Abstract
Abstract: :
Purpose: Mutations in the myosin VIIA gene can result in the deafness–blindness disorder known as Usher Syndrome. The zebrafish mariner mutant has mutations in myosin VIIA and exhibits vestibular and hearing defects but the retinal phenotype is unknown. The purpose of this study was to examine the consequences of a myosin VIIA nonsense mutation on photoreceptor histology, physiology, and opsin localization. Methods: Visual thresholds in zebrafish mariner mutants were measured using the optokinetic response (OKR) behavioral assay. To assess retinal function, full–field ERGs were performed under both scotopic and photopic conditions on 5, 7, and 10 day–old larvae. Photoreceptor morphology was examined under light– and transmission electron microscopy. To study the effects on opsin transport, the mariner mutation was bred into a transgenic line that expresses GFP fused to the C–terminal tail of rhodopsin. Immunohistochemistry was used to determine the localization of several photoreceptor proteins. Results: The mariner mutant exhibited a consistent increase in visual threshold with behavioral testing and a significant decrease in the amplitude of both the b– and d–wave of the ERG. In mariner mutants, light microscopy revealed a slight increase in cell death in the outer nuclear layer whereas electron microscopy showed mild abnormalities in the photoreceptor outer segments. Significant redistribution of rod opsin or UV, blue, or green cone opsin immunoreactivity was not observed in mariner mutants. Conclusions: Zebrafish deficient in myosin VIIA exhibit many of the characteristics of Usher Syndrome, including reduced ERG amplitudes and histological abnormalities. The mariner mutant provides a useful model for studying the retinal phenotype of Usher Syndrome.
Keywords: retinal degenerations: cell biology • transgenics/knock–outs • retina