The role of cholesterol in rod outer segment membranes

Paradoxes: Cholesterol and Hypoxia in Preeclampsia

Preeclampsia, a hypertensive disease of pregnancy of unknown etiology, is intensely studied as a model of cardiovascular disease (CVD) not only due to multiple shared pathologic elements but also because changes that develop over decades in CVD appear and resolve within days in preeclampsia. Those affected by preeclampsia and their offspring experience increased lifetime risks of CVD. At the systemic level, preeclampsia is characterized by increased cellular, membrane, and blood levels of cholesterol; however, cholesterol-dependent signaling, such as canonical Wnt/βcatenin, Hedgehog, and endothelial nitric oxide synthase, is downregulated indicating a cholesterol deficit with the upregulation of cholesterol synthesis and efflux. Hypoxia-related signaling in preeclampsia also appears to be paradoxical with increased Hypoxia-Inducible Factors in the placenta but measurably increased oxygen in maternal blood in placental villous spaces. This review addresses the molecular mechanisms by which excessive systemic cholesterol and deficient cholesterol-dependent signaling may arise from the effects of dietary lipid variance and environmental membrane modifiers causing the cellular hypoxia that characterizes preeclampsia.

Shotgun lipidomics of human subretinal fluids under rod-dominant retina reveals cone-dominated lipids

Subretinal fluid (SRF) accumulates between photoreceptor outer segments and retinal pigment epithelium during rhegmatogenous retinal detachment. Biomolecular components such as lipids originate from cells surrounding the SRF. Knowledge of the composition of these molecules in SRF potentially provides mechanistic insight into the physiologic transfer of lipids between retinal tissue compartments. Using mass spectrometry and tandem mass spectrometry analysis on an electrospray ionization quadrupole-time-of-flight mass spectrometer, we identified a total of 115 lipid molecular species of 11 subclasses and 9 classes in two samples from two patients with rhegmatogenous retinal detachment. These included 47 glycerophosphocholines, 6 glycerophosphoethanolamines, 1 glycerophosphoinositol, 18 sphingomyelins, 9 cholesteryl esters, free cholesterol, 3 ceramides, 22 triacylglycerols and 8 free fatty acids. Glycerophosphocholines were of the highest intensity. By minimizing the formation of different adduct forms or clustering ions of different adducts, we determined the relative intensity of lipid molecular species within the same subclasses. The profiles were compared with those of retinal cells available in the published literature. The glycerophosphocholine profile of SRF was similar to that of cone outer segments, suggesting that outer segment degradation products are constitutively released into the interphotoreceptor matrix, appearing in SRF during detachment. This hypothesis was supported by the retinal distributions of corresponding lipid synthases' mRNA expression obtained from an online resource based on publicly available single-cell sequencing data. In contrast, based on lipid profiles and relevant gene expression in this study, the sources of free cholesterol and cholesteryl esters in SRF appeared more ambiguous, possibly reflecting that outer retina takes up plasma lipoproteins. Further studies to identify and quantify lipids in SRF will help better understand etiology of diseases relevant to outer retina.

A cholesterol switch controls phospholipid scrambling by G protein-coupled receptors

Class A G protein-coupled receptors (GPCRs), a superfamily of cell membrane signaling receptors, moonlight as constitutively active phospholipid scramblases. The plasma membrane of metazoan cells is replete with GPCRs yet has a strong resting trans-bilayer phospholipid asymmetry, with the signaling lipid phosphatidylserine confined to the cytoplasmic leaflet. To account for the persistence of this lipid asymmetry in the presence of GPCR scramblases, we hypothesized that GPCR-mediated lipid scrambling is regulated by cholesterol, a major constituent of the plasma membrane. We now present a technique whereby synthetic vesicles reconstituted with GPCRs can be supplemented with cholesterol to a level similar to that of the plasma membrane and show that the scramblase activity of two prototypical GPCRs, opsin and the β1-adrenergic receptor, is impaired upon cholesterol loading. Our data suggest that cholesterol acts as a switch, inhibiting scrambling above a receptor-specific threshold concentration to disable GPCR scramblases at the plasma membrane.

A cholesterol switch controls phospholipid scrambling by G protein-coupled receptors

Class A G protein-coupled receptors (GPCRs), a superfamily of cell membrane signaling receptors, moonlight as constitutively active phospholipid scramblases. The plasma membrane of metazoan cells is replete with GPCRs, yet has a strong resting trans-bilayer phospholipid asymmetry, with the signaling lipid phosphatidylserine confined to the cytoplasmic leaflet. To account for the persistence of this lipid asymmetry in the presence of GPCR scramblases, we hypothesized that GPCR-mediated lipid scrambling is regulated by cholesterol, a major constituent of the plasma membrane. We now present a technique whereby synthetic vesicles reconstituted with GPCRs can be supplemented with cholesterol to a level similar to that of the plasma membrane and show that the scramblase activity of two prototypical GPCRs, opsin and the β1-adrenergic receptor, is impaired upon cholesterol loading. Our data suggest that cholesterol acts as a switch, inhibiting scrambling above a receptor-specific threshold concentration to disable GPCR scramblases at the plasma membrane.

Clearance phagocytosis by the retinal pigment epithelial during photoreceptor outer segment renewal: Molecular mechanisms and relation to retinal inflammation

Mammalian photoreceptor outer segment renewal is a highly coordinated process that hinges on timed cell signaling between photoreceptor neurons and the adjacent retinal pigment epithelial (RPE). It is a strictly rhythmic, synchronized process that underlies in part circadian regulation. We highlight findings from recently developed methods that quantify distinct phases of outer segment renewal in retinal tissue. At light onset, outer segments expose the conserved "eat-me" signal phosphatidylserine exclusively at their distal, most aged tip. A coordinated two-receptor efferocytosis process follows, in which ligands bridge outer segment phosphatidylserine with the RPE receptors αvβ5 integrin, inducing cytosolic signaling toward Rac1 and focal adhesion kinase/MERTK, and with MERTK directly, additionally inhibiting RhoA/ROCK and thus enabling F-actin dynamics favoring outer segment fragment engulfment. Photoreceptors and RPE persist for life with each RPE cell in the eye servicing dozens of overlying photoreceptors. Thus, RPE cells phagocytose more often and process more material than any other cell type. Mutant mice with impaired outer segment renewal largely retain functional photoreceptors and retinal integrity. However, when anti-inflammatory signaling in the RPE via MERTK or the related TYRO3 is lacking, catastrophic inflammation leads to immune cell infiltration that swiftly destroys the retina causing blindness.

Retina and RPE lipid profile changes linked with ABCA4 associated Stargardt's maculopathy

Stargardt maculopathy, caused predominantly by mutations in the ABCA4 gene, is characterized by an accumulation of non-degradable visual pigment derivative, lipofuscin, in the retinal pigment epithelium (RPE) - resulting in RPE atrophy. RPE is a monolayer tissue located adjacent to retinal photoreceptors and regulates their health and functioning; RPE atrophy triggers photoreceptor cell death and vision loss in Stargardt patients. Previously, ABCA4 mutations in photoreceptors were thought to be the major contributor to lipid homeostasis defects in the eye. Recently, we demonstrated that ABCA4 loss of function in the RPE leads to cell-autonomous lipid homeostasis defects. Our work underscores that an incomplete understanding of lipid metabolism and lipid-mediated signaling in the retina and RPE are potential causes for lacking treatments for this disease. Here we report altered lipidomic in mouse and human Stargardt models. This work provides the basis for therapeutics that aim to restore lipid homeostasis in the retina and the RPE.

