OVERVIEW

Optogenetics and the future of

Edward S Boyden Over the last 10 years, optogenetics has become widespread in neuroscience for the study of how specific cell types contribute to functions and brain disorder states. The full impact of optogenetics will emerge only when other toolsets mature, including neural connectivity and cell phenotyping tools and neural recording and imaging tools. The latter tools are rapidly improving, in part because optogenetics has helped galvanize broad interest in development.

It was precisely 10 years ago that this journal timescale pulses of light, without the need for the use of light-sensitive chloride pumps to published a study1 describing how a microbial chemical supplementation. This of course mediate neural silencing with light6,7. Over , a natural light-sensitive ion-transport- was serendipitous: there was no a priori rea- the last few years, the optogenetics toolset has ing membrane protein, could be expressed in son for an algal protein to successfully express expanded to include molecules that mediate to make their electrical activity con- and operate in mammalian neurons. This ser- neural activation with green and even red trollable by light. This special issue of Nature endipity was the result of the intersection of light8–10, neural silencing with red light11, neu- Neuroscience reflects on how this field, now decades of basic curiosity about how microbial ral silencing via light-driven chloride conduc- known as optogenetics, has developed over the proteins use light for energy storage and sen- tances12–14, and high speed and light-sensitive ensuing decade. The journal has asked scien- sation, and modern neuroscience-question- neural activation10, among other advances. tists for their personal thoughts on the past, driven technology need. Arguably, the optogenetic tools are matur- present and future of optogenetics in neuro- Another reason for skepticism may have ing, and in some cases may even be near their science, in a Q&A2 and in a Commentary by been that many technologies for neuroscience physical limits. Usage of optogenetic tools Nature America, Inc. All rights reserved. America, Inc. © 201 5 Nature Deisseroth3. Here I reflect on the impact opto- had been published in the years before, some spread throughout the genetic model organ- has had on neuroscience, not just in of which were not robust nor easy to use, and isms of neuroscience—, the answering of specific scientific questions skepticism about technology seemed, to me melanogaster, zebrafish, mice—and npg but on the direction of the field as a whole. anyway, to abound in neuroscience. “Don’t later nongenetic model organism such as non- The study, which originated in ideas and develop tools, just learn to answer scientific human primates. experiments generated by and questions,” one person advised me when I What have we learned from optogenetics? myself, collaborating with and first arrived at Stanford in 1999. Thus, there As far as direct scientific impact goes, one and later with the assistance was some question about the actual utility of could argue that no major paradigm shift in of , was not immediately a smash optogenetics. In addition, neuroengineering neuroscience has resulted from the use of opto- hit. Rejected by Science, then Nature, the dis- was not the fast-growing field that it is now. genetic tools in their first 10 years—in com- covery perhaps seemed too good to be true. There was no BRAIN initiative (http://www. parison with, say, the discovery of neurons Could you really just express a single natural whitehouse.gov/share/brain-initiative), and in the first place, or of synaptic release. What algal gene, -2 (ChR2)4, in one could argue that neurotechnology devel- optogenetics has done so far is make the study neurons to make them light-activatable? Our opment was not then a fully respectable profes- of circuits more tractable, especially when paper showed that the gene product was safe sion. Indeed, after our first optogenetics paper it comes to causally probing what a in neurons, and had appropriate kinetics and appeared, my faculty job search was hit-or- means to a circuit, since neural activation and current amplitude to cause neurons to fire miss. The MIT Media Lab, which had decided silencing enable the testing of the sufficiency action potentials upon delivery of millisecond- that one of its mandates was to hire eccentric and necessity of specific cell populations in misfits, thankfully took a chance on me. the generation of behaviors and pathologies. I’ve discussed the early history of the opto- Optogenetics has also greatly helped with the MIT Media Lab and McGovern Institute, genetics toolset previously5. In the years fol- linkage of cellular types and mechanisms to cir- Departments of Brain and Cognitive Sciences and lowing our first paper, the toolset rapidly cuit- and systems-level emergent phenomena. Biological Engineering, Massachusetts Institute of expanded, and adoption of the tools began But we still cannot fully explain any sensation, Technology, Cambridge, Massachusetts, USA. to spread—first slowly, then faster and faster. behavior, emotion, movement or cognitive e-mail: [email protected] In 2007, neural silencing was achieved with process. Some of the studies done in the first

