Abca4, mutated in Stargardt disease, is required for structural integrity of cone outer segments

Stargardt disease (STGD), the leading cause of inherited childhood blindness, is primarily caused by mutations in the ABCA4 gene; yet, the underlying mechanisms of photoreceptor degeneration remain elusive, partly due to limitations in existing animal disease models. To expand our understanding, we mutated the human ABCA4 paralogues abca4a and abca4b in zebrafish, which has a cone-rich retina. Our study unveiled striking dysmorphology and elongation of cone outer segments (COS) in abca4a;abca4b double mutants, alongside reduced phagocytosis by the retinal pigmented epithelium (RPE). We report that zebrafish Abca4 protein forms a distinctive stripe along the length of COS, suggesting a potential structural role. We further show that, in wild-type zebrafish, outer segments of cone cells constitutively present externalized phosphatidylserine, an apoptotic 'eat-me' signal, and that this pattern is disrupted in abca4a;abca4b double mutants, potentially contributing to reduced RPE phagocytic activity. More broadly, constitutive presentation of the 'eat-me' signal by COS - if conserved in humans - might have important implications for other retinal degenerative diseases, including age-related macular degeneration. Our zebrafish model provides novel insights into cone dysfunction and presents a promising platform for unraveling the mechanisms of STGD pathogenesis and advancing therapeutic interventions.