Manipulating Memory through Optogenetics: A Conversation with Neuroscientists Xu Liu and Steve Ramirez
Over the past few years, MIT neuroscientists Xu Liu and Steve Ramirez have been bringing ideas that might only seem possible in science fiction to reality. In their lab, Liu and Ramirez were able to create artificial memories in mice. Their work illustrates the increasing ability of neuroscientists to control, manipulate, and engineer learning and memory in the brain. In their TEDx Boston talk, Ramirez explained “The mind, with its seemingly mysterious properties, is actually made of physical stuff that we can tinker with.”
The tool that allows Liu and Ramirez to tinker, explore, and manipulate the brain is optogenetics — the process whereby researchers insert the DNA of a light-sensitive protein into specific neurons, allowing them to switch those particular neurons on and off using flashes of light. (For a brief introduction on how optogenetics works check out this video from MIT’s McGovern Institute for Brain Research).
Optogenetics, which developed over the past decade, has shown tremendous potential to help researchers understand brain function and treat brain disorders, because unlike electrical stimulation or pharmacological methods, it allows for very precise activation and deactivation of neurons. It’s this precision that allowed Liu and Ramirez to carry out their groundbreaking experiments. In addition to learning and memory, optogenetics has been used to study pain, addiction, sleep, and social behavior, and has been highlighted as a technology crucial to the success of the White House’s new BRAIN initiative.
We recently sat down with Liu and Ramirez at the Tonegawa Lab at MIT to find out more about how they were able to create false memories in mice, as well as discuss what the future may hold for optogenetics research.
Anne West: Can you describe the logic behind how you were able to establish false fear memories in mice?
Xu Liu: Memory is not a solid, unchanging thing. Actually, whenever you recall something, you make some changes to that memory, which is stored back into your brain. That’s why memory evolves throughout time. In this case, Steve [Ramirez] and I induced a recall memory, but at the same time, we also provided the animal with new information. That new information is incorporated into this reactivated memory and when it’s stored back, it changes the memory. That’s how we implanted this so-called “artificial” or “false” memory. It’s based on a normal phenomenon that happens to all sorts of memories.
AW: How were you able to implant a false fear memory in a mouse?
XL: We can find ways to label cells active in the formation of one memory and later use a technology called optogenetics to activate the exact same cell population in the brain. Doing this, we can recall a memory. In the 2012 study, we activated the recall of a neutral memory about one context or one particular environment – a box – and then at the same time, when we use light to reactivate that memory, we also provide a shock. This all takes place in a completely different environment compared to the first one. We want to artificially associate the reactivated memory of box 1 with the real shock in box 2. In reality, the animal was shocked in box 2. But through this experiment, the animal was made to believe it was shocked in box 1. Later, when you put the animal back into box 1, it starts to show freezing behaviors, displaying behavior that indicates it has a fear memory because it associates box 1 with the shock (see image below).
AW: When you designed this experiment creating a false fear memory, what broader questions about learning and memory were you looking to answer?
XL: The whole reason why this [works] is based on a property of memory that you can form a false memory while presenting new information.
SR: That’s exactly my answer. In this case, it tells us 2 things. We already know from human and animal studies that memory is not a tape recorder of the past. It’s actually a re-constructive process that is from you re-pasting bits of the past into that memory, to make it not a fully accurate, veridical representation of the past. The term that a lot of behavioral neuroscientists use is that memory is “labile” – it’s subject to modification. But could we artificially render a memory labile and introduce new information in the form of foot-shocks to it? That could tell us a lot about what the hippocampus is doing – that if the hippocampus is involved in bringing these memories back to life, back to their labile state, then the concurrent introduction of new information could bring new information into that memory.
AW: Why did you choose to employ optogenetics to test learning and memory?
Steve Ramirez: Optogenetics gives unprecedented control over certain sets of cells. No tool before optogenetics could act on the millisecond timescale that your brain uses, except for maybe electricity, but the problem is that if you want to target a specific set of cells within the hippocampus that support a certain memory, targeting electricity to just those cells is almost impossible, because electricity is the main thing the brain uses to communicate already. If you go there and indiscriminately zap part of the brain, you’ll be zapping a lot more than just that memory. The nifty part about optogenetics is that brain cells don’t normally respond to light, so in this case, the only ones that respond to light are the ones that you trick to respond to light. In [this] case, you can actually shoot light into the brain, leave neighboring cells intact, and the only ones that will be responsive are the ones that you selected already. So you’re probing the brain on the timescale that it communicates on, which is the “ace in the hole” that makes optogenetics the amazing thing that it is.
