Shedding Light on the Brain: Neuroscientists Reflect on Optogenetics Nobel
On October 5, three scientists—Peter Hegemann, Georg Nagel, and Karl Deisseroth—were awarded the Nobel Prize in Physiology or Medicine for the development of optogenetics, a revolutionary tool that allows neuroscientists to use light to turn on and off individual brain cells, even those located deep within the brain. Hegemann and Nagel, the Nobel Assembly noted, "discovered a remarkable protein, channelrhodopsin, in a single-celled alga," and Deisseroth "transformed the protein into a light-controlled switch for nerve cells."
Viviana Gradinaru (BS '05), Caltech's Troendle Professor of Neuroscience and Biological Engineering and a Howard Hughes Medical Institute Investigator, was one of the first graduate students in Deisseroth's Stanford University laboratory. A Caltech professor since 2012, Gradinaru has spent much of her Caltech career further advancing the optogenetics technique.
In optogenetics, brain cells are genetically modified to carry proteins that enable researchers to excite or inhibit the cells' activity using specific wavelengths of light, even if they are located deep within the brain. Hegemann, from Humboldt University in Berlin, and Nagel, of Germany's University of Würzburg, found in the early 2000s that these proteins enable green algae to swim toward sources of light.
When Gradinaru started doctoral studies at Stanford in 2005 after graduating from Caltech, optogenetics was still a niche specialty. Deisseroth's lab had just begun to develop techniques to insert opsin genes into rodent brain cells in a dish, allowing those cells to be turned on with blue light. Gradinaru's graduate thesis was focused on developing inhibitory opsins and using optogenetics in vivo to understand how deep brain stimulation works for Parkinson's disease.
While excitatory optogenetics had advanced to working smoothly in living animals and was being utilized by more labs worldwide, inhibitory optogenetics—suppressing the activity of cells with light—was more challenging.
"In those early optogenetics days, the inhibitory opsins would aggregate and harm the cells we were trying to study," recalls Gradinaru, who is also director and Allen V. C. Davis and Lenabelle Davis Leadership Chair of the Richard N. Merkin Institute for Translational Research at Caltech. (Deisseroth is a member of the Merkin Institute's advisory board.) "Much of my early PhD was focused on how to evolve inhibitory opsins from bacteria for safe, effective use in mammalian cells. We figured out that we could guide the proteins to where they needed to go within the cell by adding small sequences acting like ZIP codes for delivering mail." This unlocked the path for inhibitory optogenetics with "light-driven chloride pumps like NpHR and its later engineered variants," as the Nobel Assembly noted in its press release.
Gradinaru's other major publication as a graduate student was the first proof-of-concept that optogenetics could be used for preclinical research to modulate behavior in a living animal, specifically a mouse model of Parkinson's disease, to better understand the mechanism underlying deep brain stimulation, a powerful therapeutic option used in clinics.
As a graduate student, Gradinaru taught courses at Stanford and at Cold Spring Harbor Laboratory in New York on optogenetics—how to synthesize the opsins, get them into the neurons of lab animals, and use the lasers. When she joined the Caltech faculty, she continued to teach optogenetics as part of a course on experimental techniques in neuroscience. Gradinaru also established the CLOVER Center (CLARITY, Optogenetics, & Vector Engineering) at Caltech's Beckman Institute for dissemination of neurotechnologies and is now also an affiliated faculty member with the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech.
Over more than a decade at Caltech, Gradinaru has continued to develop optogenetics methods. Early on, she initiated a collaboration with Frances Arnold, the Linus Pauling Professor of Chemical Engineering, Bioengineering and Biochemistry and director of the Donna and Benjamin M. Rosen Bioengineering Center. Arnold pioneered a technique called directed evolution, a lab technique that experimentally mimics the process of natural selection to produce new types of enzymes with specific traits—work that earned her the 2018 Nobel Prize in Chemistry. Together, the two labs utilized machine learning–assisted directed evolution to create new opsins that are sensitive to red light; these longer wavelengths of light can penetrate farther into the brain than the blue light, enabling recruitment of larger, deeper brain areas with distant, less damaging light sources. When paired with delivery vectors that could cross the blood-brain barrier (BBB), the team could distribute opsin genes noninvasively across the entire brain, moving one step closer to minimally invasive optogenetics.
