Early inner retinal response to acute photoreceptor loss in the non-human primate retina

Abstract

Purpose : Cell-based therapies for photoreceptor degeneration rely on transplanted photoreceptors forming synaptic connections with second-order neurons. Previously, we showed that acute photoreceptor ablation led to the loss of horizontal cells (HC), and glutamate receptors in bipolar cells (BC), as early as 3 weeks post-injury (Puthussery et al., 2023, IOVS, 64, 3804). Here, we examined earlier time points after ablation to understand the time course of the inner retinal changes in this model.

Methods : Focal photoreceptor ablations were made with a femtosecond-pulsed laser (730 nm, 27-33 J/cm²) focused on the photoreceptor inner/outer segments with an adaptive optics scanning laser ophthalmoscope in 3 adult macaques (M. fascicularis, 5-11.5 years). Lesions were made ~1-20 degrees from the foveal center and eyes were processed for fluorescence immunohistochemistry at 1, 4 and 8 days post-ablation. HC and BC integrity were assessed with cell-type specific markers and synaptic markers. Lesioned areas were compared with unlesioned regions from the same retinas.

Results : At one day post-laser ablation, the photoreceptor inner and outer segments were absent whereas their nuclei appeared pyknotic and disorganized. The presence of VGLUT1 in the outer plexiform layer indicated remaining photoreceptor synaptic terminals in the lesioned area. The morphology and density of the underlying HCs and BCs appeared normal, and there was persistent expression of glutamate receptors in ON-BC (mGluR6) and OFF-BC (GluK1) dendritic tips. By four days post-ablation, there was a complete loss of photoreceptors in the area of laser injury. Expression of dendritic mGluR6 and GluK1 was present but disorganized compared to adjacent healthy areas. At this time point, HCs were still present but showed membrane blebbing. By 8 days post-ablation, the HCs showed signs of apoptosis, including cell shrinkage and pyknotic nuclei, and synaptic expression of mGluR6 and GluK1 was markedly reduced.

Conclusions : Degeneration of HCs and loss of synaptic glutamate receptors in BCs begins around 1 week post laser induced photoreceptor ablation. Further studies are needed to determine whether similar changes occur in chronic photoreceptor degenerations and whether synaptic protein expression can be restored after photoreceptor transplantation.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.