Dexamethasone rescues cones and the RPE in a mouse model of Retinitis Pigmentosa

Abstract

Purpose : To delay photoreceptor degeneration and vision loss in a mouse model of typical Retinitis Pigmentosa (RP), slowing down the bystander death of cones by pharmacological targeting retinal inflammation.

Methods : Animal experimentation adhered to ARVO and EU laws; protocols were approved by Institutional committees. rd10 mutant mice (n=30) were used for the study. Ozurdex® tabs (700 µg dexamethasone, Abbvie) were cut in 7 fragments, 1mm long, and re-inserted in their surgical dispenser. Single fragments were injected intravitreally in mice aged 25 days (P25), near the corneal margin. Control mice received identical implants that did not contain the drug. Mice were harvested at 45 days. Photoreceptor survival, microglial reactivity and Retinal Pigment Epithelium (RPE) integrity were assessed by immunostaining retinal and RPE whole mounts with antibodies respectively against cone arrestin, Ccl2, Iba1 and ZO-1. Western blot and immunostaining for glucocorticoid receptors (GR) were done on retina/RPE samples of wt mice using GR antibodies. Cone and microglial cell counts counted with Metamorph or Image J on z stacks of the outer retina obtained by confocal microscopy. The integrity of ZO-1 positive arrays of RPE cells was measured automatically by image analysis with a custom-made script. The expression of retinal inflammatory genes was assessed by qRT-PCR on rd10 mice that received daily intraperitoneal dexamethasone in the P25-P45 period.

Results : Cone counts showed significantly higher (27%) survival rates in Ozurdex-treated mice. Cones were also better preserved morphologically, with longer outer segments; activated microglial cells were 50% less numerous and the RPE showed statistically lower numbers of discontinuities in ZO-1 stained profiles compared to controls. Inflammatory responses downregulation was confirmed by gene expression analysis on the retina of rd10 mice which received systemic dexamethasone daily. Immunohistochemistry of wt mice showed GR positive profiles in Muller, photoreceptors and RPE cells.

Conclusions : Dexamethasone can limit microglial activation and RPE breakdown associated to photoreceptor degeneration in a mouse model of RP, extending cone survival. These data pave the way to clinical trials aimed at repurposing approved ophthalmological devices for long-term release of corticosteroids for human RP, expectantly providing a mutation-agnostic, therapeutic option for this orphan disease.

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