Aberrant Arl3 activity drives nuclear migration defect in rod photoreceptors
The small GTPase Arl3 is important for the enrichment of lipidated proteins to primary cilia, including the outer segment of photoreceptors. Human mutations ARL3 cause both autosomal recessive and dominant inherited retinal dystrophies. We discovered that dominant mutations result in increased active G-protein—Arl3-D67V has constitutive activity and Arl3-Y90C is fast cycling—and their expression in mouse rods resulted in a displaced nuclear phenotype due to an aberrant Arl3-GTP gradient. We go on to show that removing or restoring the Arl3-GTP gradient within the cilium is sufficient to rescue the nuclear migration defect. Our results reveal that an Arl3 ciliary gradient is involved in proper positioning of photoreceptor nuclei during retinal development.
Photoreceptor discs: built like ectosomes
Spencer WJ, Lewis TR, Pearring JN, Arshavsky VY.
The light-sensitive outer segment organelle of the vertebrate photoreceptor cell is a modified cilium filled with hundreds of flattened “disc” membranes that provide vast light-absorbing surfaces. In this review, we describe recent breakthroughs in our understanding of photoreceptor disc morphogenesis, with a focus on the molecular mechanisms responsible for initiating disc formation from the ciliary membrane. We highlight the discoveries that this mechanism evolved from an innate ciliary process of releasing small extracellular vesicles, or ectosomes, and that both disc formation and ectosome release rely on the actin cytoskeleton.
Photoreceptor discs form through peripherin-dependent suppression of ciliary ectosome release.
Salinas RY, Pearring JN, Ding JD, Spencer WJ, Hao Y, Arshavsky VY.
The primary cilium is a highly conserved organelle housing specialized molecules responsible for receiving and processing extracellular signals. A recently discovered property shared across many cilia is the ability to release small vesicles called ectosomes, which are used for exchanging protein and genetic material among cells. In this study, we report a novel role for ciliary ectosomes in building the elaborate photoreceptor outer segment filled with hundreds of tightly packed "disc" membranes. We demonstrate that the photoreceptor cilium has an innate ability to release massive amounts of ectosomes. However, this process is suppressed by the disc-specific protein peripherin, which enables retained ectosomes to be morphed into discs. This new function of peripherin is performed independently from its well-established role in maintaining the high curvature of disc edges, and each function is fulfilled by a separate part of peripherin's molecule. Our findings explain how the outer segment structure evolved from the primary cilium to provide photoreceptor cells with vast membrane surfaces for efficient light capture.
Peripherin diverts ciliary ectosome release to photoreceptor disc morphogenesis.
Spotlight article in Journal of Cell Biology. 2017; 216(5):1227-1229.
Guanylate cyclase 1 relies on rhodopsin for intracellular stability and ciliary trafficking.
Pearring JN, Spencer WJ, Lieu EC, Arshavsky VY.
Sensory cilia are populated by a select group of signaling proteins that detect environmental stimuli. How these molecules are delivered to the sensory cilium and whether they rely on one another for specific transport remains poorly understood. Here, we investigated whether the visual pigment, rhodopsin, is critical for delivering other signaling proteins to the sensory cilium of photoreceptor cells, the outer segment. Rhodopsin is the most abundant outer segment protein and its proper transport is essential for formation of this organelle, suggesting that such a dependency might exist. Indeed, we demonstrated that guanylate cyclase-1, producing the cGMP second messenger in photoreceptors, requires rhodopsin for intracellular stability and outer segment delivery. We elucidated this dependency by showing that guanylate cyclase-1 is a novel rhodopsin-binding protein. These findings expand rhodopsin's role in vision from being a visual pigment and major outer segment building block to directing trafficking of another key signaling protein.