Editorial

Toward the Miracle of Retinal Reanimation Russell N. Van Gelder, MD, PhD - Seattle, Washington

“Any sufficiently advanced technology is indistinguishable ganglion cells of the intact retina is problematic because the from magic.” current necessary to depolarize cells extracellularly generates Arthur C. Clarke, “Hazards of Prophecy: The Failure of substantial heat. Present-day technologies also are challenged Imagination,” 1962 to provide power, which currently requires a hard-wired The restoration of vision to the blind isdquite literallyd connection between the intraocular device and an extra- miraculous. The Catholic church long has recognized vision ocular power source. Additionally, implantation typically restoration as a miracle worthy of canonization, and de- requires a challenging vitreoretinal surgery. Nonetheless, this scriptions of miraculous recovery of vision are found in approach has established the feasibility of retinal reanimation nearly every major religion. As ophthalmologists, we and its potential for vision restoration. perhaps have become a bit inured to this miracle; nearly every practitioner has had the privilege of restoring vision to individuals with cataracts, corneal disease, retinal detach- Cell Replacement Therapy ment, and a host of other maladies. However, for those with hereditary or acquired retinal degeneration, the notion of The biochemical code that directs stem cells to become ocular restoring visiondreanimating the retinadin which the rods tissues largely has been delineated.2 In 2011, Eiraku et al3 and cones have died still seems the stuff of miracles or at published the remarkable results of growing an entire least science fiction. primordial eyecup from a stem cell, establishing the potential Yet 4 approachesdoptoelectronics, cell replacement, gene of this approach. Retinal pigment epithelium is one of the therapy, and photopharmacologydhave seen remarkable more readily differentiated cell types, and nearly a dozen progress in the past decade. These The value of vision restoration to the clinical trials are now underway to techniques exploit the persistence implant stem cell-derived retinal of retinotopically mapped, func- patient will be enormous, as will the pigment epithelium in the sub- tioning retinal ganglion cells after value to society. retinal space to treat dry age-related degeneration of the rods and cones. macular degeneration and other By conferring light sensitivity to these cells through exogenous retinal degenerativediseasessuch means, researchers have succeeded with each in restoring some as Stargardt’sdisease.4 Nascent photoreceptors and neural retina visual function, at least in animal models of retinal degenera- also can be differentiated, and Chao et al5 recently demonstrated tion. I wish to give a brief update for the practicing ophthal- survivalandevenganglioncellprojectiontothebraininstem mologist of the progress toward this objective that has occurred cell-derived retinal precursors implanted subretinally in pri- in the past decade, as well as detailing some of the remaining mates. A recent remarkable study by Sanges et al6 highlights an challenges for achieving this noble goal. alternative approach. This group transplanted hematopoietic stem and progenitor cells into degenerating mouse retinas and observed these cells to fuse with Müller glia cells and Optoelectronics subsequently to reprogram into photoreceptors within the retina. (A similar, Müller cell-dependent mechanism is Optoelectronics is the most advanced approach of the 4, thought to underlie the ability of fish retinas to regenerate because it has achieved proof of principle in humans and Food naturally.7) Although in animal models, restoration of some and Drug Administration approval. In this approach, a mi- visual function (as measured by electroretinography) has been croelectronic device is implanted epiretinally or subretinally shown, demonstration of the correctness of synapse formation, and directly stimulates the retinal ganglion cells, using local long-term persistence of cells, and absence of immune responses field potentials. The image is fed into the device by an for allograft transplants remains to be established. Additionally, exogenous camera. Exemplified by the ARGUS II implant some early reports of generation of photoreceptors in the outer (Second Sight Medical Products, Inc., Sylmar, CA), this retina now seem to be the result of transfer of proteins between approach has been shown to restore the ability to distinguish cells, rather than persistence and differentiation of photorecep- high-contrast letters in patients with advanced degeneration, tors.8 Cell replacement therapy shows great promise; the as well as to improve mobility and independence.1 Although ongoing trials with stem cell-derived retinal pigment epithe- the ARGUS II has only 60 stimulating electrodes (limiting lium cell replacement will provide valuable data as to the resolution), newer-generation implants in development have viability of this approach. However, significant additional basic at least a 10-fold increase. However, increasing electrode science studies will be needed before photoreceptor-restorative density to match the approximately 1.3 million retinal approaches can be attempted in humans.

