Control of protein and lipid composition of photoreceptor outer segments-Implications for retinal disease

Vision is arguably our most important sense, and its loss brings substantial limitations to daily life for affected individuals. Light is perceived in retinal photoreceptors (PRs), which are highly specialized neurons subdivided into several compartments with distinct functions. The outer segments (OSs) of photoreceptors represent highly specialized primary ciliary compartments hosting the phototransduction cascade, which transforms incoming light into a neuronal signal. Retinal disease can result from various pathomechanisms originating in distinct subcompartments of the PR cell, or in the retinal pigment epithelium which supports the PRs. Dysfunction of primary cilia causes human disorders known as "ciliopathies", in which retinal disease is a common feature. This chapter focuses on PR OSs, discussing the mechanisms controlling their complex structure and composition. A sequence of tightly regulated sorting and trafficking events, both upstream of and within this ciliary compartment, ensures the establishment and maintenance of the adequate proteome and lipidome required for signaling in response to light. We discuss in particular our current understanding of the role of ciliopathy proteins involved in multi-protein complexes at the ciliary transition zone (CC2D2A) or BBSome (BBS1) and how their dysfunction causes retinal disease. While the loss of CC2D2A prevents the fusion of vesicles and delivery of the photopigment rhodopsin to the ciliary base, leading to early OS ultrastructural defects, BBS1 deficiency results in precocious accumulation of cholesterol in mutant OSs and decreased visual function preceding morphological changes. These distinct pathomechanisms underscore the central role of ciliary proteins involved in multiple processes controlling OS protein and lipid composition.

Expansion Microscopy of Mouse Photoreceptor Cilia

The small size of ciliary structures that underlies photoreceptor function and inherited ciliopathies requires imaging techniques adapted to visualizing them at the highest possible resolution. In addition to powerful super-resolution imaging modalities, emerging approaches to sample preparation, including expansion microscopy (ExM), can provide a robust route to imaging specific molecules at the nanoscale level in the retina. We describe a protocol for applying ExM to whole retinas in order to achieve nanoscale fluorescence imaging of ciliary markers, including tubulin, CEP290, centrin, and CEP164. The results are consistent with those from other super-resolution fluorescence techniques and reveal new insights into their arrangements with respect to the subcompartments of photoreceptor cilia. This technique is complimentary to other imaging modalities used in retinal imaging, and can be carried out in virtually any laboratory, without the need for expensive specialized equipment.

The Role of the Prph2 C-Terminus in Outer Segment Morphogenesis

Peripherin 2 (also known as RDS/Prph2) is localized to the rims of rod and cone outer segment (OS) discs. The C-terminus of Prph2 is a critical functional domain, but its exact role is still unknown. In this mini review, we describe work on the Prph2 C-terminus, highlighting its role as a regulator of protein trafficking, membrane curvature, ectosome secretion, and membrane fusion. Evidence supports a role for the Prph2 C-terminus in these processes and demonstrates that it is necessary for the initiation of OS morphogenesis.

Rhodopsin Trafficking and Mistrafficking: Signals, Molecular Components, and Mechanisms

Rhodopsin is a seven-transmembrane G protein-coupled receptor (GPCR) and is the main component of the photoreceptor outer segment (OS), a ciliary compartment essential for vision. Because the OSs are incapable of protein synthesis, rhodopsin must first be synthesized in the inner segments (ISs) and subsequently trafficked across the connecting cilia to the OSs where it participates in the phototransduction cascade. Rapid turnover of the OS necessitates a high rate of synthesis and efficient trafficking of rhodopsin to the cilia. This cilia-targeting mechanism is shared among other ciliary-localized GPCRs. In this review, we will discuss the process of rhodopsin trafficking from the IS to the OS beginning with the trafficking signals present on the protein. Starting from the endoplasmic reticulum and the Golgi apparatus within the IS, we will cover the molecular components assisting the biogenesis and the proper sorting. We will also review the confirmed binding and interacting partners that help target rhodopsin toward the connecting cilium as well as the cilia-localized components which direct proteins into the proper compartments of the OS. While rhodopsin is the most critical and abundant component of the photoreceptor OS, mutations in the rhodopsin gene commonly lead to its mislocalization within the photoreceptors. In addition to covering the trafficking patterns of rhodopsin, we will also review some of the most common rhodopsin mutants which cause mistrafficking and subsequent death of photoreceptors. Toward the goal of understanding the pathogenesis, three major mechanisms of aberrant trafficking as well as putative mechanisms of photoreceptor degeneration will be discussed.