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Optogenetics and Its Potential for Use in Research and the Clinic Brain Stimulation (2011) 4, 1–6 www.brainstimjrnl.com ORIGINAL ARTICLES A new technique for controlling the brain: optogenetics and its potential for use in research and the clinic Ryan T. LaLumiere Department of Psychology, University of Iowa, Iowa City, Iowa The recent development of optogenetic techniques has generated considerable excitement in neuroscience research. Optogenetics uses light to control the activity of neurons which have been modified to express light-sensitive proteins. Some proteins, such as channelrhodopsin, are cation channels that produce depolarization of neurons when illuminated. In other cases, neuronal activity can be inhibited through illumination of proteins, such as the chloride pump halorhodopsin, that hyperpolarize neurons. Because these proteins can be selectively expressed in specific cell types and/or in specific locations, optogenetics avoids several of the non-specific effects of electrical or pharmacological brain stimulation. This short review will explain the physiology of this technique, describe the basic and technical aspects of the method, and highlight some of the research as well as the clinical potential of optogenetics. Ó 2011 Elsevier Inc. All rights reserved. Keywords channelrhodopsin; halorhodopsin; ChR2; NpHR Over the past few years, optogenetics has begun to optogenetics and the potential for its use in research as revolutionize the field of neuroscience as well as opening well as in the clinic. up new possibilities in the field of brain stimulation. The optogenetic technique uses light to control the activity of neurons expressing light-sensitive proteins, thus enabling Physiology of light-sensitive proteins those using the technique to avoid the nonspecific effects of for control of neuronal activity electrical stimulation or pharmacologic manipulation. Moreover, because the expression of the proteins is genet- In the first report on the use of the light-gated cation ically targetable, optogenetics limits the population of channel channelrhodopsin-2 (ChR2; Figure 1 for a depiction neurons that respond to the light, providing an important of ChR2), Boyden et al.1 showed that pulses of blue light at level of resolution. This review will provide a brief approximately 470 nm wavelengths induce spikes of action explanation of the physiology and technical aspects potentials in cells expressing the ChR2 protein. Hippo- involved in this technique and address recent uses of campal neurons expressing these channels can follow trains of light pulses up to 50 Hz with 95% fidelity. 2 Importantly, long-term ChR2 expression does not appear to alter the Correspondence: Ryan T. LaLumiere, Department of Psychology, 11 basal physiology of transduced neurons or show evidence Seashore Hall E, University of Iowa, Iowa City, IA 52242. of toxicity,1 suggesting that neuronal expression of ChR2 E-mail address: [email protected] Submitted June 17, 2010; revised September 3, 2010. Accepted for produces optically controllable, physiologically relevant publication September 24, 2010. systems. Studies in mice with retinal photoreceptor 1935-861X/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.brs.2010.09.009 2 LaLumiere Na+ Cl- Channelrhodopsin Halorhodopsin Figure 1 Channelrhodopsin (left, in blue) is a light-sensitive, membrane-bound cation channel that opens and closes with the onset and offset of w470 nm (blue) light, respectively. When activated, channelrhodopsin produces depolarization of the expressing neuron, leading to temporally precise control of action potential firing. Halorhodopsin (right, in yellow) is a light-sensitive, membrane-bound chloride pump that, like channelrhodopsin, works with excellent temporal fidelity to the onset and offset of w580 nm (yellow) light. When activated, halorhodopsin hyperpolarizes the neuron, preventing action potentials. degeneration have demonstrated that virally delivered inactivation that can occur with NpHR.8 As new opsins ChR2 in retinal cells restores the ability of the neurons to are developed, they will continue to expand the kinds of transmit information to the visual cortex in response to light questions that are tractable with optogenetics. in a safe, stable long-term manner.3 Since the development of ChR2, other opsins have been identified or engineered and used to control neural activity. Technical aspects in the use of optogenetics Halorhodopsin (NpHR) is a light-sensitive chloride pump (see Figure 1 for a depiction of halorhodopsin) that, when Opsin expression activated, provides significant hyperpolarization of the transduced neuron.4 Although early work with NpHR Transgenic animals can be created with opsin expression demonstrated problems with efficacy and intracellular under the control