ª 2017 by the American Academy of Ophthalmology http://dx.doi.org/10.1016/j.ophtha.2017.08.021 1 Published by Elsevier Inc. ISSN 0161-6420/17

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Opsin Gene Therapy reversibility or upgradability of compounds (whereby the optimal agent and its dosing could be titrated for the indi- The discovery and characterization of channelopsins opened vidual), and a more familiar small-molecule Food and Drug the era of optogenetics. Channelopsins are opsin-family Administration approval path. Challenges include maintain- proteins that, rather than stimulating a G-protein to signal, ing continuous delivery and open questions on long-term directly open, or close cell membrane ion channels. Chla- toxicity. mydomonas channelrhodopsindisolated from green algaedcan depolarize mammalian ganglion cells directly when expressed by gene transfer, causing them to fire in Common Challenges response to light. Delivery of these proteins is accomplished through virally mediated gene therapy, a technology Perhaps the largest challenge facing all these technologies is currently in numerous clinical trials for conditions including our collective lack of understanding of the neural code. The Leber congenital amaurosis, X-linked juvenile retinoschisis, retina performs substantial local processing of the nascent achromotopsia, and choroideremia, among others. At least image, extracting information about motion, direction, co- partial restoration of retinal function has been demonstrated lor, contrast, and intensity. The question of how these at- using this approach in multiple mouse models of degener- tributes are encoded in the firing of approximately 1.3 ation.9 Perhaps most promising is the virally-mediated gene million retinal ganglion cells remains fundamentally un- transfer of rhodopsin (and, presumably soon, cone opsin) to solved. Initial attempts to replicate the natural encoding of 17 bipolar cells. Achieved by 2 laboratories in 2015 in vision in mice have shown promising results, but until the mice,10,11 this approach offers recapitulation of some of the human code is cracked, attempts at retinal reanimation likely cell signaling pathways in the retina with a photopigment will not restore normal vision, and instead will create usable capable of high-yield quantum efficiency and marked signal information for the cerebral cortex that can be interpreted to amplification. (That these pigments work in cells they were allow function. Such has been the case with the cochlear not intended to be expressed in is yet another minor implantdthe pioneer of sensory restorationdwith which miracle!) Gene therapy-mediated opsin expression also has recipients report the auditory experience as being distinct been used in nonhuman primates to correct the most com- from native hearing, but sufficient for functional hearing mon genetic vision defect of all, red-green color blindness.12 restoration. Other challenges common to these approaches Challenges for gene therapy include optimization of the include the central scotoma problem (methods restoring viral vectors for efficient infection of the appropriate cell ganglion cell function will create a central scotoma types (particularly after intravitreal rather than subretinal because of the slight displacement of retinal ganglion cells injection), the permanence of treatment (i.e., it may be from the fovea), the color-encoding challenge (there is no difficult to retreat individuals and upgrades to new way at present to determine which ganglion cells encode constructs may not be possible), maintaining long-term which color channels), and the retinal rewiring problem expression of proteins, and limiting inflammation after (whereby substantial remodeling of intraretinal connections 18 treatment. Nonetheless, this is a most promising avenue for occurs in outer retinal degenerative disease, a problem that vision restoration and has achieved the most impressive is particularly challenging for cell replacement results in animals to date. technologies). None of these hurdles are insurmountable, but substantial research will be needed to address each. Finally, there is the issue of cost. The Argus II implant Photopharmacology costs more than $100 000 (exclusive of surgical implantation costs). Gene therapies in the past have been Photopharmacology is the newcomer to the group. Organic priced even higher; Glybera (alipogene tiparvovec, chemists have modified existing drugs with a molecular light [uniQure, Amsterdam, Netherlands], a therapy for switch (an azobenzene moiety) that creates a light-dependent lipoprotein lipase deficiency) was the first approved AAV steric confirmation.13 Existing drugs thus may become light- gene therapeutic (approved in Europe in 2012), at a cost or dark-activated. When linked to a potassium channel of $1 million per treatment (!). The value of vision blocker (which will lead to depolarization and firing of restoration to the patient will be enormous, as will the retinal ganglion cells), this allows cell firing to be driven by value to society. However, the market forces that light, thus achieving the same result as optoelectronic or determine its pricing (at least in the United States) are not opsin gene therapy approaches. Variants include the 1 yet clear. Cost could prove a barrier to advancement or component (i.e., photoswitched drug alone) or 2 component adoption of promising technologies. (gene therapy with a modified channel or receptor that Although the optoelectronic approach to vision restora- reacts only to photoswitched compound) methods. These tion has an almost 25-year history, all the remaining ap- approaches have restored light responsiveness and some proaches are less than a decade old. The progress in this visual function to mice and rats blind from outer retinal domain has been remarkable, and it is likely that several of degeneration.14 Newer-generation compounds feature long these approaches will be in clinical trials before the next half-lives (on the orders of weeks)15 and activation of decade dawns. These approaches offer great hope to our upstream cellular components such as bipolar cells.16 patients with currently untreatable retinal disease and Advantages of this approach include the need only for eventually will bring treatments to one of the few areas of small-molecule treatment (in the 1-component system), the ophthalmology that have no treatment options at present.

