Defective Photoreceptors Underlie Inherited Blindness in the Raifteirí Mutant

Abstract

Purpose: : To characterise the basis of the inherited visual function defect in the Raifteirí mutant.

Methods: : Optokinetic response (OKR) screens were performed on N-ethyl-N-nitrosourea (ENU)-mutagenised zebrafish to identify families with recessive defects in visual function. Retinal differentiation and lamination was characterised by light microscopy, and immunohistochemistry of retinal sections using the cell-specific markers zpr1, zpr3, 5e11 and zn-5. Cell death levels were determined by TUNEL staining. The integrity of the inner retina (hyaloid) and trunk vasculature was analysed by alkaline phosphatase staining. Circadian locomotion and shadow-response behaviours were ascertained using Zebralab (Viewpoint Zebrafish Tracking System).

Results: : A novelmutant designated Raifteirí (raf) was identified by OKR screens. Raf mutants show a significantly reduced or absent OKR and are paler with slightly smaller eyes. The locomotor activity levels of raf mutants are ~50% of normal siblings, however, raf mutants exhibit circadian behaviour and a shadow reponse. No difference in vasculature morphology or levels of cell death is observed between mutants and siblings. Light microscopy and DAPI staining show raf mutants to have normal retinal lamination. 5e11 and zn-5 staining of the inner retina suggests that amacrine and ganglion cell differentiation is equivalent in siblings and raf mutants. In contrast, staining of the outer retina with photoreceptor markers zpr3 and zpr1 appears to be reduced overall in raf mutants. However, staining of newborn rod photoreceptors in the peripheral retina is normal. Preliminary light microscopy data suggests a morphological defect in raf photoreceptors and retinal pigment epithelium (RPE).

Conclusions: : We have identified and partially characterised a novel zebrafish model of inherited blindness. Preliminary results suggest that specific defects in raf outer retinamorphology underlie loss of visual function.

Keywords: photoreceptors • visual development • genetics