Exploration of gene modulation in a mouse model of retinitis pigmentosa

Abstract

Retinitis pigmentosa (RP) is an inherited retinal degenerative disease (RD) caused by the irreversible loss of photoreceptors. In most RPs, rod photoreceptors carry mutated genes and cones die subsequently to the initial rod loss. Due to the diversity of genetic defects in rods, gene-specific therapies are practical in a small fraction of patients with RP. In current study, we employed rd10 mice, which carries Pde6β mutation at the CAT region in rod photoreceptors, to investigate how to preserve neurons in mouse model of RP. We overexpressed the melanopsin (Opn4), ribosomal protein S6 kinase beta-1 (S6K1) and conditional deleting phosphatase and tensin homolog (PTEN) in rd10 photoreceptors and complete deleted cyclooxygenase-1 (COX-1) gene in the rd10 mice to promote the neuronal survival.

Opn4 is an endogenous light-sensitive protein found in a small population of retinal ganglion cells (RGCs) with neuroprotective effects of inner retinal neurons. To protect the photoreceptors and preserve the retinal function, plasmid electroporation was employed to introduce Opn4 into the photoreceptors of rd10 retinas at postnatal day 0 (P0) to P3. The Opn4 overexpressed in the photoreceptors of rd10 mice showed improved visual acuity in optomotor test at P21.

The mammalian target of rapamycin (mTOR) pathway regulates cell growth, proliferation, survival, autophagy, and transcription in mammals. It was involved in the photoreceptor degeneration in rd1 mice. Our study in rd10 mice showed that mTOR pathway was deactivated during retinal degeneration with up-regulation of PTEN, down regulation of serine/threonine-specific protein kinase Akt, and S6K1. When PTEN was conditional deleted in photoreceptors via a Cre-LoxP system in rd10 mice, survival of photoreceptors extended from P25 to P42. Retinal function detected by electroretinogram (ERG) test and visual acuity detected by optomotor were well preserved till P35. In addition, overexpressing S6K1 gene in photoreceptors was able to improve the neuronal survival and retinal function at P26. During photoreceptor degeneration, microglial cells are activated and produce pro-inflammatory mediators like COX-1 to exacerbate neuronal death in rd10 mice. To investigate the role of COX-1 during retinal degeneration, the COX-1 gene was completely knocked out in rd10 mice by crossing a COX-1 deficient mouse strain with rd10 mice. In COX-1-/-/rd10 mice, microglia was in non-activated status with ramified morphology. Density of photoreceptor number, retinal function, and visual acuity were significantly preserved by COX-1 deletion in rd10 mice until P35.

Taken together, over-expression of melanopsin, S6K1, and conditional deletion of PTEN in the photoreceptor cells, genetic deletion of COX-1 protected photoreceptor in rd10 mice. This gene therapeutic strategy through gene modulation might be applicable for a broad range of retinal degenerative diseases.