2009, Boyden and colleagues described optogenetics in a macaque. El-Shamayleh and others are pushing this line of research, hard. “We are definitely on the cusp” of revealing some fascinating principles of the primate brain, such as how the brain transforms signals from the eyes into perceptions, she says.

Optogenetics has evolved quickly. Scientists have engineered and optimized new light sensors and new ways of combining them with other techniques. An important reason for today’s widespread innovation, says Lerner, was the early spirit of sharing by optogenetics pioneers. At Stanford, Deisseroth would regularly run workshops to train other scientists on the technique. “In some ways, that’s as important as inventing it,” Lerner says.

So it’s worth taking a minute to appreciate the original sharers. No matter what happens next in this swiftly moving field, one thing is certain: Brain scientists will be forever in the algae’s debt.


Optogenetics: A novel light sensor built from algal enzymes

Violet light triggers a signalling chain in the light sensor protein switch-Cyclop, blue or green light stops the chain. At the end, the production of the signalling molecule cGMP is regulated by the enzyme guanylyl cyclase (GC). Credit: Shiqiang Gao / University of Würzburg

The unicellular green alga Chlamydomonas reinhardtii has already given research a massive boost: One of its light sensors, channelrhodopsin-2, founded the success of optogenetics about 20 years ago.

In this technology, the alga's light sensor is incorporated into cells or small living organisms such as threadworms. Afterwards, certain physiological processes can be triggered or stopped by light. This has already led to several new scientific findings, for example on the function of nerve cells.

Now the green alga Chlamydomonas is once again setting an accent. Once again, it is its light sensors, the rhodopsins, that have added an instrument to the toolbox of cell biology.

Light sensor produces the messenger cGMP

Researchers Yuehui Tian, Georg Nagel and Shiqiang Gao from Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany, have constructed a novel light sensor from two of the algae's rhodopsins. It has enzymatic activity and can be switched by two different light colors. UV or violet light leads to the production of cGMP, an important signaling molecule in the cell. A blue or green flash of light, on the other hand, stops the production of the signaling molecule.

The researchers present the new light sensor in the journal BMC Biology. They have given it the name switch-Cyclop.

Nagel's research group at the JMU Institute of Physiology is continuing to characterize the properties of the various rhodopsins from Chlamydomonas. The professor's team is cooperating closely with neuroscientists. The goal is to explore the possible applications of the light sensors.


The promise of optogenetics in cell biology: interrogating molecular circuits in space and time

Optogenetic modules offer cell biologists unprecedented new ways to poke and prod cells. The combination of these precision perturbative tools with observational tools, such as fluorescent proteins, may dramatically accelerate our ability to understand the inner workings of the cell.

Biology has always been primarily an observational science, and in the modern era, the development of genetically encoded fluorescent proteins such as GFP has given us the unprecedented ability to peer into the living cell and to observe its inner workings. We can now study individual cells in culture or in the context of a whole organism and directly observe where proteins are localized, their dynamics and their variability in expression level. More than ever, we now appreciate that the cell is not a bag of molecules but an anisotropic structure with highly complex spatial organization. We can see examples of how this organization shifts in dynamic processes, ranging from cell-shape changes to signal transduction propagated from the plasma membrane to the nucleus.


Talk Overview

Optogenetics is a combination of genetics and optics to achieve a gain or loss of function of biochemical events such as action potentials in a particular neuron or tissue. Opsin genes encode proteins that receive light and give rise to ion flow. This talk gives an introduction to optogenetics followed by examples of how optogenetics is being used to study the brain.

Questions

  1. Which of the following is/are true about opsins (select all that apply)?
    1. Activated by light
    2. Give rise to ion flow
    3. Activated by ions
    4. Can inhibit cells
    1. Describes the wavelength of light needed to activate the opsin
    2. Describes the time constant of deactivation of the opsin after the light is turned off
    3. Describes the time constant of activation of the opsin after light is applied
    4. Describes the number of opsins needed to be expressed on a cell to achieve activation

    Answers

    1. A, B, D
    2. Optogenetics can increase precision and specificity in excitation and inhibition. One can send these signals to a single cell or tissue if desired. The same cell/tissue can be turned on and off and controlled temporally to give control of biochemical events.
    3. B
    4. Since τ off is the time constant of deactivation of the opsin after the light is turned off, a long τ off means the cell will remain activated after the light is removed. This is useful because several pulses of low level light can be used to activate or inhibit the opsin instead of a higher intensity pulse of light. The activation/inhibition can happen without light being continuously delivered and thus is less invasive because the opsin will remain activated even when the light source is taken away since activation is persistent. These channels can be turned off with a different photon of light (e.g. on with blue light, off with green light). Lastly one can capture native activity of the cell after it is activated as opposed to prescribing a spike pattern.

