Optogenetics: Illuminating the Inner Workings of the Brain

Optogenetics meets “Three Blind Mice” and the mice become blind no more. Researchers from various institutions, including the University of Southern California (USC) and the Massachusetts Institute of Technology (MIT), have worked together and succeeded in restoring vision to blind mice [1]. In April of 2011, a paper channeling their breakthrough was published in the journal Molecular Therapy [2]. It was brought to light that a technology called optogenetics was what enabled the mice to see [1]. Led by USC’s Alan Horsager, the study worked with algae containing proteins known as channelrhodopsins that, once activated by light, stimulate their host cells [3]. After extracting the piece of DNA encoding for the channelrhodopsin-2 (ChR2) protein, it was installed in the retinas of the blind mice by way of viral carriers [2]. As expected, the mice’s retinal cells turned into functional photoreceptors, and enough vision was brought back for the mice to navigate a water maze [2]. Although the mice were unlikely to have 20/20 visual acuity [2], the study shows promise in finding a cure for the 15 million people worldwide who are blind or have retinitis pigmentosa [2], a retinal degenerative disease in which the light-sensitive cells of the retina deteriorate [1].

The cutting-edge cellular technique was discovered in 2004 by the MIT Media Lab’s Edward Boyden, Stanford University’s Karl Deisseroth, and the University of Würzburg’s Georg Nagel. As its name suggests, optogenetics integrates genetic and optical methods. It takes advantage of the algal protein channelrhodopsin to control neurons. As light-gated ion channels, channelrhodopsins transport cations across the plasma membrane when activated by light, controlling the electrical activity of the cell [1]. Researchers can implant the light-sensitive protein in whatever type of mammalian brain cells they want and turn on the lights in the animal’s brain to make the neurons fire [4]. As a result, they are able to hone in on those particular cells’ functions [4]. Not only that, if scientists can stimulate specific cell types, they certainly can suppress specific cell types. Channelrhodopsins can open or close, facilitate or block the flow of ions [1]. To make the protein channels respond accordingly, different versions of the proteins and various wavelengths of light are used. For instance, blue light might be used to activate a set of cells, while yellow light might be used to deactivate a different set of cells [4]. The field also encompasses the technologies needed for delivering light into complex, freely moving organisms to activate the channelrhodopsins and for analyzing how modulating the targeted neurons affects the organisms [5]. Optogenetics surmounts the brain’s complex properties: its delicacy, intricacy, inaccessibility, and high operating speed. With the ability to pick and choose cells of interest and control them with remarkable specificity, scientists have an unparalleled amount of control over any brain system.

Interestingly, Boyden and his colleagues were not the first to establish the need to control the electrical activity of certain neurons. In a Scientific American article published in 1979, Nobel laureate Francis Crick, co-discoverer of the structure of DNA, noted that neuroscience needed a way to stimulate or suppress one type of brain cell while preserving others [5]. That way, neuroscientists would be able to learn more about the inner workings of the brain and its neurons. Electrical stimuli would not suffice because electrodes are too crude to differentiate between cell types or silence neurons; they activate all of the brain’s circuitry at their insertion site [5]. Drugs are not specific enough to target a group of cells either, and they are much too slow to match the brain’s standard operating speed [5]. Thus, Crick hypothesized that light could be the right tool for driving or suppressing neuronal activity, just that he did not know how to make specific cells light-regulated [5].

However, more than four decades ago, even before Crick brought up the challenge facing neuroscience, biologists had already found microorganisms that produced light-activated proteins that transported ions across the cell membrane [5]. Dieter Oesterhelt and Walther Stoeckenius, both of the University of California, San Francisco, discovered the bacteriorhodopsin protein, a proton pump powered by green light, in 1971 [5]. Other microbial opsins of the same family were subsequently identified—halorhodopsins in 1977 and channelrhodopsins in 2002 [5].

In retrospect, had the two realms of biology concerning the brain and opsins converged, Crick’s challenge would have been solved. Of course, one can only say this in hindsight. In reality, it would take more than thirty years for the domains to fuse into a new technology, optogenetics.

It all began in 2000, as just another late night brainstorming session between Ed Boyden and Karl Deisseroth, who were PhD/MD-PhD students and labmates at Stanford at the time [6]. Boyden had been thinking about innovating a neurotechnology that would be able to control specific neurons. Initially, he and Deisseroth planned on adapting stretch-activated ion channels for certain cell types and then binding magnetic beads to the channels. When a magnetic force big enough to make the beads move is exerted, the ion channels would open and turn on the neurons [6]. Not long after, though, Boyden became drawn towards implementing light-gated ion channels to increase or decrease the electrical activity of neurons. Upon reading a paper authored by Georg Nagel and colleagues, which highlighted the discovery of channelrhodopsin-2 and how it could depolarize mammalian cells when exposed to light, Boyden suggested that he and Deisseroth reach out to Nagel to see if he would be willing to send them the construct of ChR2 [6]. A month later, Deisseroth received the construct and delivered the gene into a neural expression vector [6]. Boyden then spent all of July working on the project, debugging the optical filters, and coding the program that would shine blue light in precisely timed intervals [6]. On August 4, 2004, at around 1 AM, he went into the lab for the first night of experimentation, ready to test the technology he, Deisseroth, and Nagel had put together [6]. He ran the program that pulsed blue light on a patch-clamped ChR2-expressing neuron, and the neuron fired action potentials in response to the beams of light [6]. Boyden concluded that the neurons safely expressed ChR2 and precisely responded to light, sending nerve impulses [6].

