Affinity of rhodopsin to raft enables the aligned oligomer formation from dimers: Coarse-grained molecular dynamics simulation of disk membranes

Dynamic lipid turnover in photoreceptors and retinal pigment epithelium throughout life

The retinal pigment epithelium-photoreceptor interphase is renewed each day in a stunning display of cellular interdependence. While photoreceptors use photosensitive pigments to convert light into electrical signals, the RPE supports photoreceptors in their function by phagocytizing shed photoreceptor tips, regulating the blood retina barrier, and modulating inflammatory responses, as well as regenerating the 11-cis-retinal chromophore via the classical visual cycle. These processes involve multiple protein complexes, tightly regulated ligand-receptors interactions, and a plethora of lipids and protein-lipids interactions. The role of lipids in maintaining a healthy interplay between the RPE and photoreceptors has not been fully delineated. In recent years, novel technologies have resulted in major advancements in understanding several facets of this interplay, including the involvement of lipids in phagocytosis and phagolysosome function, nutrient recycling, and the metabolic dependence between the two cell types. In this review, we aim to integrate the complex role of lipids in photoreceptor and RPE function, emphasizing the dynamic exchange between the cells as well as discuss how these processes are affected in aging and retinal diseases.

A hybrid stochastic/deterministic model of single photon response and light adaptation in mouse rods

•

A hybrid stochastic/deterministic model of mouse rod phototransduction is presented.

•

Rod photocurrent to photovoltage conversion in darkness is accurately characterized.

•

Photoresponses to dim and bright stimuli and in various mutants are well reproduced.

•

Recently debated molecular mechanisms of the phototransduction cascade are examined.

The phototransduction cascade is paradigmatic for signaling pathways initiated by G protein-coupled receptors and is characterized by a fine regulation of photoreceptor sensitivity and electrical response to a broad range of light stimuli. Here, we present a biochemically comprehensive model of phototransduction in mouse rods based on a hybrid stochastic and deterministic mathematical framework, and a quantitatively accurate description of the rod impedance in the dark. The latter, combined with novel patch clamp recordings from rod outer segments, enables the interconversion of dim flash responses between photovoltage and photocurrent and thus direct comparison with the simulations. The model reproduces the salient features of the experimental photoresponses at very dim and bright stimuli, for both normal photoreceptors and those with genetically modified cascade components. Our modelling approach recapitulates a number of recent findings in vertebrate phototransduction. First, our results are in line with the recently established requirement of dimeric activation of PDE6 by transducin and further show that such conditions can be fulfilled at the expense of a significant excess of G protein activated by rhodopsin. Secondly, simulations suggest a crucial role of the recoverin-mediated Ca²⁺-feedback on rhodopsin kinase in accelerating the shutoff, when light flashes are delivered in the presence of a light background. Finally, stochastic simulations suggest that transient complexes between dark rhodopsin and transducin formed prior to light stimulation increase the reproducibility of single photon responses. Current limitations of the model are likely associated with the yet unknown mechanisms governing the shutoff of the cascade.

Supramolecular organization of rhodopsin in rod photoreceptor cell membranes

Rhodopsin is the light receptor in rod photoreceptor cells that initiates scotopic vision. Studies on the light receptor span well over a century, yet questions about the organization of rhodopsin within the photoreceptor cell membrane still persist and a consensus view on the topic is still elusive. Rhodopsin has been intensely studied for quite some time, and there is a wealth of information to draw from to formulate an organizational picture of the receptor in native membranes. Early experimental evidence in apparent support for a monomeric arrangement of rhodopsin in rod photoreceptor cell membranes is contrasted and reconciled with more recent visual evidence in support of a supramolecular organization of rhodopsin. What is known so far about the determinants of forming a supramolecular structure and possible functional roles for such an organization are also discussed. Many details are still missing on the structural and functional properties of the supramolecular organization of rhodopsin in rod photoreceptor cell membranes. The emerging picture presented here can serve as a springboard towards a more in-depth understanding of the topic.

Light or tyrosine phosphorylation recruits retinal rod outer segment proteins to lipid rafts

Lipid rafts are localized liquid-ordered regions of the plasma membrane that contain high levels of cholesterol and glycosphingolipids, and are resistant to extraction with nonionic detergents. Retinal photoreceptor cells contain detergent-resistant membrane microdomains (DRM), which were isolated here from bovine rod outer segments (ROS) under dark and light conditions. Rhodopsin (R) was present in both DRM and detergent soluble fractions (DSF), and detergent-insoluble ROS rafts were enriched in caveolin 1 (Cav-1) and c-Src. In the dark, arrestin and its 44-kDa truncated form (p44) were present mainly in DSF; however, p44 was translocated to DRM under illumination. Similarly, transducin (T) was mainly present in DSF in the dark, but it was recruited toward the DRM fraction following photolysis. DRM were also prepared in the absence or presence of Mg-ATP, guanosine 5'-3-O-(thio)triphosphate (GTPγS), or both. Although GTPγS released T into DSF in the light, GTPγS-activated T was retained in DRM when Mg²⁺ and ATP were added. Moreover, T was always tyrosine-phosphorylated under light conditions, which suggested that T phosphorylation prevents its GTPγS-induced release from DRM. In addition, treatment with the tyrosine kinase inhibitor genistein prevented the segregation of T to the rafts. In contrast, no localization difference was seen in the presence of Mg-ATP for Cav-1, c-Src, R and both forms of arrestin. Interestingly, immunoprecipitation assays followed by Western blot analyses under light conditions showed the formation of multimeric complexes containing R, T, c-Src, p44 and Cav-1 in DRM, where T and c-Src were tyrosine-phosphorylated.