Structural Impact of Selected Retinoids on Model Photoreceptor Membranes

Photoreceptor membranes have a unique lipid composition. They contain a high level of polyunsaturated fatty acids including the most unsaturated fatty acid in nature, docosahexaenoic acid (22:6), and are enriched in phosphatidylethanolamines. The phospholipid composition and cholesterol content of the subcellular components of photoreceptor outer segments enables to divide photoreceptor membranes into three types: plasma membranes, young disc membranes, and old disc membranes. A high degree of lipid unsaturation, extended exposure to intensive irradiation, and high respiratory demands make these membranes sensitive to oxidative stress and lipid peroxidation. Moreover, all-trans retinal (AtRAL), which is a photoreactive product of visual pigment bleaching, accumulates transiently inside these membranes, where its concentration may reach a phototoxic level. An elevated concentration of AtRAL leads to accelerated formation and accumulation of bisretinoid condensation products such as A2E or AtRAL dimers. However, a possible structural impact of these retinoids on the photoreceptor-membrane properties has not yet been studied. In this work we focused just on this aspect. The changes induced by retinoids, although noticeable, seem not to be significant enough to be physiologically relevant. This is, however, an positive conclusion because it can be assumed that accumulation of AtRAL in photoreceptor membranes will not affect the transduction of visual signals and will not disturb the interaction of proteins engaged in this process.

GPCRs in Intracellular Compartments: New Targets for Drug Discovery

The architecture of eukaryotic cells is defined by extensive membrane-delimited compartments, which entails separate metabolic processes that would otherwise interfere with each other, leading to functional differences between cells. G protein-coupled receptors (GPCRs) are the largest class of cell surface receptors, and their signal transduction is traditionally viewed as a chain of events initiated from the plasma membrane. Furthermore, their intracellular trafficking, internalization, and recycling were considered only to regulate receptor desensitization and cell surface expression. On the contrary, accumulating data strongly suggest that GPCRs also signal from intracellular compartments. GPCRs localize in the membranes of endosomes, nucleus, Golgi and endoplasmic reticulum apparatuses, mitochondria, and cell division compartments. Importantly, from these sites they have shown to orchestrate multiple signals that regulate different cell pathways. In this review, we summarize the current knowledge of this fascinating phenomenon, explaining how GPCRs reach the intracellular sites, are stimulated by the endogenous ligands, and their potential physiological/pathophysiological roles. Finally, we illustrate several mechanisms involved in the modulation of the compartmentalized GPCR signaling by drugs and endogenous ligands. Understanding how GPCR signaling compartmentalization is regulated will provide a unique opportunity to develop novel pharmaceutical approaches to target GPCRs and potentially lead the way towards new therapeutic approaches.

Non-Viral Delivery of CRISPR/Cas Cargo to the Retina Using Nanoparticles: Current Possibilities, Challenges, and Limitations

The discovery of the CRISPR/Cas system and its development into a powerful genome engineering tool have revolutionized the field of molecular biology and generated excitement for its potential to treat a wide range of human diseases. As a gene therapy target, the retina offers many advantages over other tissues because of its surgical accessibility and relative immunity privilege due to its blood-retinal barrier. These features explain the large advances made in ocular gene therapy over the past decade, including the first in vivo clinical trial using CRISPR gene-editing reagents. Although viral vector-mediated therapeutic approaches have been successful, they have several shortcomings, including packaging constraints, pre-existing anti-capsid immunity and vector-induced immunogenicity, therapeutic potency and persistence, and potential genotoxicity. The use of nanomaterials in the delivery of therapeutic agents has revolutionized the way genetic materials are delivered to cells, tissues, and organs, and presents an appealing alternative to bypass the limitations of viral delivery systems. In this review, we explore the potential use of non-viral vectors as tools for gene therapy, exploring the latest advancements in nanotechnology in medicine and focusing on the nanoparticle-mediated delivery of CRIPSR genetic cargo to the retina.

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.

Cholesterol occupies the lipid translocation pathway to block phospholipid scrambling by a G protein-coupled receptor

Class A (rhodopsin-like) G protein-coupled receptors (GPCRs) are constitutive phospholipid scramblases as evinced after their reconstitution into liposomes. Yet phospholipid scrambling is not detectable in the resting plasma membrane of mammalian cells that is replete with GPCRs. We considered whether cholesterol, a prominent component of the plasma membrane, limits the ability of GPCRs to scramble lipids. Our previous Markov State Model (MSM) analysis of molecular dynamics simulations of membrane-embedded opsin indicated that phospholipid headgroups traverse a dynamically revealed hydrophilic groove between transmembrane helices (TM) 6 and 7 while their tails remain in the bilayer. Here, we present comparative MSM analyses of 150-μs simulations of opsin in cholesterol-free and cholesterol-rich membranes. Our analyses reveal that cholesterol inhibits phospholipid scrambling by occupying the TM6/7 interface and stabilizing the closed groove conformation while itself undergoing flip-flop. This mechanism may explain the inability of GPCRs to scramble lipids at the plasma membrane.

Functional expression of opioid receptors and other human GPCRs in yeast engineered to produce human sterols

The yeast Saccharomyces cerevisiae is powerful for studying human G protein-coupled receptors as they can be coupled to its mating pathway. However, some receptors, including the mu opioid receptor, are non-functional, which may be due to the presence of the fungal sterol ergosterol instead of cholesterol. Here we engineer yeast to produce cholesterol and introduce diverse mu, delta, and kappa opioid receptors to create sensitive opioid biosensors that recapitulate agonist binding profiles and antagonist inhibition. Additionally, human mu opioid receptor variants, including those with clinical relevance, largely display expected phenotypes. By testing mu opioid receptor-based biosensors with systematically adjusted cholesterol biosynthetic intermediates, we relate sterol profiles to biosensor sensitivity. Finally, we apply sterol-modified backgrounds to other human receptors revealing sterol influence in SSTR5, 5-HTR4, FPR1, and NPY1R signaling. This work provides a platform for generating human G protein-coupled receptor-based biosensors, facilitating receptor deorphanization and high-throughput screening of receptors and effectors.

Loss of the Bardet-Biedl protein Bbs1 alters photoreceptor outer segment protein and lipid composition

Primary cilia are key sensory organelles whose dysfunction leads to ciliopathy disorders such as Bardet-Biedl syndrome (BBS). Retinal degeneration is common in ciliopathies, since the outer segments (OSs) of photoreceptors are highly specialized primary cilia. BBS1, encoded by the most commonly mutated BBS-associated gene, is part of the BBSome protein complex. Using a bbs1 zebrafish mutant, we show that retinal development and photoreceptor differentiation are unaffected by Bbs1-loss, supported by an initially unaffected transcriptome. Quantitative proteomics and lipidomics on samples enriched for isolated OSs show that Bbs1 is required for BBSome-complex stability and that Bbs1-loss leads to accumulation of membrane-associated proteins in OSs, with enrichment in proteins involved in lipid homeostasis. Disruption of the tightly regulated OS lipid composition with increased OS cholesterol content are paralleled by early functional visual deficits, which precede progressive OS morphological anomalies. Our findings identify a role for Bbs1/BBSome in OS lipid homeostasis, suggesting a pathomechanism underlying retinal degeneration in BBS.

