Unexpected pairings
19.10.2021
Being able to precisely turn on or off particular neurons in the brain at will was a major challenge for the neuro- science field, and few could have anticipated that the solution would come from algae. The 2021 Albert Lasker Basic Medical Research Award recognizes the contributions of NeuroCure PI Peter Hegemann, Dieter Oesterhelt, and Karl Deisseroth for their discovery of light-sensitive microbial proteins that can activate or silence brain cells. Cell editor Nicole Neuman had a conversation with Peter Hegemann about his role in bridging the two fields of microbial phototaxis and neuroscience and his perspective on the nature and future of interdisci- plinary science. Excerpts from this conversation are presented below, and the full conversation is available with the article online
Nicole Neuman: You’re sharing this award with Dr. Karl Deisser- oth and Dr. Dieter Oesterhelt. Can you tell me about the signifi- cance of being awarded with these two?
Peter Hegemann: This is a very special award. First of all, because it’s one of the highest American awards in medical- related basic science. Second, to receive the prize together with my long-standing colleague and friend, Karl, which is something extraordinary, and with basically my only teacher, Dieter Oesterhelt, is certainly a special joy and pleasure for me.
NN: The award itself is for not only the discovery of light- sensitive microbial proteins, but also for the use of these proteins to activate or silence individual brain cells, better known as optogenetics. That’s a big leap from basic discovery to application. Can we go back to the beginning of the story for you? Tell me about the beginning of your career and what scientific questions you were interested in.
PH: So, the discovery of bacterial rhodopsin, which was the first light-activated microbial rhodopsin and the first light- activated ion pump, revolutionized the topic of membrane protein research. The advantage of bacteriorhodopsin, which was discovered by Dieter, was that it is available in large amounts. By using bacteriorhodopsin, many different technologies have been introduced and modified for membrane protein research over the last 30 years. Among these technologies are membrane protein sequencing. The first membrane protein to be sequenced, and to be cloned, was bacteriorhodopsin. It was also used for the first electron microscopy structure for the first membrane protein infrared spectroscopy and the first Raman spectroscopy for membrane proteins.
This protein has established many, many methods that later on were transferred to the light-activated ion channels that we discovered. So, Dieter was the first in the world [to discover a light-activated microbial rhodopsin], and then my group discovered the channelrhodopsin (that is, the light activated ion channel), and then Karl transferred these technologies to the neuroscience world. I think it’s a fantastic selection, and I’m certainly very proud to be part of this.
NN: So, your piece in this was in characterizing the channelrhodopsins. Tell me about any challenges or hurdles that you had to overcome in characterizing them.
PH: I had some experience from my time in Dieter’s lab because my PhD project was to work on the light-activated sodium pump. The hypothesis was wrong, and it turned out to be a chloride pump, which is now named halorhodopsin and is also used in optogenetic projects. However, when I finished my PhD, I only could stay as an independent researcher if I would do something new, which is not correlated to Dieter’s work.
There was a publication in ‘84 from Ken Foster’s lab, and he proposed that also in the plant world, rhodopsin would be present and it would serve as photoreceptors for phototaxis— so, to mediate the movement responses and these movement responses in green algae, which had been studied already for more than 100 years.
There is a publication from Famintzin, a researcher in St. Petersburg in the nineteenth century, that described the behavior pretty well. Almost a hundred years passed and none of the plant physiologists were able to identify the photoreceptor until Ken Foster proposed it was a rhodopsin. So, I spent a year in Ken’s lab to characterize the photoreceptor, but it turned out to be much more difficult than we expected. Then I came back to Martinsried, to the Max Planck Institute, where I worked before in Dieter’s lab, and I got my independent small research group. It was very generous of Dieter to let me work independently. The big question was how can we further characterize it, and we realized very quickly that we needed electrophysiology. However, there was no electrophysiology available at that time for green algae.