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    Synthetic photobiology: why use light?

    In synthetic biology and bioengineering, scientists seek to directly probe and build new systems. Such goals require methods to directly activate and inactivate biological processes. Chemical effectors are suboptimal for a number of reasons, including their potential toxicity and cross-reactivity with other pathways. Since these effectors are diffusible, they’re also not suitable for spatially limited studies and can fluctuate with changing culture conditions.

    The use of low intensity light solves each of these problems inexpensive tools such as LEDs can be engineered to deliver precise, consistent, and controllable pulses of light. Complicated patterns of light intensity and wavelength can be used to study dynamic processes. Conversely, varied input patterns may also be used to engineer new pathways with sophisticated gene expression controls. Most cell types do not respond to low-intensity light, so off-target effects should be limited.

    Phytochromes are light-responsive systems found in some bacteria, but not E. coli . They fall under the heading of two-component systems (TCSs). TCSs consist of a histidine kinase that phosphorylates a response regulator (RR). TCS-induced responses are varied one outcome is directed transcription from a given promoter.

    To harness the power of light, Christopher Voigt’s lab created the first E. coli light-sensitive two-component system (TCS). Levskaya et al. fused a photosensory domain from cyanobacteria to a common E. coli histidine kinase. This hybrid construct senses light, and an obligate chromophore allows the system to respond to various light inputs. In far-red light or dark conditions, the chromophore activates the RR (OmpR) via phosphorylation, promoting transcription (see figure below). Subsequent exposure to red light rapidly deactivates the system. Voigt’s lab used this system to develop a bacterial camera that prints a chemical image, as well as a genetic method for the computational problem of edge detection . Tabor et al. subsequently engineered a second photosensitive TCS activated by green light these two systems can be coexpressed for sophisticated control of gene expression.

    An E. coli light-sensitive two-component system (TCS). A hybrid histidine kinase/photosensor senses various light inputs, and an obligate chromophore allows the system to respond. In far-red light or dark conditions, the chromophore activates the response regulator via phosphorylation, promoting transcription of a GFP reporter. Subsequent exposure to red light rapidly deactivates the system.

    Version 1 of this far-red/red light-responsive system is spread across three plasmids, with the response regulator encoded in the genome. To make the system less bulky and easier to use with other strains of E. coli, Schmidl et al. condensed the system to two streamlined plasmids.

    The utility of these TCSs was limited by a few factors. These bulky systems were spread over a number of plasmids, with one RR (ompR) encoded chromosomally. Leakiness was also an issue some promoter activity persisted in the inactive state. The dynamic range, defined as the difference between the lowest and highest level of “output” signal, was only about 10-fold, precluding the study of gene expression at very high or very low levels.

    Schmidl et al. chose to optimize these systems to make them more tunable and user-friendly. They experimented with the promoter strength of various components, removing inducible promoters that could crosstalk with other pathways. They also reduced system leakiness and increased dynamic range to around 100-fold. These improvements will allow researchers to better fine-tune the level of output gene expression based on the input light intensity. In addition to these improvements, the constructs have been greatly streamlined only two plasmids are required per system.


    Optogenetics: Illuminating Our Head Space

    The brain serves as a computer for the body: it regulates body functions and systems, sends signals, stores memories and learned behaviors, and adapts as new behaviors and concepts are learned. One of the most complex organs in the human body, the brain does all this through a dense network of several billion neurons that constantly interact with each other.

    Defined by their shape and the molecules they contain, neurons can be categorized into hundreds of different types. When some of these types are compromised, either by injury or chemical imbalance, some of the brain's specific computations can be thrown off. Neurological conditions such as Parkinson's and psychiatric disorders such as schizophrenia can be the result. These types of illnesses affect billions of people around the world and are some of the most problematic medical issues we face today.

    In order treat such disorders, scientists are trying to understand how the elements of a neural circuit work together. One method is to selectively stimulate the activity of specific neurons within the circuit, and observe the results of that stimulation on neural computation and behavior.

