Abstract
Purpose :
A number of vision restoration therapies aim to restore light sensitivity to retinal ganglion cells (RGCs) following extended periods of blindness. In rodent models of retinal degeneration, physiological changes in RGCs after photoreceptor (PR) loss have been reported but this has not been investigated in primates. By expressing both a calcium indicator (GCaMP6s) and an optogenetic actuator (ChrimsonR) in foveal RGCs of a macaque, we use in vivo imaging to assess changes in RGCs in the weeks and years following PR loss.
Methods :
AAV2-CAG-GCaMP6s and AAV2-CAG-ChrimsonR-tdTomato were co-injected into the vitreous. Cones were ablated with a Mai-Tai pulsed laser (0.8 x 0.7o, 106 ms, 120-150mW, 730 nm) delivered through an adaptive optics scanning light ophthalmoscope (AOSLO). GCaMP fluorescence from 218 RGCs in two eccentricity matched regions of the fovea was collected using AOSLO and tracked over 10 weeks. In the first location (long-term deafferented) PR input was removed 2 years prior to recording, while in the second location (short-term deafferented) PR input was removed 1 week prior to recording. A 0.5 s, optogenetic stimulus (1 mW, 640 nm) was delivered to the RGCs, the GCaMP fluorescence was recorded for 90 s, and the decay fitted with an exponential model.
Results :
Optogenetic responses in RGCs persisted for over 2 years following PR ablation. The mean time to peak calcium response did not differ significantly between the long-term (1.43±0.17s SD) and the short-term (1.45±0.30s SD) deafferented RGCs (unpaired t-test, p=0.05, n=654). The mean decay constant of the calcium response decreased 2.1 fold (2.5±0.5 s to 1.2±0.2 s SD) in the 8 weeks post PR ablation (p<0.001, paired t-test, n=109), with 87% of the decrease occurring within the first 5 weeks. The decay constant did not decrease further from week 8-10.
Conclusions :
By expressing both an optogenetic actuator and a calcium indicator in foveal RGCs, we tracked the development of altered RGC physiology in vivo in the weeks and years following PR loss. The presence of optogenetic responses 2 years after PR loss and the stability of the rise time are promising for vision restoration therapies targeting RGCs, however, the 2.1 fold reduction of the calcium response decay constant suggests that restored activity may be impacted by changes in the inner retina weeks after PR loss.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.