Ciliary tip actin dynamics regulate the cadenceof photoreceptor disc formation

As signalling organelles, primary cilia regulate their membrane G protein-coupled receptor (GPCR) content by ectocytosis, a process requiring localised actin dynamics at their tip to alter membrane shape.(1, 2) Mammalian photoreceptor outer segments comprise an expanse of folded membranes (discs) at the tip of highly-specialised connecting cilia (CC), in which photosensitive GPCRs like rhodopsin are concentrated. In an extraordinary feat of biology, outer segment discs are shed and remade daily.(3) Defects in this process, due to genetic mutations, cause retinitis pigmentosa (RP), an untreatable, blinding disease. The mechanism by which photoreceptor cilia generate outer segments is therefore fundamental for vision yet poorly understood. Here, we show the membrane deformation required for outer segment disc genesis is driven by dynamic changes in the actin cytoskeleton in a process akin to ectocytosis. Further, we show RPGR, a leading causal RP gene, regulates activity of actin binding proteins crucial to this process. Disc genesis is compromised in Rpgr mouse models, slowing the actin dynamics required for timely disc formation, leading to aborted membrane shedding as ectosome-like vesicles, photoreceptor death and visual loss. Manipulation of actin dynamics partially rescues the phenotype, suggesting this pathway could be targeted therapeutically. These findings help define how actin-mediated dynamics control outer segment turnover.