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. Author manuscript; available in PMC: 2016 Jun 1.
Published in final edited form as: Vision Res. 2015 Feb 11;111(0 0):134–141. doi: 10.1016/j.visres.2015.02.001

Photochemical approaches to vision restoration

Russell N Van Gelder 1
PMCID: PMC4444397  NIHMSID: NIHMS663143  PMID: 25680758

Abstract

Photoswitches are traditional pharmacologic agonists, antagonists, or channel blockers that are covalently modified with an azobenzene derivative. Azobenzene undergoes wavelength-dependent isomerization between cis and trans conformation. For some photoswitches, only one of these configurations is biologically active, resulting in light-dependent activation or inhibition of function. Photoswitches that feature a quaternary ammonium coupled to the azobenzene moiety cause light-dependent neuronal depolarization due to blockage of voltage-gated potassium channels. Two photoswitch strategies have been pursued. In the one-component strategy, the photoswitch is applied to native receptors; in the two-component strategy, the photoswitch is combined with virally-mediated expression of a genetically modified receptor, to which the photoswitch may covalently bind. The former approach is simpler but the latter allows precise anatomic targeting of photoswitch activity. Acrylamide-azobenzene-quaternary ammonium (AAQ) is the prototypical first-generation one-component photoswitch. When applied to retinas with outer retinal degeneration, ganglion cell firing occurs in response to blue light, and is abrogated by green light. In vivo, AAQ restored pupillary light responses and behavioral light responses in blind animals. DENAQ is a prototypical second generation one-component photoswitch. It features spontaneous thermal relaxation so cell firing ceases in dark, and features a red-shifted activation spectrum. Interestingly, DENAQ only photoswitches in retinas with outer retinal degeneration. MAG is a photoswitched glutamate analog which covalently binds to a modified ionotropic glutamate receptor, LiGluR. When applied together, MAG and LiGluR also rescue physiologic and behavioral light responses in blind mice. Together, photoswitch compounds offer a potentially useful approach to restoration of vision in outer retinal degeneration.

Acquired outer retinal degeneration associated with age-related macular degeneration is the leading cause of blindness in the United States and most of the developed world, while outer retinal degeneration associated with retinitis pigmentosa is the leading inherited cause of blindness. Blindness in both conditions results from irreversible loss of the rod and cone photoreceptors from the outer retina. The remaining retinal cell types – bipolar cells, horizontal cells, amacrine cells, Muller cells, and retinal ganglion cells – remain viable (although significantly rewired (Marc and Jones, 2003)).

In theory, if one could recapitulate the native temporal firing pattern of each retinal ganglion cell in response to a dynamic visual scene, one could ‘restore’ vision to a retina devoid of rods and cones. Progress has been made on several approaches to this goal. Embryonic or inducible progenitor stem cells can be differentiated to a retinal fate and transplanted; this approach restores some light sensitivity in mouse models of retinitis pigmentosa (Lamba et al., 2009). Second, external stimulation of retinal ganglion cells by opto-electronic prosthetics has also advanced significantly in the past decade, with Food and Drug Administration approval of the Argus II device, capable of restoring some visual phenomena when implanted in profoundly blind individuals (Ahuja et al., 2011; Humayun et al., 2012). Third, virally-mediated gene therapy approaches using optogenetics, including channelrhodopsin and halorhodopsin to excite and suppress retinal cells, respectively, have advanced in the past decade, and have also been shown to restore vision-like function to mice with advanced outer retinal degeneration (Tomita et al., 2007; Lagali et al., 2008; Ivanova et al., 2010b).

Collectively, these approaches offer substantial hope for vision restoration in the foreseeable future. However, all share a common feature of effective irreversibility. Once stem cells are placed in the subretinal space, or viral gene therapy vectors introduced into the eye, these alterations are effectively permanent. Similarly, it would be challenging to replace an optoelectronic prosthesis once surgically implanted in the eye. Given the barriers to demonstration of efficacy and regulatory approval of any prosthetic or pharmacologic approach, the irreversibility of these treatments makes iterative progress in this field challenging.