"Doing research in Karl's lab was an amazing opportunity for learning, interdisciplinary discovery and lifelong friendships: Feng Zhang, first author on many optogenetics papers and inventor on much of the early patents was my graduate rotation mentor!" Gradinaru says. "I try to bring that same spirit of fluid research and preclinical-relevant work in my own lab."
Optogenetics has continued to mature and is now used widely by neuroscientists. At Caltech, it has been a fundamental tool enabling discoveries about how the brain governs social behaviors, homeostasis (internal balance in the body), sleep, and more.
David Anderson's laboratory, for example, uses optogenetics to study the neurobiology of emotion and social behaviors.
"Optogenetics has revolutionized neuroscientists' ability to move beyond simply observing correlations between brain activity and behavior to establishing cause and effect," says Anderson, the Seymour Benzer Professor of Biology, Howard Hughes Medical Institute Investigator, and Leadership Chair and director of the Tianqiao and Chrissy Chen Institute for Neuroscience. "With it, we can test whether activating certain neurons is sufficient to evoke a behavior and whether inhibiting them blocks that behavior. For example, it has allowed my lab to discover specific neurons that control basic instinctive behaviors and related emotion states including aggression, mating, and fear in both flies and mice."
In the Nobel award citation, optogenetics is credited with making possible discoveries about the neural circuits underlying thirst—research that was conducted in the Caltech laboratory of Yuki Oka, professor of biology, Heritage Medical Research Institute Investigator, and affiliated faculty member with the Tianqiao and Chrissy Chen Institute for Neuroscience.
"In our work on thirst, we used light to activate a small population of neurons in the brain, and within about a second, a fully hydrated animal began drinking water," Oka explains. "That kind of immediate, direct link between the activity of specific neurons and a motivated behavior was unimaginable before this technology. It allowed us to move beyond describing which neurons respond to thirst and to demonstrate that they drive it. In other words, optogenetics transformed neuroscience from a field of correlation to one of causality. It was a privilege to collaborate with Karl early on, and his work continues to shape how we study the brain."
The tool also can be adapted for use in many different model organisms, such as zebrafish, tiny transparent fish. Researchers in the Caltech laboratory of David Prober, professor of biology and affiliated faculty member with the Tianqiao and Chrissy Chen Institute for Neuroscience, use light to probe the zebrafish brain to answer fundamental questions about the neuroscience underlying circadian sleep rhythms.
"Optogenetics is essential for my lab's research on zebrafish," Prober says. "We routinely use optogenetics to stimulate or inhibit specific neurons while recording the behavior of almost 200 animals at the same time. This has allowed us to discover neurons that regulate sleep and determine how they do so."
When Gradinaru first met Deisseroth early in 2005 during her admissions interview at Stanford, his lab was just getting started, but, she says, there was a clear and exciting vision to use light and genes for neuroscience.
"As a student trained in optics and electronics in Romania and in molecular biology and neuroscience at Caltech, I thought I could really help—at least with some of the empty rooms that needed equipment setup," she says. "I am happy for Karl, Peter Hegemann, and Georg Nagel, and grateful to have played a part as a graduate student at Stanford and assistant professor at Caltech in the development of optogenetics, a method that is now being used beyond academia in industry and health care. Many more exciting discoveries and clinical impacts still lie ahead."
Optogenetics used to stimulate dopaminergic neurons with light, leading to the discovery of a neural circuit in the brain that controls wakefulness.
Credit: Courtesy of the Gradinaru laboratory