Primary cilia are key sensory organelles whose dysfunction leads to ciliopathy disorders such as Bardet-Biedl syndrome (BBS). Here they identify a role for Bbs1 in lipid homeostasis of photoreceptor outer segments in zebrafish, which may contribute to vision loss in patients with Bardet-Biedl syndrome.

Retinal neuroprotection by controlled release of a VCP inhibitor from self-assembled nanoparticles

Mutations in rhodopsin lead to its misfolding resulting in autosomal dominant retinitis pigmentosa (adRP). Pharmacological inhibition of the ATP-driven chaperone valosin-containing protein (VCP), a molecular checkpoint for protein quality control, slows down retinal degeneration in animal models. However, poor water-solubility of VCP inhibitors poses a challenge to their clinical translation as intravitreal injections for retinal treatment. In order to enable the delivery of VCP inhibitors, we have developed and investigated two formulations for the VCP inhibitor ML240. Nanoformulations of ML240 were obtained by using amphiphilic polymers methoxy-poly (ethylene glycol)_5kDa-cholane (mPEG_5kDa-cholane) and methoxy-poly (ethylene glycol)_5kDa-cholesterol (mPEG_5kDa-cholesterol). Both formulations increased the water-solubility of ML240 by two orders of magnitude and prolonged the drug released over ten days. In addition, encapsulation of ML240 in mPEG_5kDa-cholane showed superior photoreceptor protection at lower drug concentrations, normalized rhodopsin localization, and alleviated inflammatory microglial responses in an ex vivo rat model of retinal degeneration. The study demonstrates the potential of VCP inhibitor nanoformulations to treat adRP, a pharmacologically orphan disease.

Compartmentalization of Photoreceptor Sensory Cilia

Functional compartmentalization of cells is a universal strategy for segregating processes that require specific components, undergo regulation by modulating concentrations of those components, or that would be detrimental to other processes. Primary cilia are hair-like organelles that project from the apical plasma membranes of epithelial cells where they serve as exclusive compartments for sensing physical and chemical signals in the environment. As such, molecules involved in signal transduction are enriched within cilia and regulating their ciliary concentrations allows adaptation to the environmental stimuli. The highly efficient organization of primary cilia has been co-opted by major sensory neurons, olfactory cells and the photoreceptor neurons that underlie vision. The mechanisms underlying compartmentalization of cilia are an area of intense current research. Recent findings have revealed similarities and differences in molecular mechanisms of ciliary protein enrichment and its regulation among primary cilia and sensory cilia. Here we discuss the physiological demands on photoreceptors that have driven their evolution into neurons that rely on a highly specialized cilium for signaling changes in light intensity. We explore what is known and what is not known about how that specialization appears to have driven unique mechanisms for photoreceptor protein and membrane compartmentalization.

Differential adaptations in rod outer segment disc membranes in different models of congenital stationary night blindness

Rod photoreceptor cells initiate scotopic vision when the light receptor rhodopsin absorbs a photon of light to initiate phototransduction. These photoreceptor cells are exquisitely sensitive and have adaptive mechanisms in place to maintain optimal function and to overcome dysfunctional states. One adaptation rod photoreceptor cells exhibit is in the packing properties of rhodopsin within the membrane. The mechanism underlying these adaptations is unclear. Mouse models of congenital stationary night blindness with different molecular causes were investigated to determine which signals are important for adaptations in rod photoreceptor cells. Night blindness in these mice is caused by dysfunction in either rod photoreceptor cell signaling or bipolar cell signaling. Changes in the packing of rhodopsin within photoreceptor cell membranes were examined by atomic force microscopy. Mice expressing constitutively active rhodopsin did not exhibit any adaptations, even under constant dark conditions. Mice with disrupted bipolar cell signaling exhibited adaptations, however, they were distinct from those in mice with disrupted phototransduction. These differential adaptations demonstrate that although multiple molecular defects can lead to a similar primary defect causing disease (i.e., night blindness), they can cause different secondary effects (i.e., adaptations). The lighting environment or signaling defects present from birth and during early rearing can condition mice and affect the adaptations occurring in more mature animals. A comparison of effects in wild-type mice, mice with defective phototransduction, and mice with defective bipolar cell signaling, indicated that bipolar cell signaling plays a role in this conditioning but is not required for adaptations in more mature animals.

Differential expression of the five redox complexes in the retinal mitochondria or rod outer segment disks is consistent with their different functionality

PURPOSE

The retinal rod outer segment (OS) disk membranes, devoid of mitochondria, conducts oxidative phosphorylation (OxPhos). This study aimed at identifying which proteins expressed in the retinal rod OS disks determined the considerable adenosine-5'-triphosphate production and oxygen consumption observed in comparison with retinal mitochondria.

PROCEDURES

Characterization was conducted by immunogold transmission electron microscopy on retinal sections. OxPhos was studied by oximetry and luminometry. The proteomes of OS disks and mitochondria purified from bovine retinas were studied by mass spectrometry. Statistical and bioinformatic analyses were conducted by univariate, multivariate, and machine learning methods.

RESULTS

Weighted gene coexpression network analysis identified two protein expression profile modules functionally associated with either retinal mitochondria or disk samples, in function of a strikingly different ability of each sample to utilized diverse substrate for F1Fo-ATP synthase. The OS disk proteins correlated better than mitochondria with the tricarboxylic acids cycle and OxPhos proteins.

CONCLUSIONS

The differential enrichment of the expression profile of the OxPhos proteins in the disks versus mitochondria suggests that these proteins may represent a true proteome component of the former, with different functionality. These findings may shed new light on the pathogenesis of rod-driven retinal degenerative diseases.

The Interplay between Retinal Pathways of Cholesterol Output and Its Effects on Mouse Retina

In mammalian retina, cholesterol excess is mainly metabolized to oxysterols by cytochromes P450 27A1 (CYP27A1) and 46A1 (CYP46A1) or removed on lipoprotein particles containing apolipoprotein E (APOE). In contrast, esterification by sterol-O-acyltransferase 1 (SOAT) plays only a minor role in this process. Accordingly, retinal cholesterol levels are unchanged in Soat1^-/- mice but are increased in Cyp27a1^-/-Cyp46a1^-/- and Apoe^-/- mice. Herein, we characterized Cyp27a1^-/-Cyp46a1^-/-Soat1^-/- and Cyp27a1^-/-Cyp46a1^-/-Apoe^-/- mice. In the former, retinal cholesterol levels, anatomical gross structure, and vasculature were normal, yet the electroretinographic responses were impaired. Conversely, in Cyp27a1^-/-Cyp46a1^-/-Apoe^-/- mice, retinal cholesterol levels were increased while anatomical structure and vasculature were unaffected with only male mice showing a decrease in electroretinographic responses. Sterol profiling, qRT-PCR, proteomics, and transmission electron microscopy mapped potential compensatory mechanisms in the Cyp27a1^-/-Cyp46a1^-/-Soat1^-/- and Cyp27a1^-/-Cyp46a1^-/-Apoe^-/- retina. These included decreased cholesterol biosynthesis along with enhanced formation of intra- and extracellular vesicles, possibly a reserve mechanism for lowering retinal cholesterol. In addition, there was altered abundance of proteins in Cyp27a1^-/-Cyp46a1^-/-Soat1^-/- mice that can affect photoreceptor function, survival, and retinal energy homeostasis (glucose and fatty acid metabolism). Therefore, the levels of retinal cholesterol do not seem to predict retinal abnormalities, and it is rather the network of compensatory mechanisms that appears to determine retinal phenotype.

