Topical insulin protects against retinal ganglion cell loss in nonhuman primate model of experimental glaucoma

Abstract

Purpose : Insulin has been shown to regenerate retinal ganglion cell (RGC) dendrites and presynaptic connections in mouse optic nerve injury models. This study tests the hypothesis that treatment with insulin eyedrops protects against loss of RGCs in a nonhuman primate (NHP) model of experimental glaucoma (EG).

Methods : Twelve adult rhesus monkeys (10F/2M, ages 5-17y) underwent biweekly imaging by optical coherence tomography (OCT, Spectralis, Heidelberg Engineering, GmbH) to monitor retinal nerve fiber layer thickness (RNFLT) before and after induction of unilateral EG. At onset of EG (confirmed loss of baseline RNFLT), each animal was randomized to receive a 50 µL drop of either insulin (5 IU Humulin-R 100 IU/mL, N=6) or saline (N=6) in the EG eye 6 days per week. After 16 weeks of treatment, animals were euthanized, perfused with 4% paraformaldehyde and retinas harvested for histopathology. Each retina was cut into superior and inferior halves through the fovea and center of the optic nerve head. Each half was further divided into 4 wedges with their apex at the fovea (Fig1). The inferonasal wedge of each retina was stained with the RGC marker RBPMS and imaged on a Leica confocal microscope using a 10X objective and focus map for tile acquisition. RGCs were counted within two tiles of each image montage (1.07 mm² tile area), at mid- and peripheral locations (~ 7 and 12 mm from the fovea, respectively), using Fiji. Matched pairs ANOVA was used to compare RGC counts between saline and insulin-treated EG eyes vs. their fellow control (FC) eyes, at each retinal location sampled.

Results : At randomization, there were no significant differences in RNFLT or intraocular pressure history between EG eyes subsequently treated with insulin versus saline. After 16-weeks of treatment, there was a significant loss of RGCs in the saline group of EG eyes relative to their fellow control eyes (61% and 69% average loss at the mid- and peripheral retinal sample location, respectively), but no significant differences in RGC counts between the insulin-treated EG eyes and their fellow control eyes (Fig2). The Pearson correlation between RNFLT and the RGC counts of mid- and peripheral retina were 0.76 and 0.70, respectively.

Conclusions : Once daily topical administration of insulin provided protection against loss of RGCs in NHP EG. Insulin treatment is a promising approach for neuroprotection in glaucoma.

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