Over the past eight years, a new approach has been advanced for vision restoration. Compounds such as tetra-ethyl-ammonium which block voltage-gated potassium channels have been known for over a half century to induce neuronal cell firing when administered extracellularly (George and Johnson, 1961). By linking these agents to a photo-isomerizable moiety (azobenzene), novel compounds have been produced that can activate neurons in a light-dependent fashion. When applied to retinas with outer retinal degeneration, these compounds are capable of inducing light-dependent firing of remaining retinal ganglion cells. Such activation may form the basis for reconstitution of vision in eyes with outer retinal degeneration.

Azobenzenes

The light-isomerizable moiety utilized in these compounds is azobenzene (Figure 1). This class of compounds was among the first described in organic chemistry, having been characterized by Eilhard Mitscherlich in 1834. Azobenzenes are produced by reduction of nitrobenzene; originally this was accomplished with iron filings as catalyst (more recent methods utilize zinc). Azo dyes were extensively used throughout the late 19th and early 20th century in industry to color clothing and foodstuffs.

Figure 1.

Figure 1

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Chemical structure of azobenzene in trans and cis conformations

The property of azobenzene and its derivatives that renders it useful for photopharmacology is the photoisomerization of cis and trans isomers. The isomers can be interchanged with specific wavelengths of light. The trans-to-cis conversion utilizes short wavelength light (typically ultraviolet for unsubstituted azobenzene), while the cis-to-trans isomerization occurs under longer wavelength light (blue light for the unsubstituted azobenzene). The cis isomer is less stable than the trans and so cis-azobenzene will thermally convert back to the trans in dark. Photoisomerization of trans to cis azobenzene is extremely rapid (on the order of picoseconds), while the reversion in dark of cis to trans takes hours for unsubstituted azobenzene. However, substitution of electron-withdrawing groups can alter both the thermal stability of the cis-form, as well as the action spectrum for isomerization (Banghart et al., 2006). Because of these properties azobenzene derivatives have been used in a variety of nanotechnology applications ranging from surface holography, liquid crystal displays, and optoelectronics (Brehmer et al., 1997; Nakatsuji, 2004; Oliveira et al., 2005; Choi et al., 2007; Matharu et al., 2007).

The first successful application of azobenzene-mediated control of a potassium channel blocker was the development of the SPARK channel (Chambers et al., 2006). SPARK stands for synthetic photoisomerizable azobenzene-regulated potassium channel. In this approach, Chambers et al. synthesized a modified Shaker potassium channel (including an EGFP tag for visualization). In one version, the Shaker channel was left in its native, potassium conducting form (H-SPARK), while in another a point mutation converted the channel to a non-specific cation channel capable of conducting extracellular sodium (D-SPARK). The small molecule photoswitch employed was maleimide-azobenzene-quaternary ammonium (MAL-AZO-QA), with the latter moiety responsible for channel blockade (Figure 2). The maleimide moiety allowed for covalent coupling of the compound to a cysteine on the cell surface.

Figure 2.

Figure 2

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Structure of maleimide-azobenzene-quarternary ammonium (Mal-Azo-QA, or MAQ), the small molecule photoswitch used in the two-component SPARK channel system. A. Structure of MAQ in trans and cis conformation. B. Schematic of two-component blockade under trans conformation. C. Example of change in cell conductivity with light due to photoswitch activity. From: (Mourot et al., 2013) with permission.

SPARK-expressing CHO cells showed no change in channel function prior to addition of 300 μM MAL-AZO-QA, and light had no effect on these cells without presence of both transgenic channel and azobenzene-based photoswitch. However, after transfection and administration of MAL-AZO-QA, expected changes in the slope of the current-voltage relationship of patched clamped cells for both D-SPARK and H-SPARK were observed. Further, D-SPARK-expressing cells showed hyperpolarization when cells were changed from 390 nm light to 505 nm light, and depolarization with the opposite lighting change. The opposite effect was seen with H-SPARK channels (i.e. depolarization with the UV->blue-green lighting change). Cells expressing GFP only or without MAL-AZO-QA showed no changes. When tested in mammalian hippocampal cells in culture, cells transfected with D-SPARK and treated with MAL-AZO-QA showed light-dependent action potential firing in vitro. Taken together, these results established that an azobenzene-based small molecule potassium channel blocker is capable of inducing light-dependent cell firing in mammalian neurons.

The SPARK system constitutes a ‘two-component’ photoswitch, requiring both transgenic expression of a modified receptor and addition of the small molecule photoswitch. Subsequent research has pursued both this two-component approach, as well as a one-component approach which does not require genetically modified channels.