Membrane cholesterol depletion reduces downstream signaling activity of the adenosine A2A receptor

Cholesterol has been shown to modulate the activity of multiple G Protein-coupled receptors (GPCRs), yet whether cholesterol acts through specific interactions, indirectly via modifications to the membrane, or via both mechanisms is not well understood. High-resolution crystal structures of GPCRs have identified bound cholesterols; based on a β₂-adrenergic receptor (β₂AR) structure bound to cholesterol and the presence of conserved amino acids in class A receptors, the cholesterol consensus motif (CCM) was identified. Here in mammalian cells expressing the adenosine A_2A receptor (A_2AR), ligand dependent production of cAMP is reduced following membrane cholesterol depletion with methyl-beta-cyclodextrin (MβCD), indicating that A_2AR signaling is dependent on cholesterol. In contrast, ligand binding is not dependent on cholesterol depletion. All-atom molecular simulations suggest that cholesterol interacts specifically with the CCM when the receptor is in an active state, but not when in an inactive state. Taken together, the data support a model of receptor state-dependent binding between cholesterol and the CCM, which could facilitate both G-protein coupling and downstream signaling of A_2AR.

Binding of a Myristoylated Protein to the Lipid Membrane Influenced by Interactions with the Polar Head Group Region

Many cytoplasmic proteins contain a hydrophobic acyl chain, which facilitates protein binding to cell membranes. Hydrophobic interactions between the exposed acyl chain of the protein and hydrocarbon chains of lipids in the cell membrane are the driving force for this specific lipid-protein interaction. Recent studies point out that in addition to hydrophobic interactions the charge-charge and charge-dipole interactions between the polar head groups and basic amino acids contribute significantly to the binding process. Recoverin possesses a myristoyl chain at the N-terminus. In the presence of Ca²⁺ ions, the protein undergoes structural rearrangements, leading to the extrusion of the myristoyl chain, facilitating the protein binding to the membrane. In this work, we investigate the impact of interactions between the polar head group region of lipid molecules and recoverin which binds to the model membrane. The interaction with a planar lipid bilayer composed of phosphatidylcholine and cholesterol with myristoylated and nonmyristoylated recoverin is studied by in situ polarization modulation infrared reflection absorption spectroscopy. The binding of recoverin to the lipid bilayer depends on the transmembrane potential, indicating that the orientation of the permanent surface dipole in the supramolecular assembly of the lipid membrane influences the protein attachment to the membrane surface. Analysis of the amide I' mode indicates that the orientation of recoverin bound to the lipid bilayer is independent of the presence of myristoyl chain in the protein and of the folding of the protein into the tense or relaxed state. In contrast, it changes as a function of the membrane potential. At positive transmembrane potentials, the α-helical fragments of recoverin are oriented predominantly parallel to the bilayer surface. This orientation facilitates the insertion of the acyl chain of the protein into the hydrophobic region of the bilayer. At negative transmembrane potentials, the α-helical fragments of recoverin change their orientation with respect to the membrane surface, which is followed by the removal of the myristoyl chain from the membrane.

Opportunities and Challenges of Whole-Cell and -Tissue Simulations of the Outer Retina in Health and Disease

Visual processing starts in the outer retina, where photoreceptor cells sense photons that trigger electrical responses. Retinal pigment epithelial cells are located external to the photoreceptor layer and have critical functions in supporting cell and tissue homeostasis and thus sustaining a healthy retina. The high level of specialization makes the retina vulnerable to alterations that promote retinal degeneration. In this review, we discuss opportunities and challenges in proposing whole-cell and -tissue simulations of the human outer retina. An implicit position taken throughout this review is that mapping diverse data sets onto integrative computational models is likely to be a pivotal approach to understanding complex disease and developing novel interventions.

Impact of the protein myristoylation on the structure of a model cell membrane in a protein bound state

The neuronal calcium sensor protein recoverin is expressed in retinal rod and cone cells and is involved in the calcium-dependent control of receptor (rhodopsin) phosphorylation and receptor inactivation. In its Ca²⁺-saturated form recoverin is attached to membranes by an exposed myristoyl group and responds to oscillating changes of intracellular Ca²⁺-concentration by performing a so-called Ca²⁺-myristoyl switch. In this work we analyze changes in a liquid lipid bilayer interacting with myristoylated and non-myristoylated recoverin by employing polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) with electrochemical control. The lipid bilayer is transferred onto a polycrystalline gold electrode using Langmuir-Blodgett Langmuir-Schaefer transfer at the surface pressure π = 30 mN m^-1 which ensures, necessary for the lipid-protein interaction, liquid state of the hydrocarbon chains of phospholipids. The model lipid bilayers are adsorbed directly on the Au electrode surface at transmembrane potentials -0.2 < ∆ϕM|S < 0.25 V. The interaction with recoverin leads to a stabilization of the adsorbed state of the lipid bilayer at positive transmembrane potentials. The interaction leads to a decrease in the surface charge density that accumulates on the membrane covered electrode surface, indicating changes in the lateral interactions in the lipid membrane. In situ spectroelectrochemical studies confirm orientation changes in the hydrophobic environment of the model membrane. Insertion of the myristoyl group of recoverin into the membrane is connected with an increase in the tilt of the hydrocarbon chains with respect to the surface normal and decrease in the bilayer thickness. Potential-induced pore formation and desorption of the lipid bilayer from the membrane surface is accompanied by the removal of the acyl chains of recoverin from the membrane.

Subretinal drusenoid deposits AKA pseudodrusen

A distinction between conventional drusen and pseudodrusen was first made in 1990, and more recently knowledge of pseudodrusen, more accurately called subretinal drusenoid deposits (SDDs), has expanded. Pseudodrusen have a bluish-white appearance by biomicroscopy and color fundus photography. Using optical coherence tomography, pseudodrusen were found to be accumulations of material internal to the retinal pigment epithelium that could extend internally through the ellipsoid zone. These deposits are more commonly seen in older eyes with thinner choroids. Histologic evaluation of these deposits revealed aggregations of material in the subretinal space between photoreceptors and retinal pigment epithelium. SDDs contain some proteins in common with soft drusen but differ in lipid composition. Many studies reported that SDDs are strong independent risk factors for late age-related macular degeneration. Geographic atrophy and type 3 neovascularization are particularly associated with SDD. Unlike conventional drusen, eyes with SDD show slow dark adaptation and poor contrast sensitivity. Outer retinal atrophy develops in eyes with regression of SDD, a newly recognized form of late age-related macular degeneration. Advances in imaging technology have enabled many insights into this condition, including associated photoreceptor, retinal pigment epithelium, and underlying choroidal changes.

A new perspective on lipid research in age-related macular degeneration

There is an urgency to find new treatment strategies that could prevent or delay the onset or progression of AMD. Different classes of lipids and lipoproteins metabolism genes have been associated with AMD in a multiple ways, but despite the ever-increasing knowledge base, we still do not understand fully how circulating lipids or local lipid metabolism contribute to AMD. It is essential to clarify whether dietary lipids, systemic or local lipoprotein metabolismtrafficking of lipids in the retina should be targeted in the disease. In this article, we critically evaluate what has been reported in the literature and identify new directions needed to bring about a significant advance in our understanding of the role for lipids in AMD. This may help to develop potential new treatment strategies through targeting the lipid homeostasis.