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References function in multiple mouse models of blindness. Mol Ther. 2011;19(7):1220-1229. 10. Cehajic-Kapetanovic J, Eleftheriou C, Allen AE, et al. 1. da Cruz L, Dorn JD, Humayun MS, et al. Five-year safety and Restoration of vision with ectopic expression of human rod performance results from the Argus II Retinal Prosthesis System opsin. Curr Biol. 2015;25(16):2111-2122. Clinical Trial. Ophthalmology. 2016;123(10):2248-2254. 11. Gaub BM, Berry MH, Holt AE, et al. Optogenetic vision 2. Hyde DR, Reh TA. The past, present, and future of retinal restoration using rhodopsin for enhanced sensitivity. Mol Ther. regeneration. Exp Eye Res. 2014;123:105-106. 2015;23(10):1562-1571. 3. Eiraku M, Takata N, Ishibashi H, et al. Self-organizing optic- 12. Mancuso K, Hauswirth WW, Li Q, et al. Gene therapy for red- cup morphogenesis in three-dimensional culture. Nature. green colour blindness in adult primates. Nature. 2011;472(7341):51-56. 2009;461(7265):784-787. 4. Nommiste B, Fynes K, Tovell VE, et al. Stem cell-derived 13. Fortin DL, Banghart MR, Dunn TW, et al. Photochemical retinal pigment epithelium transplantation for treatment of control of endogenous ion channels and cellular excitability. retinal disease. Prog Brain Res. 2017;231:225-244. Nat Methods. 2008;5(4):331-338. 5. Chao JR, Lamba DA, Klesert TR, et al. Transplantation of human 14. Polosukhina A, Litt J, Tochitsky I, et al. Photochemical embryonic stem cell-derived retinal cells into the subretinal space restoration of visual responses in blind mice. Neuron. of a non-human primate. Transl Vis Sci Technol. 2017;6(3):4. 2012;75(2):271-282. 6. Sanges D, Simonte G, Di Vicino U, et al. Reprogramming 15. Tochitsky I, Trautman J, Gallerani N, et al. Restoring visual Muller glia via in vivo cell fusion regenerates murine photo- function to the blind retina with a potent, safe and long-lasting receptors. J Clin Invest. 2016;126(8):3104-3116. photoswitch. Sci Rep. 2017;7:45487. 7. Lenkowski JR, Raymond PA. Muller glia: stem cells for 16. Laprell L, Tochitsky I, Kaur K, et al. Photopharmacological generation and regeneration of retinal neurons in teleost fish. control of bipolar cells restores visual function in blind mice. Prog Retin Eye Res. 2014;40:94-123. J Clin Invest. 2017;127(7):2598-2611. 8. Pearson RA, Gonzalez-Cordero A, West EL, et al. Donor and 17. Nirenberg S, Pandarinath C. Retinal prosthetic strategy with the host photoreceptors engage in material transfer following capacity to restore normal vision. Proc Natl Acad Sci U S A. transplantation of post-mitotic photoreceptor precursors. Nat 2012;109(37):15012-15017. Commun. 2016;7:13029. 18. Jones BW, Watt CB, Frederick JM, et al. Retinal remodeling 9. Doroudchi MM, Greenberg KP, Liu J, et al. Virally delivered triggered by photoreceptor degenerations. J Comp Neurol. channelrhodopsin-2 safely and effectively restores visual 2003;464(1):1-16.

Footnotes and Financial Disclosures

Financial Disclosure(s): The author(s) have no proprietary or commercial Correspondence: interest in any materials discussed in this article. Russell N. Van Gelder, MD, PhD, Department of Ophthalmology, Uni- Supported by the National Institutes of Health, Bethesda, Maryland (grant versity of Washington School of Medicine, Campus Box 359608, 325 9th nos.: R24EY023937 and P30EY001730); an unrestricted grant from Avenue, Seattle, WA 98104. E-mail: [email protected]. Research to Prevent Blindness, Inc., New York, New York; and the Mark J. Daily, MD, research fund.

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