So, we had to establish electrophysiology, but this was a biochemistry institute, and no one really did electrophysiology. I got personal support from Dieter Lukas, a very famous Max Planck director at the psychiatry institute next door. He was very interested in this project, surprisingly, because he worked normally on neurons. My second graduate student Hartmann Harz was able to monitor electrical responses from this alga. That was the first big barrier: that is, how to establish electrical technology for this alga. It was also strange because normally these algae have a large cell wall. One of my Chlamydomonas colleagues, Ursula Goodenough from St. Louis, enthusiastically supported us because all other members of the community said it will never work—but she gave us a cell wall- deficient mutant, and only this mutant allowed us to record electrical responses from this alga.
That was one of the biggest breakthroughs we ever made. Surprisingly, these electrical responses were ultrafast. We characterized these electrical responses over many years, and we came up with a hypothesis that the rhodopsin (which is a light sensor) and the ion channel are forming one unit. That was a very difficult hypothesis because all of the rhodopsin researchers were saying, ‘‘This is impossible, an ion channel and the rhodopsin are something totally different.’’ It was difficult to overcome this criticism, and only after we identified the protein could we prove the hypothesis. This was done together with Georg Nagel because he had established the technology in his laboratory. We had proven the hypothesis and what came out was that these photoreceptors are light-gated ion channels.
This took out about 15 years between the first electrical measurement and the identification of the protein. That was a long, long way, and there were only a few reviewers that supported us in this direction. Also, to get this funded was very difficult because electrophysiology on green algae was not the most fashionable topic. Then the next important step was that Georg and I expressed this in human embryonic kidney. That was the kind of driving flame that encouraged other neuroscientists or the neuroscience community in general to try this in many different organisms.
NN: So were you thinking about the utility of channelrhodopsins when you first sought to identify them and isolate them?
PH: That was another difficulty. First, we tried to purify this protein biochemically. We purified a retinal binding protein and suggested that this was the ion channel that we were looking for because it was more hydrophilic than other membrane proteins but had a signature that was related to rhodopsin. It was also amazing. There’s only one researcher who was interested in this protein and this was Roger Tsien, and he said, ‘‘This is a very interesting protein, and it might have an enormous impact on the neuroscience sphere, if you are able to express it in a neuron.’’ Maybe the sad situation for him was that it was the wrong protein. So, the first protein we purified and gave to Roger was not the photoreceptor that we were looking for, and it took another 2 or 3 years until we identified the correct one. Then a couple of researchers started to work with this, and the first one who successfully excited the neuron was Karl Deisseroth. He was the first who demonstrated that you can fire action potentials with light rays and the action potential is following the light stimuli.
NN: One thing that’s really struck me about the origin story of optogenetics is that it has involved collaborations and contributions from scientists across multiple different fields. Tell me about your experience working in this kind of interdisciplinary science. How have you made that so productive and successful?
PH: I think in the early phase it was more difficult. People were not so enthusiastic about it, because no one really was convinced that this research will work. I think the only people who supported us at that time were Ursula Goodenough from the plant community and Dieter Oesterhelt from the Max Planck. They believed in us and supported us at least to some extent. Ursula gave us the right clone or the right strain, and Dieter supported us with equipment and support. These were the two most important people at that time. So, to work interdisciplinarily was easier after the neuroscience community had established the channelrhodopsin for their own use. Karl contacted me a year later after he published his first paper and said the biophysics that we do and the neuroscience that he does is nicely complimentary. So, over many years we have done a lot of engineering together on the channelrhodopsin.
NN: You mentioned how special it is to share this award with Dieter and from the sound of things, he’s been a mentor to you and supportive to you throughout your entire career. How has that affected your approach?
PH: First of all, when I started chemistry, I always wanted to go into biochemistry, and the Max Planck Institute in Munich is certainly one of the best institutes you can go to [for that] in Germany. What I learned about bacteriorhodopsin [working with Dieter] was very useful later for the characterization of the channelrhodopsin because these proteins are indeed related. With Karl, we engineered the channelrhodopsin to make faster and slower mutants. In this case, the basic knowledge about microbial rhodopsin that I was exposed to in Dieter’s lab over many years was certainly very helpful. It was unexpected because in the paper from Ken Foster, he proposed that this is an animal-type rhodopsin, which at the end was not true. So, for me it was an advantage when it came out that these channelrhodopsins from Chlamydomonas are a microbial rhodopsin, with a relationship to bacteriorhodopsin, that Dieter discovered.
