G protein-coupled receptor rhodopsin

Chiral vibrational structures of proteins at interfaces probed by sum frequency generation spectroscopy

We review the recent development of chiral sum frequency generation (SFG) spectroscopy and its applications to study chiral vibrational structures at interfaces. This review summarizes observations of chiral SFG signals from various molecular systems and describes the molecular origins of chiral SFG response. It focuses on the chiral vibrational structures of proteins and presents the chiral SFG spectra of proteins at interfaces in the C-H stretch, amide I, and N-H stretch regions. In particular, a combination of chiral amide I and N-H stretches of the peptide backbone provides highly characteristic vibrational signatures, unique to various secondary structures, which demonstrate the capacity of chiral SFG spectroscopy to distinguish protein secondary structures at interfaces. On the basis of these recent developments, we further discuss the advantages of chiral SFG spectroscopy and its potential application in various fields of science and technology. We conclude that chiral SFG spectroscopy can be a new approach to probe chiral vibrational structures of protein at interfaces, providing structural and dynamic information to study in situ and in real time protein structures and dynamics at interfaces.

CNTF and retina

Ciliary neurotrophic factor (CNTF) is one of the most studied neurotrophic factors for neuroprotection of the retina. A large body of evidence demonstrates that CNTF promotes rod photoreceptor survival in almost all animal models. Recent studies indicate that CNTF also promotes cone photoreceptor survival and cone outer segment regeneration in the degenerating retina and improves cone function in dogs with congenital achromotopsia. In addition, CNTF is a neuroprotective factor and an axogenesis factor for retinal ganglion cells (RGCs). This review focuses on the effects of exogenous CNTF on photoreceptors and RGCs in the mammalian retina and the potential clinical application of CNTF for retinal degenerative diseases.

Structural and functional protein network analyses predict novel signaling functions for rhodopsin

Proteomic analyses, literature mining, and structural data were combined to generate an extensive signaling network linked to the visual G protein-coupled receptor rhodopsin. Network analysis suggests novel signaling routes to cytoskeleton dynamics and vesicular trafficking.

Using a shotgun proteomic approach, we identified the protein inventory of the light sensing outer segment of the mammalian photoreceptor.

These data, combined with literature mining, structural modeling, and computational analysis, offer a comprehensive view of signal transduction downstream of the visual G protein-coupled receptor rhodopsin.

The network suggests novel signaling branches downstream of rhodopsin to cytoskeleton dynamics and vesicular trafficking.

The network serves as a basis for elucidating physiological principles of photoreceptor function and suggests potential disease-associated proteins.

Photoreceptor cells are neurons capable of converting light into electrical signals. The rod outer segment (ROS) region of the photoreceptor cells is a cellular structure made of a stack of around 800 closed membrane disks loaded with rhodopsin (Liang et al, 2003; Nickell et al, 2007). In disc membranes, rhodopsin arranges itself into paracrystalline dimer arrays, enabling optimal association with the heterotrimeric G protein transducin as well as additional regulatory components (Ciarkowski et al, 2005). Disruption of these highly regulated structures and processes by germline mutations is the cause of severe blinding diseases such as retinitis pigmentosa, macular degeneration, or congenital stationary night blindness (Berger et al, 2010).

Traditionally, signal transduction networks have been studied by combining biochemical and genetic experiments addressing the relations among a small number of components. More recently, large throughput experiments using different techniques like two hybrid or co-immunoprecipitation coupled to mass spectrometry have added a new level of complexity (Ito et al, 2001; Gavin et al, 2002, 2006; Ho et al, 2002; Rual et al, 2005; Stelzl et al, 2005). However, in these studies, space, time, and the fact that many interactions detected for a particular protein are not compatible, are not taken into consideration. Structural information can help discriminate between direct and indirect interactions and more importantly it can determine if two or more predicted partners of any given protein or complex can simultaneously bind a target or rather compete for the same interaction surface (Kim et al, 2006).

In this work, we build a functional and dynamic interaction network centered on rhodopsin on a systems level, using six steps: In step 1, we experimentally identified the proteomic inventory of the porcine ROS, and we compared our data set with a recent proteomic study from bovine ROS (Kwok et al, 2008). The union of the two data sets was defined as the ‘initial experimental ROS proteome'. After removal of contaminants and applying filtering methods, a ‘core ROS proteome', consisting of 355 proteins, was defined.