One component photoswitches

The first milestone in development of one-component photoswitches was the synthesis of AAQ (Fortin et al., 2008; Banghart et al., 2009; Fortin et al., 2011). Where MAQ had a maleamide group at its distal end, AAQ substituted an acrylamide (Figure 3). This compound was capable of photosensitizing wild-type voltage gated potassium channels. In vitro studies suggested that AAQ induced light-dependent blockade of a variety of Kv potassium channels (Fortin et al., 2008). Initially, it was thought the compound bound on the extracellular side of the receptor, where the acrylamide moeity could bind covalently to the receptor surface; however, subsequent studies suggested that AAQ acts on the internal binding site and does not require covalent attachment to function (Banghart et al., 2009). Initial studies were performed on hippocampal slice preparations (Fortin et al., 2008). Light-dependent cell firing was observed after addition of micromolar extracellular AAQ. Green light (530 nm) resulted in cell firing, while use of short wavelength light (390 nm) resulted in silencing of activity.

Figure 3.

Figure 3

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Structure of acrylamide-azobenzene-quarternary ammonium (AAQ)

These characteristics suggested that AAQ might be used to restore light-dependent firing of retinal ganglion cells in animals with outer retinal degeneration. Using isolated retina from rd1/rd1 mutant mice (with extensive outer retinal degeneration), Polosukhina and colleagues (Polosukhina et al., 2012) demonstrated that AAQ application restored light-dependent firing activity to these mice as assayed by multi-electrode array recording (Figure 4). Interestingly, the spectral sensitivity of AAQ-activated retina was reversed from that seen in hippocampus: short wavelength light (380 nm) activated cell firing, while longer wavelength light (500 nm) abrogated firing. Retinas activated by 380 nm light showed firing activity of many seconds on direct transition to dark, reflecting the slow thermal relaxation of trans to cis conformation. The observed spectral sensitivity was reversed when retinas treated with GABAergic and glycinergic blockade were tested following AAQ application. In this case, retinal ganglion cell firing was reduced in 500 nm light, and minimally affected by 380 nm light. This result suggests that the primary effect of AAQ on the retina is to light activate amacrine cells, which in turn inhibit retinal ganglion cell firing. Other interpretations are possible, however, including the possibility that the cis form of AAQ blocks channels present in retinal ganglion cells of degenerated retina.

Figure 4.

Figure 4

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Reconstitution of retinal ganglion cell firing to rd1/rd1 mouse retina by addition of AAQ, as measured by multi-electrode array (MEA) recording. Upper panel in each is multi-electrode array raster, with each line representing one ganglion cell. Note strong firing under UV light and relative silencing under green light. From: (Polosukhina et al., 2012) with permission.

The intensity of light required to activate firing was relatively bright, beginning at about 1015 photos/cm2/s and reaching midpoint at 1016 photos/cm2/s. In this study, the authors were also able to look at receptive fields of activated cells. They found a positive ‘photoswitch index’ (a measure of relative firing rates in light before and after administration of the photoswitch compound) when measuring cells within 200 μm of the light target, but did not see positive switching for cells further away. This confirms the suggestion that AAQ effect reflects the local light environment on the retina.

Several assays were employed to determine whether AAQ-mediated light-induced retinal ganglion cell firing resulted in signals interpretable by the central nervous system of blind mice. rd1/rd1;opn4−/− mice lack pupillary light responses (Panda et al., 2003). Following intravitreal administration of 1 μL of 80 mM AAQ, partial pupillary light responses were restored to approximately 35% of animals. In a second behavioral assay of negative phototaxis (modified from (Johnson et al., 2010)), in which time to turn from a bright light stimulus is measured, rd1/rd1;opn4−/− demonstrated no negative phototaxis, but the same mice showed significant light aversion following intraocular injection of AAQ.

Taken together, these results demonstrated 1.) azobenzene-based potassium channel blockers are capable of restoring light-dependent firing activity to retinal ganglion cells of animals blind from outer retinal degeneration, and 2.) these signals are at least crudely interpreted by the brain and can influence physiology and behavior.

