The cone-dominant retina and the inner ear of zebrafish express the ortholog of CLRN1, the causative gene of human Usher syndrome type 3A

Abstract

Clarin-1 (CLRN1) is the causative gene in Usher syndrome type 3A, an autosomal recessive disorder characterized by progressive vision and hearing loss. CLRN1 encodes Clarin-1, a glycoprotein with homology to the tetraspanin family of proteins. Previous cell culture studies suggest that Clarin-1 localizes to the plasma membrane and interacts with the cytoskeleton. Mouse models demonstrate a role for the protein in mechanosensory hair bundle integrity, but the function of Clarin-1 in hearing remains unclear. Even less is known of its role in vision, because the Clrn1 knockout mouse does not exhibit a retinal phenotype and expression studies in murine retinas have provided conflicting results. Here, we describe cloning and expression analysis of the zebrafish clrn1 gene, and report protein localization of Clarin-1 in auditory and visual cells from embryonic through adult stages. We detect clrn1 transcripts as early as 24h post-fertilization, and expression is maintained through adulthood. In situ hybridization experiments show clrn1 transcripts enriched in mechanosensory hair cells and supporting cells of the inner ear and lateral line organ, photoreceptors, and cells of the inner retina. In mechanosensory hair cells, Clarin-1 is polarized to the apical cell body and the synapses. In the retina, Clarin-1 localizes to lateral cell contacts between photoreceptors and is associated with the outer limiting membrane and subapical processes emanating from Müller glial cells. We also find Clarin-1 protein in the outer plexiform, inner nuclear and ganglion cell layers of the retina. Given the importance of Clarin-1 function in the human retina, it is imperative to find an animal model with a comparable requirement. Our data provide a foundation for exploring the role of Clarin-1 in retinal cell function and survival in a diurnal, cone-dominant species.