    Optogenetics, which combines optics, genetic engineering, and several other disciplines, is making this possible. In this method, specific neurons are genetically modified with light-sensitive proteins. Researchers can then use light to precisely stimulate and control those target cells. This control offers clinical benefits, especially in treating neurological disorders.


    Confocal analysis at 63x magnification, followed by the three-dimensional reconstruction of neuronal cell bodies and branches. Credit: Scripps Research

    Moving toward a light solution
    In 1979, British neuroscientist Francis Crick realized a way to address the problem of targeting individual neurons without affecting others nearby. By that time, scientists had long been able to stimulate neurons using electrical or magnetic interventions, but neither method could distinguish between individual types of neurons that might be sitting right next to each other. Certain microorganisms were known to possess proteins that responded to light, so Crick suggested that light might be the tool to control, turn on, or turn off individual neurons.

    This idea was pursued further in the 1980s when Peter Hegemann, a neuroscience professor at Humboldt University of Berlin, discovered that light-activated molecules in green algae and other simple organisms could sense light. Specifically, Hegemann and his colleagues found that channelrhodopsin molecules could be used to switch neurons on and off. Soon afterward, Gero Miesenböck, now director of the Centre for Neural Circuits and Behaviour at Oxford, tested the method on a fruit fly, showing for the first time that it was possible to control the brain using light, and offering a new way to illuminate the brain's function.

    In 2004, Karl Deisseroth, a professor of bioengineering and of psychiatry and behavioral sciences at Stanford School of Medicine, wanted to improve the lives of patients with psychiatric disorders. With grad students Feng Zhang &mdash who had a background in chemistry, molecular biology, and virology &mdash and Ed Boyden &mdash skilled in electrophysiology &mdash he conducted experiments showing the rhodopsin proteins first studied by Hegemann in single-cell organisms could be used to activate neurons. The team developed ways to both excite and inhibit neuron activity with light, and achieved the first single-cell-resolution-control of neuronal activity and behavior in living mammals using optogenetics.


    Optogenetics uses light to stimulate, inhibit, and control neuron activity in the brain. Image: Warren Alpert Foundation.

    Deisseroth is, in fact, credited with coining the phrase "optogenetics" in 2006. Between 2004 and 2009, his team at Stanford developed a series of tools that can activate or silence genetically specified neurons. These tools are optogenetic proteins encoded from natural species of algae, fungi, and archaebacteria. Like tiny, genetically encoded solar panels embedded in the membranes of neurons, the proteins respond to light by altering the voltage of the neuron. A neuron is itself an electrical device, so by using a virus to express the genes in neurons, the neuron can be activated or silenced by different colors of light.

    According to Deisseroth, the essence of optogenetics is that it uses light to make neurons active, and does this in the very language that the brain uses. While the language of the heart delivers a pump every second or so, the language of the brain uses more like a thousand words or syllables in a second. Light, of course, can operate at that speed.

    The light at the end of the synapse
    Since Deisseroth first published his team's work in 2005, optogenetics has been gaining worldwide attention. In 2010, Nature selected optogenetics as its "Method of the Year," and researchers working with in the discipline have garnered recognition with awards such as the Warren Alpert Prize the Berthold Leibinger Zukunftspreis and the Breakthrough Prize in Life Sciences.

    Today, researchers around the world are using optogenetics to define the deficits behind schizophrenia, autism, depression, addiction, Parkinson's disease and more, as well as determining the brain circuitry responsible for specific behaviors. Efforts are underway to refine optogenetic techniques, such as improving the light sensitivity of genetically modified proteins. Researchers are also finding ways of better light delivery to improve the accuracy and efficiency of optogenetic methods. These strategies include arrays of separately controllable LEDs that cover specific areas of the brain as well as infrared illumination systems that can penetrate dense brain tissue.

    "What optogenetics allows us to do is to put particular sensitizers &mdash antennas of a sort, for external sources of energy and information into specific kinds of neurons," Deisseroth explained in a 2016 video. "And then we can use light. In this case, if we make light-sensitive antennas or transducers, and make neurons of a particular kind respond to light, and we do that to allow different kinds of ions to flow into cells and turn neurons on or off depending on the experiment, well, that's the fundamental idea of optogenetics and it's something people have wanted to do for a very long time."


    Watch the video: A new way to study the brains invisible secrets. Ed Boyden (August 2022).