AAQ has several drawbacks as an agent which make further development for potential human use problematic. First, AAQ is activated by light of 380-400 nm. The human lens filters the vast majority of these wavelengths and so achieving the bright intensities necessary for AAQ activation at the level of the retina would be problematic. Second, the slow relaxation of AAQ in dark would require switching between short and long wavelengths (with an attendant auxiliary visual stimulating device). Third, the presence of the acrylamide moiety on AAQ raises the possibility of neurotoxicity (although this has primarily been associated with the monomeric form of acrylamide (Spencer and Schaumburg, 1974b, a, 1975).

Second generation one-component photoswitches

To address these limitations of AAQ, the Trauner laboratory synthesized a series of ‘second generation’ one-component photoswitches by substituting the azobenzene moiety with more electrophilic groups (Figure 5). These substitutions result in substantial red-shifting of the action spectrum these compounds, broadening of their activation spectrum, and rapid relaxation in dark (Mourot et al., 2011). One compound, with a diethylammonium substituted for the acrylamide (DENAQ) had reasonable solubility and was selected for further testing.

Figure 5.

Figure 5

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Structure and absorption spectra of second generation photoswitches. From: (Mourot et al., 2011) with permission.

In vitro, DENAQ showed good activation of retinal ganglion cells of rd1/rd1 mice with robust firing in response to light from 360 nm to over 500 nm (Figure 6) (Tochitsky et al., 2014). DENAQ-activated retina also showed higher sensitivity than AAQ, with light responses ex vivo starting in the mid 1013 photons/cm2/s range of intensity. Following a single intravitreal injection, light responses could be elicited from treated retinas for 3-4 days.

Figure 6.

Figure 6

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Conferral of light sensitivity on retinal ganglion cells of rd1/rd1 mice by DENAQ. A. Structure of DENAQ. B. Multi-electrode array recording of rd1/rd1 retina prior to DENAQ administration. C. Recofding from rd1/rd1 retina post-administration of DENAQ. Note cessation of firing in dark. (From: (Tochitsky et al., 2014) with permission)

Surprisingly, DENAQ did not affect function of wild-type retinas, and did not appear to induce photoswitching in these retinas. The same lack of effect was seen ex vivo in retinas under glutamatergic blockade, isolating the retinal ganglion cells. This strongly suggested that some aspect of retinal degeneration was creating an opportunity for DENAQ to act, either through an upregulated channel or perhaps by increased access for the drug into retinal cells. Tochitsky et al. (Tochitsky et al., 2014) found that that the Ih conductance, mediated by the HCN channel, was photoswitched in rd/rd retinas but not in retinas from wild-type animals. Use of HCN channel blockers ivabradine and cilobridine attenuated or eliminated DENAQ-induced photoswitching, providing additional supporting evidence for the HCN conductance being a major target of DENAQ activity.

In vivo, intravitreal injection of DENAQ in rd/rd mice resulted in altered free range behavior (with increasing activity in light). Additionally, rd/rd mice injected with DENAQ were able to associate light exposure with a noxious stimulus in a learning paradigm, performing equivalently to wild-type animals in this assay.

Two component photoswitches

Development has proceeded as well with two-component photoswitches. In this paradigm, treatment of blind retina entails both gene therapy with a genetically modified cell surface receptor, and administration of a photoswitch specific for that receptor. Unlike introduction of transgenic photoreceptors for treatment of blindness, this hybrid approach allows for native signal transduction and amplification mechanisms, and additionally has lower theoretical chance of eliciting a long-lasting autoimmune response to foreign protein. Given the central importance of glutamatergic signaling in the retina (Thoreson and Witkovsky, 1999; Brandstatter, 2002), development of two component photoswitches for treatment of blindness has, to date, centered on glutamate receptors and synthetic photoswitch ligands.

LiGluR is a recombinant ionotropic glutamate receptor (analogous to SPARK) with an attachment site for a synthetic ligand, MAG (maleimide-azobenzene-glutamate, Figure 7) (Volgraf et al., 2006). When transfected into HEK293 cells, LiGluR had minimal effect by itself. Similarly, application of MAG alone had no effect on untransfected 293 cells. However after conjugation of MAG in LiGluR-transfected cells, strong cationic currents were induced in cells with light stimuli. Similar results were obtained when hippocampal neurons were transfected in vitro with LiGluR6 and conjugated with MAG (Szobota et al., 2007). Even very high frequency light (> 80Hz) resulted in matching action potentials.

Figure 7.

Figure 7

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Structure of maleimide-azobenzene-glutamate (MAG, left) and proposed mechanism of action on ionotropic glutamate receptors. From: (Volgraf et al., 2006) with permission.