Modulation of the rod outer segment aerobic metabolism diminishes the production of radicals due to light absorption

Oxidative stress is a primary risk factor for both inflammatory and degenerative retinopathies. Our previous data on blue light-irradiated retinas demonstrated an oxidative stress higher in the rod outer segment (OS) than in the inner limb, leading to impairment of the rod OS extra-mitochondrial aerobic metabolism. Here the oxidative metabolism and Reactive Oxygen Intermediates (ROI) production was evaluated in purified bovine rod OS in function of exposure to different illumination conditions. A dose response was observed to varying light intensities and duration in terms of both ROI production and ATP synthesis. Pretreatment with resveratrol, inhibitor of F₁F_o-ATP synthase, or metformin, inhibitor of the respiratory complex I, significantly diminished the ROI production. Metformin also diminished the rod OS Complex I activity and reduced the maximal OS response to light in ATP production. Data show for the first time the relationship existing in the rod OS between its -aerobic- metabolism, light absorption, and ROI production. A beneficial effect was exerted by metformin and resveratrol, in modulating the ROI production in the illuminated rod OS, suggestive of their beneficial action also in vivo. Data shed new light on preventative interventions for cone loss secondary to rod damage due to oxidative stress.

Proteome of Bovine Mitochondria and Rod Outer Segment Disks: Commonalities and Differences

The retinal rod outer segment (OS) is a stack of disks surrounded by the plasma membrane, housing proteins related to phototransduction, as well as mitochondrial proteins involved in oxidative phosphorylation (OxPhos). This prompted us to compare the proteome of bovine OS disks and mitochondria to assess the significant top gene signatures of each sample. The two proteomes, obtained by LTQ-Orbitrap Velos mass spectrometry, were compared by statistical analyses. In total, 4139 proteins were identified, 2045 of which overlapping in the two sets. Nonhierarchical Spearman's correlogram revealed that the groups were clearly discriminated. Partial least square discriminant plus support vector machine analysis identified the major discriminative proteins, implied in phototransduction and lipid metabolism, respectively. Gene Ontology analysis identified top gene signatures of the disk proteome, enriched in vesiculation, glycolysis, and OxPhos proteins. The tricarboxylic acid cycle and the electron transport proteins were similarly enriched in the two samples, but the latter was up regulated in disks. Data suggest that the mitochondrial OxPhos proteins may represent a true OS proteome component, outside the mitochondrion. This knowledge may help the scientific community in the further studies of retinal physiology and pathology.

How to gather useful and valuable information from protein binding measurements using Langmuir lipid monolayers

This review presents data on the influence of various experimental parameters on the binding of proteins onto Langmuir lipid monolayers. The users of the Langmuir methodology are often unaware of the importance of choosing appropriate experimental conditions to validate the data acquired with this method. The protein Retinitis pigmentosa 2 (RP2) has been used throughout this review to illustrate the influence of these experimental parameters on the data gathered with Langmuir monolayers. The methods detailed in this review include the determination of protein binding parameters from the measurement of adsorption isotherms, infrared spectra of the protein in solution and in monolayers, ellipsometric isotherms and fluorescence micrographs.

An in silico model of retinal cholesterol dynamics (RCD model): insights into the pathophysiology of dry AMD

We developed an in silico mathematical model of retinal cholesterol (Ch) dynamics (RCD) to quantify the physiological rate of Ch turnover in the rod outer segment (ROS), the lipoprotein transport mechanisms by which Ch enters and leaves the outer retina, and the rates of drusen growth and macrophage-mediated clearance in dry age-related macular degeneration. Based on existing experimental data and mechanistic hypotheses, we estimated the Ch turnover rate in the ROS to be 1–6 pg/mm²/min, dependent on the rate of Ch recycling in the outer retina, and found comparable rates for LDL receptor-mediated endocytosis of Ch by the retinal pigment epithelium (RPE), ABCA1-mediated Ch transport from the RPE to the outer retina, ABCA1-mediated Ch efflux from the RPE to the choroid, and the secretion of 70 nm ApoB-Ch particles from the RPE. The drusen growth rate is predicted to increase from 0.7 to 4.2 μm/year in proportion to the flux of ApoB-Ch particles. The rapid regression of drusen may be explained by macrophage-mediated clearance if the macrophage density reaches ∼3,500 cells/mm². The RCD model quantifies retinal Ch dynamics and suggests that retinal Ch turnover and recycling, ApoB-Ch particle efflux, and macrophage-mediated clearance may explain the dynamics of drusen growth and regression.

Effects of NDRG1 family proteins on photoreceptor outer segment morphology in zebrafish

Rods and cones are functionally and morphologically distinct. We previously identified N-myc downstream-regulated gene 1b (ndrg1b) in carp as a cone-specific gene. Here, we show that NDRG1b and its paralog, NDRG1a-1, contribute to photoreceptor outer segment (OS) formation in zebrafish. In adult zebrafish photoreceptors, NDRG1a-1 was localized in the entire cone plasma membranes, and also in rod plasma membranes except its OS. NDRG1b was expressed specifically in cones in the entire plasma membranes. In a developing retina, NDRG1a-1 was expressed in the photoreceptor layer, and NDRG1b in the photoreceptor layer plus inner nuclear layer. Based on our primary knockdown study suggesting that both proteins are involved in normal rod and cone OS development, NDRG1a-1 was overexpressed or NDRG1b was ectopically expressed in rods. These forced-expression studies in the transgenic fish confirmed the effect of these proteins on the OS morphology: rod OS morphology changed from cylindrical to tapered shape. These taper-shaped rod OSs were not stained with N,N’-didansyl cystine that effectively labels infolded membrane structure of cone OS. The result shows that rod OS membrane structure is preserved in these taper-shaped OSs and therefore, suggests that tapered OS morphology is not related to the infolded membrane structure in cone OS.

Retinal Hypercholesterolemia Triggers Cholesterol Accumulation and Esterification in Photoreceptor Cells

The process of vision is impossible without the photoreceptor cells, which have a unique structure and specific maintenance of cholesterol. Herein we report on the previously unrecognized cholesterol-related pathway in the retina discovered during follow-up characterizations of Cyp27a1(-/-)Cyp46a1(-/-) mice. These animals have retinal hypercholesterolemia and convert excess retinal cholesterol into cholesterol esters, normally present in the retina in very small amounts. We established that in the Cyp27a1(-/-)Cyp46a1(-/-) retina, cholesterol esters are generated by and accumulate in the photoreceptor outer segments (OS), which is the retinal layer with the lowest cholesterol content. Mouse OS were also found to express the cholesterol-esterifying enzyme acyl-coenzyme A:cholesterol acyltransferase (ACAT1), but not lecithin-cholesterol acyltransferase (LCAT), and to differ from humans in retinal expression of ACAT1. Nevertheless, cholesterol esters were discovered to be abundant in human OS. We suggest a mechanism for cholesterol ester accumulation in the OS and that activity impairment of ACAT1 in humans may underlie the development of subretinal drusenoid deposits, a hallmark of age-related macular degeneration, which is a common blinding disease. We generated Cyp27a1(-/-)Cyp46a1(-/-)Acat1(-/-) mice, characterized their retina by different imaging modalities, and confirmed that unesterified cholesterol does accumulate in their OS and that there is photoreceptor apoptosis and OS degeneration in this line. Our results provide insights into the retinal response to local hypercholesterolemia and the retinal significance of cholesterol esterification, which could be cell-specific and both beneficial and detrimental for retinal structure and function.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Phospholipid scrambling by rhodopsin

Rhodopsin has been intensively characterized in its role as a visual pigment and G protein-coupled receptor responsible for dim-light vision. We recently discovered that it also functions as an ATP-independent phospholipid scramblase: when reconstituted into large unilamellar vesicles, rhodopsin accelerates the normally sluggish transbilayer translocation of common phospholipids by more than 1000-fold, to rates in excess of 10 000 phospholipids transported per rhodopsin per second. Here we summarize the work leading to this discovery and speculate on the mechanism by which rhodopsin scrambles phospholipids. We also present a hypothesis that rhodopsin's scramblase activity is necessary for the function of the ABC transporter ABCA4 that is responsible for mitigating the toxic accumulation of 11-cis-retinal and bis-retinoids in the retina.