1200 VOLUME 18 | NUMBER 9 | SEPTEMBER 2015 NATURE NEUROSCIENCE OVERVIEW

10 years of optogenetics tested old hypoth- digm. The ability to record activity and, in might be used in the clinic. As we have only eses, tying up loose ends. As the toolset gained real time, to responsively compute specific rudimentary lists of cell types of the human adoption, work exploring new pathways and patterns of illumination to be delivered to brain and their functions, solid rationale from sets of cells and functions has become more neurons in the brain would enable detailed basic science experiments is required to define prominent. Of course, 10 years is not a long studies of neural dynamics that could be more the precise neural substrates that could serve time in science terms, and we are just at the powerful than open-loop control. We are now as clinical targets in humans. Perhaps not beginning. starting to see the creation of strategies for surprisingly, the detailed knowledge of the Future optogenetics experiments, especially simultaneous recording and perturbation of cell types and wiring of the retina has helped performed in conjunction with other new neural ­dynamics—for example, in the con- make photoreceptor-loss disorders such as technologies, may realize the dream of fully text of silencing seizures in mouse models of into early candidates for understanding neural circuits with single-cell ­epilepsy20 or for precisely causing cells to fire in optogenetics treatment25. Of course, without precision. For example, many optogenetic specific patterns regardless of the inputs being human trials it is impossible to know whether studies have activated or silenced neurons as received by those neurons21. As optical three- microbial gene products will be well tolerated populations, engaging them synchronously dimensional neural activity imaging methods in the human body, especially over long tim- as an ensemble. Of course, in the brain even become faster in their volumetric acquisition escales, and thus fundamental risks remain adjacent neurons of the same kind can have speeds22,23 and as fluorescent reporters of neu- to be resolved. But it is clear that the results very divergent activity patterns, raising the ral activity also increase in speed, signal-to- emerging from the use of optogenetics in basic question of whether one could instead dial in noise ratio and fidelity24, one appealing goal is neuroscience, and from neurotechnology as a truly arbitrary, naturalistic activity pattern. the ability to image and control neural circuits a whole, will provide in the years to come a Recently, tools such as spatial light modula- continuously in real time, enabling all-optical variety of insights into new molecular targets tors have begun to enter more common use interfaces to neural circuits that can tease apart for drug development, new circuit sites for in neuroscience and are beginning to enable how individual cells work together to generate electrical brain stimulation, new protocols of optogenetics with single-cell-targeting preci- dynamics in real time. regenerative medicine, and other strategies for sion15,16. This technology improvement raises The aforementioned examples of tech- helping repair the brain. hope for individual control of multiple cells nologies that are enhancing the power of throughout neural networks. optogenetic tools hint at a broader impact of COMPETING FINANCIAL INTERESTS The authors declare competing financial interests: Many optogenetic studies have focused optogenetics: because optogenetics is so easy details are available in the online version of this paper. on classically defined cell types, such as to use and has spread so fast—in part because ­somatostatin-positive neurons or norepineph- the groups involved in optogenetic tool devel- 1. Boyden, E.S., Zhang, F., Bamberg, E., Nagel, G. & Deisseroth, K. Nat. Neurosci. 8, 1263–1268 (2005). rine-producing neurons, raising the question opment disseminated molecules freely to the 2. Adamantidis, A. et al. Nat. Neurosci. 18, XXX–XXX of whether it would be possible to control any neuroscience community—optogenetics has, (2015). desired kind of cell in the brain. Of course, through its utility to experimentalists, cata- 3. Deisseroth, K. Nat. Neurosci. 