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The majority of inherited retinal degenerations converge on the phenotype of photoreceptor cell death. Second- and third-order neurons are spared in these diseases, making it possible to restore retinal light responses using optogenetics. Viral expression of channelrhodopsin in the third-order neurons under ubiquitous promoters was previously shown to restore visual function, albeit at light intensities above illumination safety thresholds. Here, we report (to our knowledge, for the first time) activation of macaque retinas, up to 6 months post-injection, using channelrhodopsin-Ca 2+ -permeable channelrhodopsin (CatCh) at safe light intensities. High-level CatCh expression was achieved due to a new promoter based on the regulatory region of the gamma-synuclein gene (SNCG) allowing strong expression in ganglion cells across species. Our promoter, in combination with clinically proven adeno-associated virus 2 (AAV2), provides CatCh expression in peri-foveolar ganglion cells responding robustly to light under the illumination safety thresholds for the human eye. On the contrary, the threshold of activation and the proportion of unresponsive cells were much higher when a ubiquitous promoter (cytomegalovirus [CMV]) was used to express CatCh. The results of our study suggest that the inclusion of optimized promoters is key in the path to clinical translation of optogenetics., (Copyright © 2017. Published by Elsevier Inc.)

Most inherited retinal dystrophies display progressive photoreceptor cell degeneration leading to severe visual impairment. Optogenetic reactivation of retinal neurons mediated by adeno-associated virus (AAV) gene therapy has the potential to restore vision regardless of patient-specific mutations. The challenge for clinical translatability is to restore a vision as close to natural vision as possible, while using a surgically safe delivery route for the fragile degenerated retina. To preserve the visual processing of the inner retina, we targeted ON bipolar cells, which are still present at late stages of disease. For safe gene delivery, we used a recently engineered AAV variant that can transduce the bipolar cells after injection into the eye's easily accessible vitreous humor. We show that AAV encoding channelrhodopsin under the ON bipolar cell-specific promoter mediates long-term gene delivery restricted to ON-bipolar cells after intravitreal administration. Channelrhodopsin expression in ON bipolar cells leads to restoration of ON and OFF responses at the retinal and cortical levels. Moreover, light-induced locomotory behavior is restored in treated blind mice. Our results support the clinical relevance of a minimally invasive AAV-mediated optogenetic therapy for visual restoration.