Role of microRNAs in cone photoreceptor development and during retinal degeneration

Photoreceptors are the light sensitive neurons in the retina that are critical for vision. Retinal dystrophies leading to photoreceptor cell loss are a major cause of blindness and there are currently no effective treatments to restore vision. Specifically, cone photoreceptor cell death has the greatest impact on sight as these cells are essential for colour vision and visual acuity. microRNAs (miRNAs) have been associated with the aetiology of common retinal diseases and whilst increasing evidence indicates they could be acting as agents of degeneration, their role in photoreceptor cell death remains poorly understood. This thesis aims to better understand how miRNAs are involved in the mechanisms leading to cone photoreceptor cell loss. The Chrnb4-GFP reporter mouse line was characterised and its expression in developing immature cones was confirmed by immunohistochemistry and validated by RNA sequencing. Successful isolation and RNA sequencing of Chrnb4-GFP+ve cones and CD73+ve rods was achieved from the same pool of retinas, allowing for a direct comparison of the transcriptome of cones and rods, and thus identifying potential novel cone markers. To assess the role of miRNAs in cone degeneration, Chrnb4-cre driven recombination of Dicer was carried out in developing cone photoreceptors. Chrnb4-cre; Dicer flox/flox conditional knockout mice displayed an early-onset progressive cone photoreceptor degeneration, characterised by an initial segment impairment and a progressive cone cell loss. While cone morphology and survival was affected by Dicer depletion, rod photoreceptors remained unaffected. These data indicate that miRNA dysregulation can lead to a model of cone dystrophy and suggests miRNAs may be major contributors in photoreceptor degenerations. This study shows for the first time that Dicer processing in developing cones is essential for their survival, and emphasises the need of identifying the specific miRNAs that promote cone survival with the goal of developing miRNA based therapies to tackle sight loss