In step 2, proteins of the core ROS proteome were assigned to six functional modules: (1) vision, signaling, transporters, and channels; (2) outer segment structure and morphogenesis; (3) housekeeping; (4) cytoskeleton and polarity; (5) vesicles formation and trafficking, and (6) metabolism.

In step 3, a protein-protein interaction network was constructed based on the literature mining. Since for most of the interactions experimental evidence was co-immunoprecipitation, or pull-down experiments, and in addition many of the edges in the network are supported by single experimental evidence, often derived from high-throughput approaches, we refer to this network, as ‘fuzzy ROS interactome'. Structural information was used to predict binary interactions, based on the finding that similar domain pairs are likely to interact in a similar way (‘nature repeats itself') (Aloy and Russell, 2002). To increase the confidence in the resulting network, edges supported by a single evidence not coming from yeast two-hybrid experiments were removed, exception being interactions where the evidence was the existence of a three-dimensional structure of the complex itself, or of a highly homologous complex. This curated static network (‘high-confidence ROS interactome') comprises 660 edges linking the majority of the nodes. By considering only edges supported by at least one evidence of direct binary interaction, we end up with a ‘high-confidence binary ROS interactome'. We next extended the published core pathway (Dell'Orco et al, 2009) using evidence from our high-confidence network. We find several new direct binary links to different cellular functional processes (Figure 4): the active rhodopsin interacts with Rac1 and the GTP form of Rho. There is also a connection between active rhodopsin and Arf4, as well as PDEδ with Rab13 and the GTP-bound form of Arl3 that links the vision cycle to vesicle trafficking and structure. We see a connection between PDEδ with prenyl-modified proteins, such as several small GTPases, as well as with rhodopsin kinase. Further, our network reveals several direct binary connections between Ca2+-regulated proteins and cytoskeleton proteins; these are CaMK2A with actinin, calmodulin with GAP43 and S1008, and PKC with 14-3-3 family members.

In step 4, part of the network was experimentally validated using three different approaches to identify physical protein associations that would occur under physiological conditions: (i) Co-segregation/co-sedimentation experiments, (ii) immunoprecipitations combined with mass spectrometry and/or subsequent immunoblotting, and (iii) utilizing the glycosylated N-terminus of rhodopsin to isolate its associated protein partners by Concanavalin A affinity purification. In total, 60 co-purification and co-elution experiments supported interactions that were already in our literature network, and new evidence from 175 co-IP experiments in this work was added. Next, we aimed to provide additional independent experimental confirmation for two of the novel networks and functional links proposed based on the network analysis: (i) the proposed complex between Rac1/RhoA/CRMP-2/tubulin/and ROCK II in ROS was investigated by culturing retinal explants in the presence of an ROCK II-specific inhibitor (Figure 6). While morphology of the retinas treated with ROCK II inhibitor appeared normal, immunohistochemistry analyses revealed several alterations on the protein level. (ii) We supported the hypothesis that PDEδ could function as a GDI for Rac1 in ROS, by demonstrating that PDEδ and Rac1 co localize in ROS and that PDEδ could dissociate Rac1 from ROS membranes in vitro.

In step 5, we use structural information to distinguish between mutually compatible (‘AND') or excluded (‘XOR') interactions. This enables breaking a network of nodes and edges into functional machines or sub-networks/modules. In the vision branch, both ‘AND' and ‘XOR' gates synergize. This may allow dynamic tuning of light and dark states. However, all connections from the vision module to other modules are ‘XOR' connections suggesting that competition, in connection with local protein concentration changes, could be important for transmitting signals from the core vision module.

In the last step, we map and functionally characterize the known mutations that produce blindness.

In summary, this represents the first comprehensive, dynamic, and integrative rhodopsin signaling network, which can be the basis for integrating and mapping newly discovered disease mutants, to guide protein or signaling branch-specific therapies.