To test the utility of the LiGluR/MAG two component system in the treatment of blindness, Caporale et al. (Caporale et al., 2011) cloned the LiGluR receptor into an adeno-associated viral vector (AAV) under control of a synapsin promoter (driving expressin in retinal ganglion cells following intravitreal injection), and applied virus to eyes of rd1/rd1 mice. Retinas harvested from these animals demonstrated light-dependent firing in response to 380 nm light but not 500 nm light. Following intravitreal injection, MAG restored brisk and complete pupillary light responses to gnat1−/−;cnga3−/−;opn4−/− mice (mice lacking rod transducing and cone cyclic-gated nucleotide channel, and hence blind to light, as well as lacking melanopsin and non-visual signaling). Electrophysiologic visual evoked potentials (VEP) were restored to rd1/rd1 animals after LiGluR viral infection and MAG at levels approximately 50% of wild-type. Additionally, light avoidance, as measured in a water-maze paradigm, was restored in rd1/rd1 animals following treatment with LiGluR and MAG.

Relative advantages and disadvantages of photoswitch activation of retinal ganglion cells as a treatment for blindness

Taken together, results to date demonstrate that both one- and two-component photoswitch methods are capable of conferring light-dependent firing activity on retinal ganglion cells in mice with near complete outer retinal degeneration. This activity appears sufficient to restore light-dependent physiologic activity such as pupillary light response, to mice from outer retinal degeneration in vivo. Application of these techniques also restores behavioral light sensitivity, including negative phototaxis and light-dependent associative learning. It is important to note that, to date, no study has demonstrated restoration of form vision (as manifest by optokinetic reflex, for instance, or associative learning based on a true visual stimulus).

Functionally, photoswitch techniques have the same end-goal as opto-electronic prostheses (Ahuja et al., 2011; Rizzo et al., 2011; Barry et al., 2012; Humayun et al., 2012; Mandel et al., 2013; Stingl et al., 2013; Zrenner, 2013), namely converting light stimuli directly into retinal ganglion cell stimulation. The success of these implants in restoring some form vision bodes well for the potential of small molecule photoswitches to accomplish the same. Both techniques have complementary strengths and weaknesses. Opto-electronic devices can incorporate amplification circuitry to boost signal in dim light, while photoswitch compounds would require brighter light for the same effect. However, photoswitch compounds have the advantage of potentially conferring light sensitivity on every retinal ganglion cell, without concern for cross-talk induced by external electrical fields. Additionally, administration of photoswitch compounds and viral vectors will likely occur by simple intravitreal injection, as opposed to the significant epiretinal or subretinal surgery required for implantation of an optoelectric prosthesis.

Gene therapeutic approaches offer another means for conferring light sensitivity on retinal ganglion cells, bipolar cells, or non-degenerated photoreceptors (Tomita et al., 2007; Ivanova and Pan, 2009; Busskamp et al., 2010; Ivanova et al., 2010a; Ivanova et al., 2010b; Busskamp et al., 2012). Compared to one-component photoswitch these approaches allow more specific targeting to subsets of remaining cells in the retina. For instance, targeting to cone pedicles may allow substantial retinal processing of signal and reconstitution of faithful action potential trains to visual stimuli (Busskamp et al., 2010). Gene therapy approaches are also potentially permanent. However, to date channelopsin and halopsin techniques have required more light intensity than is required for photochemical approaches. The permanence of gene therapy approaches may also be a liability as there is likely not opportunity for ‘upgrading’ to an improved channel. There are still theoretical concerns as well over the possibility of long-term uveitis resulting from high level expression of foreign channel proteins in the retina, although this has not been observed in animal studies to date.

Relative to each other, one-component and two-component systems are complementary in strengths and weaknesses. The one component system is simpler, with lower potential for toxicity given the single component. However, targeting is limited by distribution of the target of the photoswitched compound, which may encompass many cell types. Conversely, the two component system allows precise targeting of the photoswitch, but requires gene therapy in addition to photoswitch compound. From a practical perspective, regulatory approval of a single component photoswitch is likely to be more expeditious than a two-component system.