AW: What implications does your work using optogenetics in the mouse brain have for the human brain and our understanding of learning and memory?
SR: The big thing here is that we were able to demonstrate that we have rapid, optical control over a defined set of cells which gives us control over a certain memory. The fact that we can even do that is a testament to the power of science. The ramifications for humans is that [this] one idea is powerful. We now have defined control over a defined set of cells that lead to the recall of a certain memory. The only thing stopping the field from doing [similar experiments] in non-human primates or in humans is technological adaptation.
We were able to demonstrate that we have rapid, optical control over a defined set of cells which gives us control over a certain memory.
The question now isn’t “Can we reactivate a memory?” because that answer is “Yes”. The question now is “How can we scale that up? How can we do that in non-human primates? And in an ethically considerate way?” But it’s no longer a question of “if” – it’s a question of “how do we do it” now because we know that we can – at least in mice.
AW: Do you picture optogenetics being used in humans in, say, 10 years?
SR: It’s already being used in nonhuman primates. I see it being used if the right question is asked – so let’s say seizures, for example. We know that sometimes, certain kinds of seizures have an original focal point in the brain. A little ball of brain cells will start firing spastically and out of control. It kind of dominoes and takes over the entire brain and you have a seizure. Now, what if you could target light to the problematic patch? Historically, you would have to go in and scoop it out and get rid of that neural tissue. What if you could make that little problematic patch light-sensitive and have some kind of light-emitter that could reach that patch, detect when it’s about to go above a threshold [meaning] it’s going to give a seizure, then you shut it off? That’s not straightforward, but conceptually, it’s easy to implement.
Things like reactivating and inactivating memory is, for me, a whole other beast. In principle, [it’s] possible, but the technology doesn’t exist yet. I’d love to see it implemented and it’s just one revolution away from happening, but currently, I don’t think it’s possible. The best thing we have to peer inside of you is an fMRI and that, we can only see at a resolution of 10’s of 1000’s of brain cells, whereas it’s hard to find a memory amidst that sea of brain cells. The technology isn’t there yet.
AW: Are there any potential applications of optogenetics for other cells inside the human body, but outside the brain?
XL: People are trying to develop all types of tools that can apply to other types of cells, not just neurons. You can use light to control the transcription and the translation of genes, or other molecular signaling pathways in the cells. By doing so, you might be able to apply light controllable manipulation to not just neurons in the brain, but also to other cells. In this, there might be a treatment of cancer or diabetes. Currently, people are developing more and more tools to expand the scope of optogenetics. That’s what I see as a very exciting beginning.
AW: What is the most frequent misconception about your work and optogenetics research in general?
SR: First, there’s a lot of sensationalism, especially surrounding our 2012 paper because we created false memories. One of the biggest confusions over our paper was that we’re “implanting” false memories by creating a memory from the ground up, that we’re creating it of the brick and mortar of “memory stuff”. It’s more that we’re recombining elements that already exist in the brain.
Also, the relationship between that experiment with mice and false memories in humans is unclear. There are ways of seeing how much they relate with one another, but there [was] a lot of sensationalism [saying], in the coming years, this could be used for humans. That’s a misconception because never does work translate that fast or that perfectly across the evolutionary ladder. There are so many leaps and bounds that have to be done from working in mice to in humans, and that’s not including the ethical ramifications of it. People who are quick to draw that parallel between mice and humans think that it’s going to happen overnight, but it’s not. In principle, possibly, but not any time soon.
AW: What was the most challenging part of your research using optogenetics to study learning and memory in mice?
XL: When you have a new concept, sometimes, it’s so new that the field will have some resistance. For example, when we first proposed this as a grant proposal and sent it to NIH trying to get funding, actually, at that time, that proposal was turned down because people had doubts. People did not think this would work. But luckily, with strong support from Dr. Susumu Tonegawa, he said that this was a good idea and that we should work on it. And within 2 years, we actually published 2 excellent papers out of this.
Sometimes, when a new idea comes up, you will face some resistance. Now that it’s possible, people in other labs all over the world are doing experiments like this. So you open a window, but when you open the window, everybody jumps in. You need to think ahead of the crowd. You need to think “What do I do next?” Clearly, you cannot be doing exactly the same thing. The fun part is gone if you don’t. You have to identify new targets, new areas to work on.
SR: Every scientist has a different philosophy on this. Xu and I could spend our careers characterizing reactivating a fear memory in the dentate. But for us, I wouldn’t say it would be boring, but it wouldn’t be as exciting anymore. After the reactivation part, we could have tested “Can you reactivate a fear memory in brain areas A, B, C, D, E, F, G?” Bam, that’s the entire career. Some people do do that. But for me and Xu, the next questions were the false memory questions. Now, the next questions are the positive memory questions, the depression questions, the social memory questions.
