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O. Biehlmaier, C. Hodel, S. C. F. Neuhauss; Localization and Function of Myosin VIIA in the Retina of Wild-Type and Mutant Zebrafish. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4495.
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© ARVO (1962-2015); The Authors (2016-present)
Myosin VIIA (Myo7a) is an unconventional myosin acting as a motor molecule that moves along actin filaments transporting specific cargo to its destination in the cell. Myo7a has been shown to be expressed in inner hair cells of the ear, retinal photoreceptors, and in the retinal pigment epithelium (RPE). Mutations in the human gene encoding for Myo7a cause Usher syndrome 1B. Affected patients suffer of deafness, vestibular dysfunction, and upcoming retinitis pigmentosa. Even though Myo7a expression has been shown in the ear, the cellular localization and the function of Myo7a in the zebrafish retina still remains unclear. Here, we describe the localization of Myo7a in the zebrafish retina by comparing wild-type and myo7a mutant fish, and probe its functionality.
We examined light and dark adapted larvae of three different mariner mutant alleles (mar ty220d, tc320b, tr202b) at different developmental stages. The morphology of dark / light adapted, and light damaged retinas was assessed by standard histology and electron microscopy. Immunocytochemistry was used to identify subpopulations of retinal neurons and a newly raised antibody was used to specifically localize zebrafish Myo7a. Furthermore, the optokinetic response (OKR) as well as standard electroretinograms (ERGs) and double flash ERGs were used to analyze functionality and light adaptation defects in the mar mutants.
In both, ERG and OKR analysis, recovery is prolonged after dark adaptation in the mutants. Morphological and morphometrical analyses show that pigment migration during light adaptation is impaired at least in two mar alleles. Immunolabeling of the adult retina with a zebrafish-specific Myo7a-antibody shows a staining in the area of the photoreceptor cilia between cone outer and inner segments as well as in presumptive microvilli of the RPE interdigitating between the cones. The cellular localization of Myo7a in wild-type and mar mutant larva will be analyzed in further experiments. Electron microscopical analyses suggest an alteration of RPE and cone cells. Additionally, light damage experiments will show whether the light adaptation defect found in our OKR and ERG-studies causes increased photoreceptor vulnerability to high light doses in the mar mutant.
Our data suggest the presence of Myo7a in photoreceptor and in RPE cells of the zebrafish retina. Our OKR and ERG data suggest that Myo7a malfunction interferes with light adaptation which is likely to be the primary defect in the visual system of mar mutant zebrafish.
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