Harmonin (Ush1c) Expression in Müller Glial Cells Is Required for Synaptic Function

Abstract

Purpose: : Mutations in the human gene encoding Harmonin cause Usher syndrome type 1C. USH1C patients have profound congenital deafness, vestibular dysfunction, and progressive vision loss usually commencing in the first decade of life. In this study, we have developed a zebrafish model of USH1C to elucidate the role of Harmonin in normal vision and hearing.

Methods: : Morpholino oligonucleotides were used to disrupt Harmonin function in larval zebrafish. The resulting retinal phenotype was analyzed by histology, immunohistochemistry, and TEM. ERGs were recorded from dark-adapted, morpholino-treated and control larvae at 5-6 days development. L-AP4 and TBOA were used to isolate photoreceptor function.

Results: : Zebrafish ush1c is expressed in the retina, ear, and lateral line from embryonic stages onward. Knockdown of Harmonin protein via antisense morpholinos results in hearing, balance and vision defects in young larvae. Although very little cell death is observed in larval retinas, ush1c knockdown results in defective synaptic connections in the OPL. ERGs from these animals show a diminished b-wave amplitude, while treatment with L-AP4 and TBOA elicit an a-wave response similar to that of control animals. In the retina, zebrafish Harmonin is expressed predominantly in Müller glia at all larval and adult stages examined, and the antibody signal is significantly diminished in morpholino treated animals.

Conclusions: : Harmonin is necessary for normal synaptic development and function in the retina. These data indicate a role for Müller glial cells in retinal synapse formation and possibly in the USH1C retinal pathology that precedes RP.

Keywords: retinal degenerations: hereditary • Muller cells • synapse