Orchestration of signaling, photoreceptor structural integrity, and maintenance needed for mammalian vision remain enigmatic. By integrating three proteomic data sets, literature mining, computational analyses, and structural information, we have generated a multiscale signal transduction network linked to the visual G protein-coupled receptor (GPCR) rhodopsin, the major protein component of rod outer segments. This network was complemented by domain decomposition of protein–protein interactions and then qualified for mutually exclusive or mutually compatible interactions and ternary complex formation using structural data. The resulting information not only offers a comprehensive view of signal transduction induced by this GPCR but also suggests novel signaling routes to cytoskeleton dynamics and vesicular trafficking, predicting an important level of regulation through small GTPases. Further, it demonstrates a specific disease susceptibility of the core visual pathway due to the uniqueness of its components present mainly in the eye. As a comprehensive multiscale network, it can serve as a basis to elucidate the physiological principles of photoreceptor function, identify potential disease-associated genes and proteins, and guide the development of therapies that target specific branches of the signaling pathway.

Light-sensitive coupling of rhodopsin and melanopsin to G(i/o) and G(q) signal transduction in Caenorhabditis elegans

Activation of G-protein-coupled receptors (GPCRs) initiates signal transduction cascades that affect many physiological responses. The worm Caenorhabditis elegans expresses >1000 of these receptors along with their cognate heterotrimeric G proteins. Here, we report properties of 9-cis-retinal regenerated bovine opsin [(b)isoRho] and human melanopsin [(h)Mo], two light-activated, heterologously expressed GPCRs in the nervous system of C. elegans with various genetically engineered alterations. Profound transient photoactivation of G(i/o) signaling by (b)isoRho led to a sudden and transient loss of worm motility dependent on cyclic adenosine monophosphate, whereas transient photoactivation of G(q) signaling by (h)Mo enhanced worm locomotion dependent on phospholipase Cβ. These transgenic C. elegans models provide a unique way to study the consequences of G(i/o) and G(q) signaling in vivo with temporal and spatial precision and, by analogy, their relationship to human neuromotor function.

Chemistry and biology of vision

Visual perception in humans occurs through absorption of electromagnetic radiation from 400 to 780 nm by photoreceptors in the retina. A photon of visible light carries a sufficient amount of energy to cause, when absorbed, a cis,trans-geometric isomerization of the 11-cis-retinal chromophore, a vitamin A derivative bound to rhodopsin and cone opsins of retinal photoreceptors. The unique biochemistry of these complexes allows us to reliably and reproducibly collect continuous visual information about our environment. Moreover, other nonconventional retinal opsins such as the circadian rhythm regulator melanopsin also initiate light-activated signaling based on similar photochemistry.

Chemical kinetic analysis of thermal decay of rhodopsin reveals unusual energetics of thermal isomerization and hydrolysis of Schiff base

The thermal properties of rhodopsin, which set the threshold of our vision, have long been investigated, but the chemical kinetics of the thermal decay of rhodopsin has not been revealed in detail. To understand thermal decay quantitatively, we propose a kinetic model consisting of two pathways: 1) thermal isomerization of 11-cis-retinal followed by hydrolysis of Schiff base (SB) and 2) hydrolysis of SB in dark state rhodopsin followed by opsin-catalyzed isomerization of free 11-cis-retinal. We solve the kinetic model mathematically and use it to analyze kinetic data from four experiments that we designed to assay thermal decay, isomerization, hydrolysis of SB using dark state rhodopsin, and hydrolysis of SB using photoactivated rhodopsin. We apply the model to WT rhodopsin and E181Q and S186A mutants at 55 °C, as well as WT rhodopsin in H(2)O and D(2)O at 59 °C. The results show that the hydrogen-bonding network strongly restrains thermal isomerization but is less important in opsin and activated rhodopsin. Furthermore, the ability to obtain individual rate constants allows comparison of thermal processes under various conditions. Our kinetic model and experiments reveal two unusual energetic properties: the steep temperature dependence of the rates of thermal isomerization and SB hydrolysis in the dark state and a strong deuterium isotope effect on dark state SB hydrolysis. These findings can be applied to study pathogenic rhodopsin mutants and other visual pigments.

