Ciliary tip actin dynamics regulate the cadence of photoreceptor disc formation

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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.

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  1. Review coordinated by Life Science Editors.

    Reviewed by: Dr. Angela Andersen, Life Science Editors

    Potential Conflicts of Interest: Dr. Mill has worked with Life Science Editors on other manuscripts.

    Background: Retinitis pigmentosa (RP) is a group of rare eye diseases that cause vision loss. Symptoms usually start in childhood, and most people eventually lose most of their sight. There is no cure for RP. Mutations in retinitis pigmentosa GTPase regulator (RPGR) cause RP and compromise the renewal of light-sensitive “disc” membranes (specialized cilia) at the outer segment of photoreceptors, resulting in the loss of these cells over time. Evidence suggests that disc formation is similar to the release of ectosomes (small extracellular vesicles) and that both rely on the actin cytoskeleton. Knockdown of RPGR in retinal pigmented epithelium cells showed stronger actin filaments and reduced cilia suggesting that it may regulate nascent photoreceptor disc formation by regulating actin-mediated membrane extension in the retina (Gakovic et al., Human Molecular Genetics, 2011). In addition, RPGR patient iPSC-retinal models displayed phenotypes consistent with abnormal actin regulation (Megaw et al., Nature Communications, 2017; Karam et al., J Personalized Medicine, 2022).

    Question: What function of RPGR is compromised in photoreceptors to cause RP?

    Advance: The authors generated novel Rpgr mutant mice harboring human disease-causing mutations that recapitulate human disease phenotypes: aborted membrane shedding as ectosome-like vesicles, photoreceptor death and visual loss. RPGR is located at the site of disc formation – to test if it plays a role in disc genesis, they engineered a novel reporter mouse to track outer segment turnover. Rhodopsin was tagged with the self-labelling peptide SNAP- Rhodopsin is the major protein component of outer segment discs, and so incubating RhodSNAP retinal slice cultures with SNAP fluorophores results in outer segment labelling. Perturbation of RPGR resulted in a slowed rate of disc formation, leading to shortened outer segments and increased vesicle shedding. To me, the breakthrough is in the last figure: the actin depolymerizing drug Cytochalasin D in PBS was injected intravitreally, and fixed retinas were analyzed 6 hours later by electron microscopy. Cytochalasin D treatment significantly reduced the number of shed vesicles from the base of the outer segment in Rpgr-mutant mice (they now look like wild-type).

    Significance: Nails down the disease-relevant function of RPGR and a molecular mechanism of RP in photoreceptor cells, in vivo, in mice. Pharmacological rescue not only demonstrates the importance of the mechanism to disease but also sheds light on a potential therapeutic avenue for RP.