Prominent role of prominin in the retina

Prominin molecules represent a new family of pentaspan membrane glycoproteins expressed throughout the animal kingdom. The name originates from its localization on membrane protrusion, such as microvilli, filopodia, lamellipodia, and microspikes. Following the original description in mouse and human, representative prominin members were found in fish (e.g., Danio rerio), amphibian (Ambystoma mexicanum, Xenopus laevis), worm (Caenorhabditis elegans), and flies (Drosophila melanogaster). Mammalian prominin-1 was identified as a marker of somatic and cancer stem cells and plays an essential role in the visual system, which contributed to increased interest of the medical field in this molecule. Here we summarize recent data from various fields, including Drosophila, which will aid to our understanding of its still elusive function.

Characterizing rhodopsin signaling by EPR spectroscopy: from structure to dynamics

Electron paramagnetic resonance (EPR) spectroscopy, together with spin labeling techniques, has played a major role in the characterization of rhodopsin, the photoreceptor protein and G protein-coupled receptor (GPCR) in rod cells. Two decades ago, these biophysical tools were the first to identify transmembrane helical movements in rhodopsin upon photo-activation, a critical step in the study of GPCR signaling. EPR methods were employed to identify functional loop dynamics within rhodopsin, to measure light-induced millisecond timescale changes in rhodopsin conformation, to characterize the effects of partial agonists on the apoprotein opsin, and to study lipid interactions with rhodopsin. With the emergence of advanced pulsed EPR techniques, the stage was set to determine the amplitude of structural changes in rhodopsin and the dynamics in the rhodopsin signaling complexes. Work in this area has yielded invaluable information about mechanistic properties of GPCRs. Using EPR techniques, receptors are studied in native-like membrane environments and the effects of lipids on conformational equilibria can be explored. This perspective addresses the impact of EPR methods on rhodopsin and GPCR structural biology, highlighting historical discoveries made with spin labeling techniques, and outlining exciting new directions in the field.

Mice that produce ApoB100 lipoproteins in the RPE do not develop drusen yet are still a valuable experimental system

PURPOSE

Mice typically produce apolipoprotein B (apoB)-48 and not apoB100. Apolipoprotein B100 accumulates in Bruch's membrane prior to basal deposit and drusen formation during the onset of AMD, raising the possibility that they are a trigger for these Bruch's membrane alterations. The purpose herein, was to determine whether mice that predominantly produce apoB100 develop features of AMD.

METHODS

The eyes of mice that produce apoB100 were examined for apoB100 synthesis, cholesteryl esterase/filipin labeling for cholesteryl esters, and transmission electron microscopy for lipid particles and phenotype.

RESULTS

Apolipoprotein B100 was abundant in the RPE-choroid of apoB100, but not wild-type mice by Western blot analysis. The apolipoprotein B100,(35)S-radiolabeled and immunoprecipitated from RPE explants, confirmed that apoB100 was synthesized by RPE. Apolipoprotein B100, but not control mice, had cholesteryl esters and lipid particles in Bruch's membrane. Immunoreactivity of ApoB100 was present in the RPE and Bruch's membrane, but not choroidal endothelium of apoB100 mice. Ultrastructural changes were consistent with aging, but not AMD when aged up to 18 months. The induction of advanced glycation end products to alter Bruch's membrane, did not promote basal linear deposit or drusen formation.

CONCLUSIONS

Mice that produce apoB100 in the RPE and liver secrete lipoproteins into Bruch's membrane, but not to the extent that distinct features of AMD develop, which suggests that either additional lipoprotein accumulation or additional factors are necessary to initiate their formation.

Pathways of cholesterol homeostasis in mouse retina responsive to dietary and pharmacologic treatments

Effects of serum cholesterol on cholesterol content in the retina are currently unknown. It is also unclear how cholesterol levels are controlled in the retina. High-cholesterol diet and oral administrations of simvastatin were used to modulate serum cholesterol in mice. These treatments only modestly affected cholesterol content in the retina and had no significant effect on retinal expression of the major cholesterol- and vision-related genes; the sterol-regulatory element binding protein pathway of transcriptional regulation does not seem to be operative in the retina under the experimental conditions used. Evidence is obtained that posttranslational mechanisms play a role in the control of retinal cholesterol. Retinal genes were only upregulated by oral administrations of TO901317 activating liver X receptors. Three of the upregulated genes could be of particular importance (apoD, Idol, and Rpe65) and have not yet been considered in the context of cholesterol homeostasis in the retina. Collectively, the data obtained identify specific features of retinal cholesterol maintenance and suggest additional therapies for age-related macular degeneration, a blinding disease characterized by cholesterol and lipid accumulations in chorioretinal tissues.

Cholesterol in the retina: the best is yet to come

Historically understudied, cholesterol in the retina is receiving more attention now because of genetic studies showing that several cholesterol-related genes are risk factors for age-related macular degeneration (AMD) and because of eye pathology studies showing high cholesterol content of drusen, aging Bruch's membrane, and newly found subretinal lesions. The challenge before us is determining how the cholesterol-AMD link is realized. Meeting this challenge will require an excellent understanding these genes' roles in retinal physiology and how chorioretinal cholesterol is maintained. In the first half of this review, we will succinctly summarize physico-chemical properties of cholesterol, its distribution in the human body, general principles of maintenance and metabolism, and differences in cholesterol handling in human and mouse that impact on experimental approaches. This information will provide a backdrop to the second part of the review focusing on unique aspects of chorioretinal cholesterol homeostasis, aging in Bruch's membrane, cholesterol in AMD lesions, a model for lesion biogenesis, a model for macular vulnerability based on vascular biology, and alignment of AMD-related genes and pathobiology using cholesterol and an atherosclerosis-like progression as unifying features. We conclude with recommendations for the most important research steps we can take towards delineating the cholesterol-AMD link.

Mechanism of inflammation in age-related macular degeneration: an up-to-date on genetic landmarks

Age-related macular degeneration (AMD) is the most common cause of irreversible visual impairment among people over 50 years of age, accounting for up to 50% of all cases of legal blindness in Western countries. Although the aging represents the main determinant of AMD, it must be considered a multifaceted disease caused by interactions among environmental risk factors and genetic backgrounds. Mounting evidence and/or arguments document the crucial role of inflammation and immune-mediated processes in the pathogenesis of AMD. Proinflammatory effects secondary to chronic inflammation (e.g., alternative complement activation) and heterogeneous types of oxidative stress (e.g., impaired cholesterol homeostasis) can result in degenerative damages at the level of crucial macular structures, that is photoreceptors, retinal pigment epithelium, and Bruch's membrane. In the most recent years, the association of AMD with genes, directly or indirectly, involved in immunoinflammatory pathways is increasingly becoming an essential core for AMD knowledge. Starting from the key basic-research notions detectable at the root of AMD pathogenesis, the present up-to-date paper reviews the best-known and/or the most attractive genetic findings linked to the mechanisms of inflammation of this complex disease.

