Optogenetic activation of retinal ganglion cells persists for at least a year in primate fovea following photoreceptor loss

Abstract

Purpose : Vision restoration therapies such as optogenetics aim to confer light sensitivity to retinal ganglion cells (RGCs) when photoreceptor input has been lost. The success of these retinal approaches in humans requires that RGCs retain the capacity to send signals to the brain after extended periods without photoreceptor input. By non-invasively imaging RGCs expressing both a calcium indicator and an optogenetic channel we evaluated restored RGC response in the living primate hours to years after the loss of photoreceptor input.

Methods : AAV2-CAG-G-CaMP6s and AAV2-CAG-ChrimsonR-tdTomato were co-injected into the vitreous of two normal macaques (M. fascicularis). Fluorescence from transduced foveal RGCs was visualized using a fundus camera and SLO. Optogenetic responses were measured at the retinal level using adaptive optics ophthalmoscopy (AOSLO) calcium imaging of RGCs while 640 nm, 4.5 Hz spatiotemporal stimuli were focused on the RGC layer. A 0.8^ο scotoma was produced in one retinal hemifield by a 106 ms exposure to a 150 mW, 730 nm fs-laser delivered to foveal cones through the AOSLO. RGC responsivity was measured at two locations, one with photoreceptor input removed and a second control location with photoreceptors intact, weekly for 5 weeks in one animal, and at 1 year in another.

Results : We observed an abrupt 71% decrease in GCaMP6s fluorescence in a segment of the foveal RGC ring anatomically consistent with the loss of input from foveolar photoreceptors. Over 5 weeks there was a gradual 40% decrease in tdTomato fluorescence in this area relative to an eccentricity matched location with intact photoreceptor input. Despite these changes, optogenetic responses in RGCs were present at the end of this five week period and persisted more than one year after photoreceptor ablation. The sensitivity index D’ of the optogenetic mediated GCaMP6s response was 15.7 at 1 year.

Conclusions : Calcium imaging in the living eye paired with localized photoreceptor ablation provides a pre-clinical development platform to study long term changes in restored RGC activity at the retinal level in the living primate. While this study does not address possible post-retinal impairment following prolonged photoreceptor loss, it does demonstrate that retinal ganglion cells lacking photoreceptor input retain the capacity to respond to optogenetic stimulation for at least a year in primate.

This is a 2020 ARVO Annual Meeting abstract.