Since then, optogenetics has revolutionized neuroscience. Its applications go far beyond reversing blindness; thousands of laboratories around the world have adopted the technology to study and find potential therapies for neurological disorders, such as Parkinson’s disease, epilepsy, and addiction; and psychiatric diseases, such as depression, anxiety, and posttraumatic stress disorder.

Parkinson’s disease is a chronic and degenerative movement disorder of the nervous system that arises when nerve cells in the brain do not produce enough dopamine. In a study published January 2015 in Nature Biotechnology, a team of researchers from Columbia University implanted opsins in dopaminergic neurons derived from human pluripotent stem cells [7]. The neuronal cells of the midbrain were transplanted in mouse models of Parkinson’s disease, and the mice fully recovered from their motor deficits [7]. However, once the cells were turned off with a laser light, the mice’s motor deficits returned within minutes [8]. Optogenetics unveiled the mechanisms behind recovery and relapse [7] and showed that the neurons need to be continuously firing in order to relieve the mice of their symptoms and increase dopamine release [8].

Researchers have also unraveled some of the complex brain circuitry behind depression and anxiety. Understanding that it is linked to a lack of motivation, or psychomotor retardation, Karl Deisseroth and Melissa Warden worked backward from the brainstem with optogenetics to pinpoint the neuronal pathway in the prefrontal cortex and to trace the interconnected map of cells that control motivation. Upon triggering the dorsal raphe nucleus region of the brainstem in particular, Deisseroth and Warden found that mice displayed an immediate increase in motivational movement. They were also able to reverse the process by triggering cells in the lateral habenula region [8]. Scientists at another institute took another approach—stimulating neurons associated with positive memories. After giving mice a pleasant experience, labelling the cells in the hippocampus that retain the memory of the experience with a light-sensitive protein that can be activated with optogenetic techniques and subjecting the mice to chronic stress, they turned on the hippocampal cells and discovered that they acted as though they had never been distressed and that this effect was reproducible over time. Comparable to deep-brain stimulation except targeting specific memory-storing cells instead of entire regions of the brain, this treatment could be a much more successful procedure for curing neurological and psychiatric conditions [9].

Optogenetics is a promising avenue that has shed light on and has allowed researchers to probe the intricacies and mysteries of the brain. Soon, scientists will be able to produce a neuron-by-neuron mapping of the brain that will lead to the discovery of optogenetic treatments or novel drug targets for a multitude of conditions. Soon, optogenetics will be extended to clinical trials; companies aiming to do just that, like RetroSense Therapeutics, are on the rise [10]. Soon, we will be able to treat diseases with a flip of a light switch.

References:

  1. Anne Trafton. “Seeing the light: Optogenetic technology restores visual behavior in mice, holds promise for treating human blindness”. MIT News, April 20, 2011. http://news.mit.edu/2011/blindness-boyden-0420
  2. M Mehdi Doroudchi, Kenneth P Greenberg, Jianwen Liu, Kimberly A Silka, Edward S Boyden, Jennifer A Lockridge, A Cyrus Arman, Ramesh Janani, Shannon E Boye, Sanford L Boye, Gabriel M Gordon, Benjamin C Matteo, Alapakkam P Sampath, William W Hauswirth and Alan Horsager. “Virally delivered Channelrhodopsin-2 Safely and Effectively Restores Visual Function in Multiple Mouse Models of Blindness”. Molecular Therapy. April, 2011. Volume 19, issue 7, 1220–1229
  3. MIT TechTV. “Optogenetics: Controlling the brain with light”. May 20, 2011. http://video.mit.edu/watch/optogenetics-controlling-the-brain-with-light-7659/
  4. Jon Bardin. “Scientists control monkey behavior with light for the first time”. Los Angeles Times. July 27, 2012. http://articles.latimes.com/2012/jul/27/science/la-sci-sn-scientists-control-monkey-behavior-with-light-for-the-first-time-20120726
  5. Karl Deisseroth. “Optogenetics: Controlling the Brain with Light”. Scientific American. October 20, 2010. http://www.scientificamerican.com/article/optogenetics-controlling/
  6. Edward S. Boyden. “The Birth of Optogenetics: An account of the path to realizing tools for controlling brain circuits with light”. July 1, 2011. http://www.the-scientist.com/?articles.view/articleNo/30756/title/The-Birth-of-Optogenetics/
  7. Julius A Steinbeck, Se Joon Choi, Ana Mrejeru, Yosif Ganat, Karl Deisseroth, David Sulzer, Eugene V Mosharov and Lorenz Studer. “Optogenetics enables functional analysis of human embryonic stem cell–derived grafts in a Parkinson’s disease model”. Nature Biotechnology. Feburary, 2015. Volume 33, No. 2, 204–209
  8. Kelly Servick, “Optogenetics illuminates pathways of motivation through brain”. Stanford Medicine News. Nov. 18, 2012. https://med.stanford.edu/news/all-news/2012/11/optogenetics-illuminates-pathways-of-motivation-through-brain-study-shows.html
  9. Steve Ramirez, Xu Liu, Christopher J. MacDonald, Anthony Moffa, Joanne Zhou, Roger L. Redondo and Susumu Tonegawa. “Activating positive memory engrams suppresses depression-like behavior”. Nature. June, 2015. Volume 522, No. 7556, 335–339
  10. Susan Young Rojahn. “Company Aims to Cure Blindness with Optogenetics”. MIT Technology Review. August 28, 2012. https://www.technologyreview.com/s/429010/company-aims-to-cure-blindness-with-optogenetics/

Image References:

  1. Adapted from Reference 6.
  2. Adapted from Reference 6.

 

Amy is a sophomore at The Harker School in San Jose, California. She likes to explore how STEM intersects with various disciplines.

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