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Jennifer Phillips, Sabrina Toro, Marcel Rockwell, Monte Westerfield; Usher proteins contribute to cell-cell interactions between photoreceptors in the zebrafish retina. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1191.
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© ARVO (1962-2015); The Authors (2016-present)
Vision loss in Usher syndrome is due to progressive retinal degeneration. Previously, we showed that the zebrafish Usher type 1C protein, Harmonin, localizes in the apical termini of Müller Glial Cells (MGCs) that are thought to form cell junctions between glia and cone photoreceptors. Here, we explore the potential roles of Usher proteins in this subapical region (SAR) and between cone inner segments.
We used antibodies against zebrafish harmonin (ush1c), sans (ush1g), whirlin (ush2d) and clrn1 (ush3a) to immunolabel cryosectioned retinas from larval, juvenile and adult zebrafish wild-type, ush1c-/-, or transgenic lines expressing GFP driven by uv opsin, blue cone opsin or GFAP promoters. Tissues were co-labeled with antibodies against Crumbs (crb2a or crb2b), rod and cone markers ZPR1 and ZPR3, Müller cell marker Glutamine synthetase and phalloidin to label f-actin. Images were obtained by confocal microscopy.
Whirlin and clrn1 both localize to lateral contacts between cone inner segments apical to the outer limiting membrane (OLM), closely associated with the SAR. In larval retinas, whirlin is highly restricted to this lateral cell boundary and shares a localization pattern with crb2b. In addition to localization at cell membranes in or near the SAR, clrn1 has a broader localization domain in the cytoplasm of all cone inner segments, a distribution shared by sans. These distributions are unchanged in ush1c mutants. In wild-type adult retinas, clrn1 localizes to lateral cell contacts between inner segments of all cone subtypes, and is also present in rods. Whirlin remains associated with glial processes apical to the OLM and at the junctional interfaces of blue and uv cones.
The colocalization of Usher and Crumbs proteins to lateral cell contacts in the outer retina is consistent with previously reported interactions between whirlin and the Crumbs interacting protein, mpp5. Mutations resulting in loss of Usher proteins produce slow, progressive vision loss, in contrast to the severe, early retinal defects seen with loss of function mutations in CRB and associated factors. Nevertheless, the presence of Usher proteins in these regions may indicate a role in cell adhesion, junction formation or maintenance, such that loss of Usher proteins at these lateral interfaces could contribute to progressive vision loss.
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