Protein sorting, targeting and trafficking in photoreceptor cells

Abstract

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a lightsensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

Heterotrimeric Kinesin-2 (KIF3) Mediates Transition Zone and Axoneme Formation of Mouse Photoreceptors

“…Because of continuous OS turnover, rod trafficking requires a flow of ϳ70 rhodopsin molecules per second to be advanced through the PTZ. In Xenopus, an animal with very large photoreceptors, the flow increases to several hundred rhodopsins/ second (64). Alternative trafficking pathways have been proposed in which rhodopsin-containing vesicles stream toward the OS, without motor activity, inside the axonemal shaft to eventually expand into large disk membranes at the proximal OS (33,65).…”

Beyond spectral tuning: human cone visual pigments adopt different transient conformations for chromophore regeneration

“…Bright light activates the photosensitive opsin pigments which are embedded in the outer segment membrane discs of cone retinal cells thereby initiating the complex process of e.Proofing http://eproofing.springer.com/journals/printpage.php?token=CL_CUU... photopic vision with a fast response [ 3 ]. The cone visual pigments correspond to blue opsin (SWL, short wavelength), green opsin (MWL, medium wavelength) and red opsin (LWL, long wavelength).…”

Accumulation of non-outer segment proteins in the outer segment underlies photoreceptor degeneration in Bardet–Biedl syndrome

“…One remarkable feature of the OS is its continuous renewal to prevent accumulation of damaged proteins caused by photooxidative stress. Older components are constantly shed at the distal end of the OS, and new proteins are delivered at the proximal end, which is linked to the cell body [the inner segment (IS)], where proteins are produced (1)(2)(3). This OS renewal is a daunting task for the trafficking system, transporting nearly 700 rhodopsin (Rho) molecules per second in frogs and 80 in mice (1,4).…”

“…The mouse rod OS is completely renewed within 10 days (2). Not surprisingly, mutations impairing protein trafficking to the OS cause various types of photoreceptor degeneration, and considerable research efforts have been made to elucidate mechanisms of protein trafficking to the OS (1,5,6).…”