Localization of PDZD7 to the stereocilia ankle-link associates this scaffolding protein with the usher syndrome protein network

Usher syndrome is the leading cause of genetic deaf-blindness. Monoallelic mutations in PDZD7 increase the severity of Usher type II syndrome caused by mutations in USH2A and GPR98, which respectively encode usherin and GPR98. PDZ domain-containing 7 protein (PDZD7)isaparalogofthe scaffolding proteins harmonin and whirlin, which are implicatedinUsher type1and type2syndromes. While usherin and GPR98 have been reportedtoform hair cell stereocilia ankle-links, harmonin localizestothe stereocilia upper tip-link density and whirlin localizes to both tip and ankle-link regions. Here, we used mass spectrometry to show that PDZD7 is expressed in chick stereocilia at a comparable molecular abundance to GPR98. We also show by immunofluorescence and by overexpression of tagged proteinsin rat and mouse hair cells that PDZD7 localizes to the ankle-link region, overlapping with usherin, whirlin, and GPR98. Finally, we show in LLC-PK1 cells that cytosolic domains of usherin and GPR98 can bind to both whirlin and PDZD7. These observations are consistent with PDZD7 being a modifier and candidate gene for USH2, and suggest that PDZD7 isa second scaffolding component of the ankle-link complex.