New findings in ATP supply in rod outer segments: insights for retinopathies

BACKGROUND INFORMATION

The rod outer segment (OS) is the specialised organelle where phototransduction takes place. Our previous proteomic and biochemical analyses on purified rod disks showed the functional expression of the respiratory chain complexes I-IV and F1 Fo -ATP synthase in OS disks, as well as active soluble tricarboxylic acid cycle enzymes. Here, we focussed our study on the whole OS that contains the cytosol and plasma membrane and disks as native flattened saccules, unlike spherical osmotically intact disks.

RESULTS

OS were purified from bovine retinas and characterised for purity. Oximetry, ATP synthesis and cytochrome c oxidase (COX) assays were performed. The presence of COX and F₁F₀-ATP synthase (ATP synthase) was assessed by semi-quantitative Western blotting, immunofluorescence or confocal laser scanning microscopy on whole bovine retinas and bovine retinal sections and by immunogold transmission electron microscopy (TEM) of purified OS or bovine retinal sections. Both ATP synthase and COX are catalytically active in OS. These are able to consume oxygen (O₂) in the presence of pyruvate and malate. CLSM analyses showed that rhodopsin autofluorescence and MitoTracker Deep Red 633 fluorescence co-localise on rod OS. Data are confirmed by co-localisation studies of ATP synthase with Rh in rod OS by immunofluorescence and TEM in bovine retinal sections.

CONCLUSIONS

Our data confirm the expression and activity of COX and ATP synthase in OS, suggestive of the presence of an extra-mitochondrial oxidative phosphorylation in rod OS, meant to supply ATP for the visual transduction. In this respect, the membrane rich OS environment would be meant to absorb both light and O₂. The ability of OS to manipulate O₂ may shed light on the pathogenesis of many retinal degenerative diseases ascribed to oxidative stress, as well as on the efficacy of the treatment with dietary supplements, presently utilised as supporting therapies.

Subretinal drusenoid deposits in non-neovascular age-related macular degeneration: morphology, prevalence, topography, and biogenesis model

PURPOSE

To characterize the morphology, prevalence, and topography of subretinal drusenoid deposits, a candidate histological correlate of reticular pseudodrusen, with reference to basal linear deposit (BlinD), a specific lesion of age-related macular degeneration, and to propose a biogenesis model for both lesion.

METHODS

Donor eyes with median death-to-preservation of 2:40 hours were postfixed in osmium tannic acid paraphenylenediamine and prepared for macula-wide high-resolution digital sections. Annotated thicknesses of 21 chorioretinal layers were determined at standard locations in sections through the fovea and the superior perifovea.

RESULTS

In 22 eyes of 20 white donors (83.1 ± 7.7 years), SDD appeared as isolated or confluent drusenoid dollops punctuated by tufts of retinal pigment epithelium apical processes and associated with photoreceptor perturbation. Subretinal drusenoid deposits and BlinD were detected in 85 and 90% of non-neovascular age-related macular degeneration donors, respectively. Subretinal drusenoid deposit was thick (median, 9.4 μm) and more abundant in the perifovea than in the fovea (P < 0.0001). BlinD was thin (median, 2.1 μm) and more abundant in the fovea than in the perifovea (P < 0.0001).

CONCLUSION

Subretinal drusenoid deposits and BlinD prevalence in age-related macular degeneration eyes are high. Subretinal drusenoid deposits organized morphology, topography, and impact on surrounding photoreceptors imply specific processes of biogenesis. Contrasting topographies of subretinal drusenoid deposits and BlinD suggest relationships with differentiable aspects of rod and cone physiology, respectively. A 2-lesion 2-compartment biogenesis model incorporating outer retinal lipid homeostasis is presented.

Functional expression of oxidative phosphorylation proteins in the rod outer segment disc

The rod Outer Segment (OS) disc, an organelle devoid of mitochondria, is specialized in phototransduction, a process requiring a continual chemical energy supply. We have shown that OS discs express functional mitochondrial electron transport chains, Fo F1 -ATP synthase and the tricarboxylic acid cycle enzymes, all mitochondrial features. Here, we focus on oxygen consumption and adenosine triphosphate (ATP) synthesis by OS discs analysing electron transport chain I-III-IV and II-II-IV pathways, supported by reduced nicotinamide adenine dinucleotide and succinate, respectively. Interestingly, respiratory capacity of discs was measurable also in the presence of 3-hydroxy-butyrrate, a typical metabolic substrate for the brain. Data were supported by a two-dimensional electrophoresis analyses conducted as our previous one, but focused to those mitochondrial proteins that are involved in oxidative phosphorylation. Carbonic anhydrase was also found active in OS discs. Moreover, colocalization of Rhodopsin with respiratory complex I and ATP synthase seems a further step in the characterization of some proteins typical of the mitochondrial inner membranes that are expressed in the rod discs. The existence of oxygen utilization in the outer retina, likely supplying ATP for phototransduction, may shed light on some retinal pathologies related to oxidative stress of the outer retina.

Photoreceptor inner and outer segments

Photoreceptors are exquisitely adapted to transform light stimuli into electrical signals that modulate neurotransmitter release. These cells are organized into several compartments including the unique outer segment (OS). Its whole function is to absorb light and transduce this signal into a change of membrane potential. Another compartment is the inner segment where much of metabolism and regulation of membrane potential takes place and that connects the OS and synapse. The synapse is the compartment where changes in membrane potentials are relayed to other neurons in the retina via release of neurotransmitter. The composition of the plasma membrane surrounding these compartments varies to accommodate their specific functions. In this chapter, we discuss the organization of the plasma membrane emphasizing the protein composition of each region as it relates to visual signaling. We also point out examples where mutations in these proteins cause visual impairment.

A Prospective Genetic Marker of the Visual-Perception Disorder Meares-Irlen Syndrome

Prior investigations of scotopic sensitivity or Meares-Irlen syndrome have identified several features also found in attention deficit/hyperactivity disorder, chronic fatigue syndrome, and a subtype of dyslexia in which visual recognition is the primary deficit. In particular, anomalies in lipid metabolism, including low essential fatty acid status and decreased serum cholesterol, have been identified in all three disorders. Genetic expression of the transporter molecule apolipoprotein B-100 (APOB) has been correlated with abnormal lipid metabolism, particularly in relation to levels of cholesterol. Cholesterol esters are important carriers of essential fatty acids entering the retina. The APOB gene coding for apolipoprotein B-100 is located on the short arm of Chromosome 2, and closely neighbours a gene (DYX3) known to confer susceptibility to dyslexia. The APOB locus is also recognised as being one of the most highly polymorphic regions of the human genome, and thus provides a promising tool for genetic researchers. In this pilot study, certain allelic variants of the APOB gene were more common in participants diagnosed with Meares–Irlen syndrome than in individuals without the condition. This study appears to be a first in which a condition known to cause reading difficulties has been associated with the APOB gene.

Spatial distribution of the pathways of cholesterol homeostasis in human retina

Background

The retina is a light-sensitive tissue lining the inner surface of the eye and one of the few human organs whose cholesterol maintenance is still poorly understood. Challenges in studies of the retina include its complex multicellular and multilayered structure; unique cell types and functions; and specific physico-chemical environment.

