Enhancement of light sensitivity in retinal degeneration in mice by use of novel optogenetic approaches

Abstract

BackgroundInherited retinal degenerations (including retinitis pigmentosa) that lead to irreversible blindness due to progressive loss of rods and cones in the outer retina affect one in 2500 people worldwide. Despite this high prevalence, there is no cure. The therapeutic approaches that are being explored in clinical trials have low efficacy and high complication rates (gene augmentation) or low resolution (artificial retinal implants). In this study we investigated the effect of ectopic expression of two light sensitive proteins, human melanopsin and human rhodopsin, on visual function in a mouse model (rd1) of advanced retinal degeneration using enhanced gene therapy. We hypothesised that this combined gene therapy and optogenetic approach might render surviving inner retinal cells photosensitive and enhance visual function. MethodsThe melanopsin or rhodopsin expressing adeno-associated virus serotype 2 (AAV2) vector was injected intravitreally into the eyes of adult rd1 mice with a combination of glycosidic enzymes, which we have previously shown to improve retinal transduction efficiency. Mouse visual function was assessed with pupillometry at 6 weeks post treatment. The downstream effects of more complex visual processing were examined by in-vivo electrophysiology recordings from a mouse lateral geniculate nucleus (LGN), a primary input for retinal ganglion cells. FindingsWe found expression of ectopic human melanopsin in a variety of surviving retinal cells in rd/rd mouse eyes treated with melanopsin, including ganglion and bipolar cells. The pupillary light reflex showed an enhancement in visual sensitivity, by two orders of magnitude, in eyes treated with melanopsin. We also found that rhodopsin treatment led to a similar shift in sensitivity. InterpretationMelanopsin and rhodopsin, expressed in degenerating retina, have potential to enhance light sensitivity. We are now assessing the benefits of this enhanced sensitivity in terms of changes in visual responses in mouse LGN. Both receptors operate within physiological light conditions, unlike present optogenetic approaches (such as microbial channel rhodopsin-2 [Ch2]), which require stimulation with very high light intensities that are potentially toxic to the retina. Furthermore, both melanopsin and rhodopsin are native mammalian proteins with minimum risk of adverse immune responses compared with Ch2, making them preferred candidates for use in future human trials. FundingUK Medical Research Council.