It’s not our style to ask the small questions that are just going to characterize a tiny bit of what this memory might look like. It’s more of “What’s the next big question that could potentially open up a whole new novel way of studying this field?” Those are super exciting. They’re hard and they’re difficult, and they’re really hard to execute and there’s a lot of resistance, absolutely. But they’re the ones that give you the biggest jolt of adrenaline when you make positive contact with reality and you discover something and you’re asking a question that big.
AW: Is there anything more you would like to add?
SR: The one thing I would advise to anyone is that these seemingly sci-fi-y questions that we asked and to some extent executed in mice was the result of a giant collaboration. I know how competitive and how tough the academic environment can be nowadays, but it doesn’t have to be that way. The reason why these projects worked out at the speed that they worked out at is that Xu and I are team players. We helped each other out so much for both of these projects. It was a giant, team-oriented endeavor. That made it fun, first and foremost, and it made it go twice as fast–two brains are better than one. Now, there’s a whole team working on this project. I want to carry through that team oriented spirit for the rest of my career because I’ve had too much fun doing it.
XL: You can hold onto your own little secret, but you can hold onto it for only so long. We have our so-called “Team X” expanding tremendously. [Here], you have people using similar methods, but asking creatively different types of questions or questions that we have never even considered before. Everybody had brilliant ideas, but if we put our ideas together, we can collaborate and that will create many more exciting moments for us.
About Xu Liu and Steve Ramirez
Xu Liu received his Ph. D. from Baylor College of Medicine, Houston, Texas. During his Ph. D., he studied the mechanism of learning and memory using fruit fly as a model system. By changing the expression of certain genes in the fly brain, he generated smart flies that can learn many times faster than their peer. Using live imaging, he also detected learning induced changes in the brain cells and observed memory formation inside the brain with light. After graduation, he moved to MIT and joined Dr. Susumu Tonegawas lab as a postdoctoral associate. He continued his pursuit of memory with light there. Instead of just watching memory formation, he developed a system in mice where one can not only identify and label cells in the brain for a particular memory, but also turn these cells on and off with light to activate this memory at will. His work has received coverage by media worldwide, including The Boston Globe, Scientific American, Science & Vie, Daily Mail, and The Guardian. (Photo: Josh Sarinana; Bio courtesy TEDxBoston)
Steve Ramirez is a graduate student at MIT’s Brain and Cognitive Sciences department pursuing a Ph.D. in neuroscience. His work focuses on finding where single memories are located throughout the brain, genetically tricking the brain cells that house these memories to respond to brief pulses of light, and then using these same flickers of light to reactivate, erase, and implant memories. The goals of his research are twofold: to figure out how the brain gives rise to the seemingly ephemeral process of memory, and to predict what happens when specific brain pieces breakdown to impair cognition. In the past 2 years, his projects have been covered by New Scientist, Discover, Scientific American, and Gizmodo. (Bio courtesy TEDxBoston)
- Liu, X., Ramirez, S., Pang, P. T., Puryear, C. B., Govindarajan, A., Deisseroth, K., & Tonegawa, S. Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature, 484(7394), 381.
- Ramirez, S., Liu, X., Lin, P. A., Suh, J., Pignatelli, M., Redondo, R. L., … & Tonegawa, S. (2013). Creating a false memory in the hippocampus. Science,341(6144), 387-391.
- Ramirez, Steve, Susumu Tonegawa, and Xu Liu. “Identification and optogenetic manipulation of memory engrams in the hippocampus”. Frontiers in Behavioral Neuroscience 7.226 (2013): 1-9.
- Deisseroth, K. (2011). Optogenetics. Nature methods, 8(1), 26-29.
- Boyden, E. S., Zhang, F., Bamberg, E., Nagel, G., & Deisseroth, K. (2005). Millisecond-timescale, genetically targeted optical control of neural activity. Nature neuroscience, 8(9), 1263-1268.
- Miesenböck, G. (2011). Optogenetic control of cells and circuits. Annual review of cell and developmental biology, 27, 731.
- Optogenetics: Controlling the Brain with Light (2010), Karl Deisseroth, Scientific American.
- Xu Liu and Steve Ramirez at TEDxBoston 2013
- Brief video on optogenetics from MIT’s McGovern Institute for Brain Research
- Edward Boyden at TED 2011
- Gero Miesenboeck’s at TEDGlobal 2010
- Brief video on optogenetics from Nature
Disclosure: Anne West previously interned with Xu Liu’s Lab.