Zebrafish Rpgr is required for normal retinal development and plays a role in dynein-based retrograde transport processes

Mutations in the human RPGR gene cause one of the most common and severe forms of inherited retinal dystrophy but the function of its protein product remains unclear. We have identified two genes resembling human RPGR (ZFRPGR1, ZFRPGR2) in zebrafish (Danio rerio), both of which are expressed within the nascent and adult eye as well as more widely during development. ZFRPGR2 appears to be functionally orthologous to human RPGR since it encodes similar protein isoforms (ZFRPGR2(ORF15), ZFRPGR2(ex1-17)) and causes developmental defects similar to other ciliary proteins, affecting gastrulation, tail and head development after morpholino-induced knockdown (translation suppression). These defects are consistent with a ciliary function and were rescued by human RPGR but not by RPGR mutants causing retinal dystrophy. Unlike mammals, RPGR knockdown in zebrafish resulted in both abnormal development and increased cell death in the dysplastic retina. Developmental abnormalities in the eye included lamination defects, failure to develop photoreceptor outer segments and a small eye phenotype, associated with increased cell death throughout the retina. These defects could be rescued by expression of wildtype but not mutant forms of human RPGR. ZFRPGR2 knockdown also resulted in an intracellular transport defect affecting retrograde but not anterograde transport of organelles. ZFRPGR2 is therefore necessary both for the normal differentiation and lamination of the retina and to prevent apoptotic retinal cell death, which may related to its proposed role in dynein-based retrograde transport processes.