NN: Do you see a potential for optogenetics outside of neuroscience? What do you think the full potential of this core technology is of using exogenous light driven ion channels to control cell function? Where’s the limit?
PH: You never know where the limits are. The limits are set by the next generation. They will probably find things out that we cannot imagine, in a similar way as we did. Very recently the channelrhodopsin has been successfully applied to plants. That is a new development, because plants do not have retinal. To co-express the channelrhodopsin with a dioxygenase, that cleaves beta-carotene into retinal, was a kind of breakthrough, and now it is nicely used in the plant world as well. So, you never know what will happen. All the experiments the past 10 years in plants were not successful, until someone came up with the right solution. This was done in a collaboration between Rainer Hedrich and Georg Nagel.
So, there are new developments always in sight, and probably the next application will be in hearing, to cure deafness. No one knows whether it will work or not, but it looks very promising. So, there might be applications in very different fields; plant science and the human brain are pretty far apart, but still it depends on the ideas of individual researchers and how they move the field forward. If you ask me for the perspective of the next, let’s say 10 or 20 years, I’m sure that other media will be involved, because light is not ideal for the human brain because it’s too large and dark. Probably the next generation of researchers will use different media—ultrasound, radio waves, magnetism—to activate proteins in the brain. You never know.
NN: You started your research with a very basic biological question in algae, and this eventually led to a transformative applied technology. In the current era of science, do you think that there are more of these basic discoveries to be found that will be similarly transformational? And if so, where do you think we will find them?
PH: It’s very hard to predict, but for example, yesterday in our student seminar, we talked about magnetoreception in birds. They fly from north to south and back by using their magnetic compass, and this is a topic that is under investigation for at least 30 years, but no one identified the protein which is used as a compass. So, if this is elucidated or will be elucidated in the future, maybe you can use this magnetic system for other purposes in biology. It would be very interesting. I think we shouldn’t be too shortsighted. [The next transformative basic science goal] is not to optimize the channelrhodopsin. This is easy, and I’m sure another 500 channelrhodopsins will be found in the near future, but is that what we need? Probably not.
NN: In many ways optogenetics is one area that’s really emblematic of how understanding the biophysical mechanisms of a process can help speed the progress of science. What role do you see for biophysics in 2021?
PH: During the past, let’s say 10 or 20 years, we certainly made use of genomics and proteomics to analyze large data collections. This also brought up on the table, several hundred channelrhodopsins now and even thousands of rhodopsins, but to understand the difficulties of the mechanisms, biophysics is absolutely necessary. So, one thing is we need a structure. The structure of the channelrhodopsin was solved in a collaboration with Osamu Nureki from Japan, but maybe we have to start going into the biophysics to understand the details. Then we might go back to the organism and do the mutations that we did in vitro again in vivo to understand why the channelrhodopsin is like it is. In the past 10 or 20 years, we asked how the channelrhodopsin functions. Probably the upcoming years, by combining biophysics and basic biology, we could ask, ‘‘why it is that way?’’ Why do some either use a cation conducting channelrhodopsin and others use anion channelrhodopsin?
It has to do with the environment, ecological situation, and many other things. We can learn from this and also transfer this information back to the application field. That is, go back to the neuroscience world and talk to the colleague, and say, ‘‘Under this and this conditions, you need this ion selectivity and for the others you need that. To cover a large range of light intensity, you need small conductance, and if you want to depolarize fast at dim light, you need a large conducting channelrhodopsin.’’ So it depends on the different purposes and communication between biophysics on one hand and the plant physiology on the other. I was educated as a chemist, then I studied biochemistry, and then I worked in plant physiology for a couple of years. First, I became a biochemistry professor in Regensburg and later, I got a biophysics professorship here in Berlin, and now I’m a neuroscience professor. This drive and this journey from one to the other field was quite fun. I met many, many, highly interesting individuals, each totally different, and I very much appreciate that the Lasker Foundation also awarded the prize to three very different individuals that contributed, from totally different perspectives, to the development of the optogenetics.
Source: CellPress Interview
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.cell. 2021.08.009.