Methodology/Principal Findings

We isolated specimens of the neural retina (NR) and underlying retinal pigment epithelium (RPE)/choroid from six deceased human donors and evaluated them for expression of genes and proteins representing the major pathways of cholesterol input, output and regulation. Eighty-four genes were studied by PCR array, 16 genes were assessed by quantitative real time PCR, and 13 proteins were characterized by immunohistochemistry. Cholesterol distribution among different retinal layers was analyzed as well by histochemical staining with filipin. Our major findings pertain to two adjacent retinal layers: the photoreceptor outer segments of NR and the RPE. We demonstrate that in the photoreceptor outer segments, cholesterol biosynthesis, catabolism and regulation via LXR and SREBP are weak or absent and cholesterol content is the lowest of all retinal layers. Cholesterol maintenance in the RPE is different, yet the gene expression also does not appear to be regulated by the SREBPs and varies significantly among different individuals.

Conclusions/Significance

This comprehensive investigation provides important insights into the relationship and spatial distribution of different pathways of cholesterol input, output and regulation in the NR-RPE region. The data obtained are important for deciphering the putative link between cholesterol and age-related macular degeneration, a major cause of irreversible vision loss in the elderly.

Regulation of presynaptic strength by controlling Ca2+ channel mobility: effects of cholesterol depletion on release at the cone ribbon synapse

Synaptic communication requires proper coupling between voltage-gated Ca(2+) (Ca(V)) channels and synaptic vesicles. In photoreceptors, L-type Ca(V) channels are clustered close to synaptic ribbon release sites. Although clustered, Ca(V) channels move continuously within a confined domain slightly larger than the base of the ribbon. We hypothesized that expanding Ca(V) channel confinement domains should increase the number of channel openings needed to trigger vesicle release. Using single-particle tracking techniques, we measured the expansion of Ca(V) channel confinement domains caused by depletion of membrane cholesterol with cholesterol oxidase or methyl-β-cyclodextrin. With paired whole cell recordings from cones and horizontal cells, we then determined the number of Ca(V) channel openings contributing to cone Ca(V) currents (I(Ca)) and the number of vesicle fusion events contributing to horizontal cell excitatory postsynaptic currents (EPSCs) following cholesterol depletion. Expansion of Ca(V) channel confinement domains reduced the peak efficiency of release, decreasing the number of vesicle fusion events accompanying opening of each Ca(V) channel. Cholesterol depletion also inhibited exocytotic capacitance increases evoked by brief depolarizing steps. Changes in efficiency were not due to changes in I(Ca) amplitude or glutamate receptor properties. Replenishing cholesterol restored Ca(V) channel domain size and release efficiency to control levels. These results indicate that cholesterol is important for organizing the cone active zone. Furthermore, the finding that cholesterol depletion impairs coupling between channel opening and vesicle release by allowing Ca(V) channels to move further from release sites shows that changes in presynaptic Ca(V) channel mobility can be a mechanism for adjusting synaptic strength.

Inhibition of biogenic membrane flippase activity in reconstituted ER proteoliposomes in the presence of low cholesterol levels

Biogenic membranes or self-synthesizing membranes are the site of synthesis of new lipids such as the endoplasmic reticulum (ER) in eukaryotes. Newly synthesized phospholipids (PLs) at the cytosolic leaflet of ER need to be translocated to the lumen side for membrane biogenesis and this is facilitated by a special class of lipid translocators called biogenic membrane flippase. Even though ER is the major site of cholesterol synthesis, it contains very low amounts of cholesterol, since newly synthesized cholesterol in ER is rapidly transported to other organelles and is highly enriched in plasma membrane. Thus, only low levels of cholesterol are present at the biosynthetic compartment (ER), which results in loose packing of ER lipids. We hypothesize that the prevalence of cholesterol in biogenic membranes might affect the rapid flip-flop. To validate our hypothesis, detergent solubilized ER membranes from both bovine liver and spinach leaves were reconstituted into proteoliposomes with varying mol% of cholesterol. Our results show that (i) with increase in the cholesterol/PL ratio, the half-life time of PL translocation increased, suggesting that cholesterol affects the kinetics of flipping, (ii) flipping activity was completely inhibited in proteoliposomes reconstituted with 1 mol% cholesterol, and (iii) FRAP and DSC experiments revealed that 1 mol% cholesterol did not alter the bilayer properties significantly and that flippase activity inhibition is probably mediated by interaction of cholesterol with the protein.

Extra-mitochondrial aerobic metabolism in retinal rod outer segments: new perspectives in retinopathies

Vertebrate retinal rods are photoreceptors for dim-light vision. They display extreme sensitivity to light thanks to a specialized subcellular organelle, the rod outer segment. This is filled with a stack of membranous disks, expressing the proteins involved in visual transduction, a very energy demanding process. Our previous proteomic and biochemical studies have shed new light on the chemical energy processes that supply ATP to the outer segment, suggesting the presence of an extra-mitochondrial aerobic metabolism in rod outer segment, devoid of mitochondria, which would account for a quantitatively adequate ATP supply for phototransduction. Here the functional presence of an oxidative phosphorylation in the rod outer limb is examined for its relationship to many physiological and pathological data on the rod outer segment. We hypothesize that the rod outer limb is at risk of oxidative stress, in any case of impairment in the respiratory chain functioning, or of blood supply. In fact, the electron transfer chain is a major source of reactive O(2) species, known to produce severe alteration to the membrane lipids, especially those of the outer segment that are rich in polyunsaturated fatty acids. We propose that the disk membrane may become the target of reactive oxygen species that may be released by the electron transport chain under pathologic conditions. For example, during aging reactive oxygen species production increases, while cellular antioxidant capacity decreases. Also the apoptosis of the rod observed after exposure to bright or continuous illumination can be explained considering that an overfunctioning of phototransduction may damage the disk membrane to a point at which cytochrome c escapes from the intradiskal space, where it is presently supposed to be, activating a putative caspase 9 and the apoptosome. A pathogenic mechanism for many inherited and acquired retinal degenerations, representing a major problem in clinical ophthalmology, is proposed: a number of rod pathologies would be promoted by impairment of energy supply and/or oxidative stress in the rod outer segment. In conclusion we suppose that the damaging role of oxygen, be it hypoxia or hyperoxia invoked in most of the blinding diseases, acquired and even hereditary is to be seeked for inside the photoreceptor outer segment that would conceal a potential for cell death that is still to be recognized.

The role of the lipid matrix for structure and function of the GPCR rhodopsin

Photoactivation of rhodopsin in lipid bilayers results within milliseconds in a metarhodopsin I (MI)-metarhodopsin II (MII) equilibrium that is very sensitive to the lipid composition. It has been well established that lipid bilayers that are under negative curvature elastic stress from incorporation of lipids like phosphatidylethanolamines (PE) favor formation of MII, the rhodopsin photointermediate that is capable of activating G protein. Furthermore, formation of the MII state is favored by negatively charged lipids like phosphatidylserine and by lipids with longer hydrocarbon chains that yield bilayers with larger membrane hydrophobic thickness. Cholesterol and rhodopsin-rhodopsin interactions from crowding of rhodopsin molecules in lipid bilayers shift the MI-MII equilibrium towards MI. A variety of mechanisms seems to be responsible for the large, lipid-induced shifts between MI and MII: adjustment of the thickness of lipid bilayers to rhodopsin and adjustment of rhodopsin helicity to the thickness of bilayers, curvature elastic deformations in the lipid matrix surrounding the protein, direct interactions of PE headgroups and polyunsaturated hydrocarbon chains with rhodopsin, and direct or lipid-mediated interactions between rhodopsin molecules. This article is part of a Special Issue entitled: Membrane protein structure and function.