Frontiers in photochemical restoration of blindness

For one-component systems, a major challenge to translation for human use is the solubility of photo-switch compounds. Future development will be targeted at improving the solubility of these compounds, perhaps by using uncharged moieties rather than the charged, quaternary ammonium. Ultimately, a sustained release delivery device, similar to a Retisert (Jaffe et al., 2000, might be required to maintain a steady and sufficient supply of photoswitch to the retina (Jaffe et al., 2000). Delivery of one-component compounds could also be improved by utilizing large cellular pores, an approach which has been used successfully to load the QAQ photoswitch (containing two, charged quarternary ammonia) (Mourot et al., 2012).

A range of additional compounds, targeting other pathways including GABAergic and TRP channel pathways, may be rendered photoswitchable as well (Stein et al., 2012; Kienzler et al., 2013; Reiter et al., 2013; Stein et al., 2013; Schonberger and Trauner, 2014). Some of these may have high specificity for certain visual processing pathways. An ideal target might be an mGluR6-specific photoswitch, given its specificity for specific bipolar pathways (Snellman et al., 2008).

For two-component system, MAG’s activation at 380 nm is too short to be useful in a phakic human eye (van de Kraats and van Norren, 2007). Red shifted MAG has now been synthesized and shown to be efficacious (Kienzler et al., 2013), which will alleviate this shortcoming. Improvements in viral vector targeting through directed selection will help improve cell-type specificity following intravitreal injection (Dalkara et al., 2013). And, as was the case for the one-component switches, additional channels may be targeted for two component function, potentially including metabotropic receptors (Schonberger and Trauner, 2014) which would allow signal amplification, which in turn could improve sensitivity.

For all vision restoration techniques, one major hurdle is determining the extent to which a non-native action potential series can be interpreted by the vision processing centers of the brain. The normal retina performs a great deal of signal processing, including directional motion detection, color surround processing, and widespread adaptation to lighting conditions, which are not intrinsic features of ‘reanimated’ (and functionally reprogrammed) retinal ganglion cells. While techniques that target upstream cells such as bipolar cells may result in more native visual processing and ultimately action potential firing, it still remains to be determined if these signal sequences can be correctly interpreted as dynamic visual scenes. It is conceivable that first-generation use of photoswitch compounds will also include a ‘visual processing unit’ that would interpret a visual scene and stimulate photoswitch-activated retinal ganglion cells with particular light stimuli designed to more faithfully reproduce normal retinal ganglion cell output. In making such a processing unit user-tunable, it may be possible to achieve better visual results. As an example, in primates the retinal ganglion cells subserving the fovea, while having very high resolution due to their one-to-one mapping the cones, are radially displaced from the fovea proper (Curcio and Allen, 1990). As such, in response to a visual scene presented normally to the retina, direct stimulation of retinal ganglion cells would likely mis-map perifoveal stimulation to the center of vision, while producing a relative scotoma for central vision (due to the lack of retinal ganglion cells in the fovea proper). This could be corrected with a device that converted a digital video image of the outside world to a raster-scan of appropriate wavelength, which would displace the true foveal image to be projected on the appropriate retinal ganglion cells. Similarly, the nature of color vision following photoswitch-mediated retinal reanimation is not apparent (as all color-opponent ganglion cells would be stimulated equivalently), but again any detriment to vision might be mitigated by an altered retinal stimulation pattern to mimic native retinal firing. This challenge will be an active area for research (for all modes of vision restoration) for years to come.

Conclusion

In a relatively brief period of time since the first proof-of-concept that photoswitched channel blockers could induce light-dependent neuronal firing activity, the field has progressed to demonstration of partial reversal of blindness in animal models using both one- and two-component systems. This approach is complementary to other approaches in vision restoration, but potentially offers very high resolution (conceivably every ganglion cell), as well as (for the one-component system) the potential for sequential improvement in agents for a single patient. Significant hurdles with respect to demonstration of safety, drug delivery, and optimization of signaling to the visual pathways of the brain remain before this system can be effectively applied to human blindness.

Highlights.

  • Photoswitch compounds use azobenzene moieties to confer light activation

  • Photoswitched voltage-gated K+ channels confer light-activated cell firing to neurons

  • Photoswitches can confer light-dependent retinal ganglion cell firing to blind retinas

  • Two component photoswitches offer the possibility of targeted activity in the retina

  • These molecules offer an additional approach to vision restoration

Acknowledgments

This work supported in part by PN2 EY018241, P30 EY01730, and an unrestricted grant from Research to Prevent Blindness

Footnotes

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