18, XXX–XXX (2015). 4. Nagel, G. et al. Proc. Natl. Acad. Sci. USA 100, complete cell taxonomies for almost all spe- lyzed a desire for more kinds of powerful tool. 13940–13945 (2003). cies and brain circuits do not yet exist, much Indeed, neuroscience studies performed in the 5. Boyden, E.S. F1000 Biol. Rep. 3, 11 (2011). less good genetic handles that enable specific first decade of optogenetics have often helped 6. Han, X. & Boyden, E.S. PLoS ONE 2, e299 (2007). 7. Zhang, F. et al. Nature 446, 633–639 (2007). targeting of those cell types for gene expres- to raise awareness of, and sometimes even 8. Yizhar, O. et al. Nature 477, 171–178 (2011). Nature America, Inc. All rights reserved. America, Inc. © 201 5 Nature sion. Recently, many groups have begun to accelerate development of, other toolsets to 9. Lin, J.Y., Knutsen, P.M., Muller, A., Kleinfeld, D. & Tsien, R.Y. Nat. Neurosci. 16, 1499–1508 (2013). develop strategies for enumerating, describing meet complementary needs in neuroscience. 10. Klapoetke, N.C. et al. Nat. Methods 11, 338–346 and achieving molecular handles on cell types, This cultural shift in neuroscience toward (2014). npg ranging from new ways of assessing cell genetic celebrating tool development has resulted in 11. Chuong, A.S. et al. Nat. Neurosci. 17, 1123–1129 17 (2014). programs and transcriptomes to new ways of efforts like the US BRAIN initiative, which 12. Berndt, A., Lee, S.Y., Ramakrishnan, C. & Deisseroth, K. describing neural morphologies and molecular aims to catalyze new , and Science 344, 420–424 (2014). 18,19 13. Wietek, J. et al. Science 344, 409–412 (2014). locations . If complete descriptions of cell the creation of many centers and institutes 14. Govorunova, E.G., Sineshchekov, O.A., Janz, R., Liu, X. & types are achievable, and in particular if selec- for neuroengineering throughout the world. Spudich, J.L. Science 349, 647–650 (2015). tive genetic handles that allow specific cells to This indirect impact of optogenetics, making 15. Papagiakoumou, E. et al. Nat. Methods 7, 848–854 (2010). be targeted for are found, it the neuroscience world safe, as it were, for 16. Andrasfalvy, B.K., Zemelman, B.V., Tang, J. & Vaziri, A. might be possible to optogenetically activate tool development has helped create a sense of Proc. Natl. Acad. Sci. USA 107, 11981–11986 (2010). or silence any specific kind of neuron that is optimism and confidence that new technolo- 17. Macosko, E.Z. et al. Cell 161, 1202–1214 (2015). 18. Chen, F., Tillberg, P.W. & Boyden, E.S. Science 347, hypothesized to be involved in a behavior or gies can assist in making the big mysteries of 543–548 (2015). a disease. If connectivity maps of neural cir- the brain more tractable. 19. Peng, H., Ruan, Z., Long, F., Simpson, J.H. & Myers, E.W. Nat. Biotechnol. 28, 348–353 (2010). cuits can be derived, that may help provide new The impact of optogenetics in basic neu- 20. Krook-Magnuson, E., Armstrong, C., Oijala, M. & hypotheses of neural circuit functions that can roscience has been significant, and it will Soltesz, I. Nat. Commun. 4, 1376 (2013). then be causally tested with optogenetics. probably only grow as the aforementioned 21. Newman, J.P. et al. Optogenetic feedback control of neural activity. 4, e07192 (2015). Many optogenetic studies have been focused synergistic tools yield, in the years to come, 22. Prevedel, R. et al. Nat. Methods 11, 727–730 (2014). on open-loop control of neurons, activating or potentially near-complete powers for map- 23. Ahrens, M.B., Orger, M.B., Robson, D.N., Li, J.M. & silencing cells or sets of cells without involving ping, recording the dynamics of, and control- Keller, P.J. Nat. Methods 10, 413–420 (2013). 24. Chen, T.-W. et al. Nature 499, 295–300 (2013). neural recording in conjunction with real-time ling the dynamics of neural circuits. An open 25. Chow, B.Y. & Boyden, E.S. Sci. Transl. Med. 5, 177ps5 data analysis to sculpt the stimulation para- question remains as to how optogenetic tools (2013).

NATURE NEUROSCIENCE VOLUME 18 | NUMBER 9 | SEPTEMBER 2015 1201