of general or specific promoters,9,10 aggregates with high levels of NpHR expression, alterations though, with cell class-specific promoters, the protein to the NpHR protein have produced second generation expression is often weak. To achieve the high opsin expres- (eNpHR) and third generation (eNpHR3.0) NpHRs that sion levels necessary for optogenetic control, a mouse with do not produce aggregates and have improved hyperpola- the transgene Cre-recombinase under a cell-specific rizing functioning.5 NpHR has the advantage of being promoter can be used. This system then uses a viral vector sensitive to a wavelength spectrum with a peak at approx- that transduces the neuron with the opsin gene, requiring imately 580 nm, producing minimal overlap with the the action of Cre for opsin expression.11 This system optimal wavelength spectrum for stimulation of ChR2 enables the opsin itself to be under the control of a strong (w470 nm). As a result, it is possible to transduce neurons but general promoter, as the neuronal specificity is with both opsins and stimulate or inhibit neuronal activity conferred by the promoter for the Cre-recombinase and with pulses of light of different wavelengths.4 opsin expression cannot occur without the action of Cre. Considerable advances continue to be made in the This has permitted experiments in which the opsin expres- development of opsins for neurobiologic use. For example, sion is limited to dopaminergic neurons of the ventral optoXRs are optically controllable G-protein-coupled tegmental area.11 The major disadvantage of this method receptors that trigger traditional intracellular signaling is that it requires the use of transgenic animals, thus pathways.6 Recent engineering has produced a ChR2 limiting it to certain species and nonclinical use. mutant labeled ChETA that reduces double-spiking in For optogenetics in other species that are less tractable response to light, a problem with ChR2, and can follow for transgenic models, such as rats, or for potential human trains up to 200 Hz, far beyond the 40-50 Hz ceiling for therapeutic use, viral transduction of cells provides the best many cells expressing ChR2.7 Recent screening of micro- route for opsin expression. In particular, the specificity and bial opsins has led to the discovery and characterization localization of virally mediated expression would be highly of archaerhodopsin-3 (Arch) and the Leptosphaeria macu- desirable in clinical use. Adeno-associated virus (AAV) has lans opsin (Mac), two light-activated proton pumps that already been used to deliver genes into neurons of humans provide powerful inhibition of neurons without the as part of gene therapy,12-14 making AAV a highly feasible Optogenetics and neuroscience 3 A B M P X N Z Y O Figure 2 A, One method of targeting specific neurons using optogenetics uses cell-specific promoters that control the expression of the opsin. In this case, neurons X and Y are different classes of neurons but are located in the same structure. Thus, a virus containing channelrhodopsin and a promoter specific to neurons like X can be injected into the structure, but only neuron X will express channelrho- dopsin, enabling cell-type specific control of activity in the structure. B, In another method for gaining specificity, the structure on the left (in yellow) is transduced with a channelrhodopsin-containing virus but only the axon terminal region for neuron M is illuminated with the appropriate light. This method allows for selective control of a specific pathway without affecting other efferent pathways from the transduced structure (e.g., neuron N) or other afferent pathways entering the illuminated structure (e.g., neuron O). vector for delivery of opsins in future clinical studies. In used to those small enough to be packaged with the opsin optogenetics, viruses such as AAV or lentivirus (LV) can gene in the virus. be packaged with the transgenic elements containing the With optogenetic methods, one can thus stimulate sequence for the opsin as well as a promoter.15 Both viruses neurons of a particular cell type in a specific location produce long-term expression of the opsin protein. Full, (anatomy) with tremendous temporal precision and control. stable in vivo expression of the opsins in rodents is depen- This kind of precision outstrips traditional electrical dent on both vector and promoter choice and typically stimulation methods and has already provided valuable requires 1-3 weeks for the cell body and 6 or more weeks information on thorny neuroscience questions such as the for the distal ends of axons, depending on the distance possible mechanisms of action of deep brain stimulation and virus used.16 If the virus is administered via microin- (DBS) for Parkinson’s Disease (PD) and the origins of the jections
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