UVA phototransduction drives early melanin synthesis in human melanocytes

Exposure of human skin to solar ultraviolet radiation (UVR), a powerful carcinogen [1] comprising ~95% ultraviolet A (UVA) and ~5% ultraviolet B (UVB) at the Earth's surface, promotes melanin synthesis in epidermal melanocytes [2, 3], which protects skin from DNA damage [4, 5]. UVB causes DNA lesions [6] that lead to transcriptional activation of melanin-producing enzymes, resulting in delayed skin pigmentation within days [7]. In contrast, UVA causes primarily oxidative damage [8] and leads to immediate pigment darkening (IPD) within minutes, via an unknown mechanism [9, 10]. No receptor protein directly mediating phototransduction in skin has been identified. Here we demonstrate that exposure of primary human epidermal melanocytes (HEMs) to UVA causes calcium mobilization and early melanin synthesis. Calcium responses were abolished by treatment with G protein or phospholipase C (PLC) inhibitors or by depletion of intracellular calcium stores. We show that the visual photopigment rhodopsin [11] is expressed in HEMs and contributes to UVR phototransduction. Upon UVR exposure, significant melanin production was measured within one hour; cellular melanin continued to increase in a retinal- and calcium-dependent manner up to 5-fold after 24 hr. Our findings identify a novel UVA-sensitive signaling pathway in melanocytes that leads to calcium mobilization and melanin synthesis and may underlie the mechanism of IPD in human skin.

Molecular mechanisms of disease for mutations at Gly-90 in rhodopsin

Two different mutations at Gly-90 in the second transmembrane helix of the photoreceptor protein rhodopsin have been proposed to lead to different phenotypes. G90D has been classically associated with congenital night blindness, whereas the newly reported G90V substitution was linked to a retinitis pigmentosa phenotype. Here, we used Val/Asp replacements of the native Gly at position 90 to unravel the structure/function divergences caused by these mutations and the potential molecular mechanisms of inherited retinal disease. The G90V and G90D mutants have a similar conformation around the Schiff base linkage region in the dark state and same regeneration kinetics with 11-cis-retinal, but G90V has dramatically reduced thermal stability when compared with the G90D mutant rhodopsin. The G90V mutant also shows, like G90D, an altered photobleaching pattern and capacity to activate Gt in the opsin state. Furthermore, the regeneration of the G90V mutant with 9-cis-retinal was improved, achieving the same A(280)/A(500) as wild type isorhodopsin. Hydroxylamine resistance was also recovered, indicating a compact structure around the Schiff base linkage, and the thermal stability was substantially improved when compared with the 11-cis-regenerated mutant. These results support the role of thermal instability and/or abnormal photoproduct formation in eliciting a retinitis pigmentosa phenotype. The improved stability and more compact structure of the G90V mutant when it was regenerated with 9-cis-retinal brings about the possibility that this isomer or other modified retinoid analogues might be used in potential treatment strategies for mutants showing the same structural features.

Rhodopsin-transducin heteropentamer: three-dimensional structure and biochemical characterization

The process of vision is initiated when the G protein-coupled receptor, rhodopsin (Rho), absorbs a photon and transitions to its activated Rho(∗) form. Rho(∗) binds the heterotrimeric G protein, transducin (G(t)) inducing GDP to GTP exchange and G(t) dissociation. Using nucleotide depletion and affinity chromatography, we trapped and purified the resulting nucleotide-free Rho(∗)·G(t) complex. Quantitative SDS-PAGE suggested a 2:1 molar ratio of Rho(∗) to G(t) in the complex and its mass determined by scanning transmission electron microscopy was 221±12kDa. A 21.6Å structure was calculated from projections of negatively stained Rho(∗)·G(t) complexes. The molecular envelope thus determined accommodated two Rho molecules together with one G(t) heterotrimer, corroborating the heteropentameric structure of the Rho(∗)·G(t) complex.

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.

Topological and functional characterization of an insect gustatory receptor

Insect gustatory receptors are predicted to have a seven-transmembrane structure and are distantly related to insect olfactory receptors, which have an inverted topology compared with G-protein coupled receptors, including mammalian olfactory receptors. In contrast, the topology of insect gustatory receptors remains unknown. Except for a few examples from Drosophila, the specificity of individual insect gustatory receptors is also unknown. In this study, the total number of identified gustatory receptors in Bombyx mori was expanded from 65 to 69. BmGr8, a silkmoth gustatory receptor from the sugar receptor subfamily, was expressed in insect cells. Membrane topology studies on BmGr8 indicate that, like insect olfactory receptors, it has an inverted topology relative to G protein-coupled receptors. An orphan GR from the bitter receptor family, BmGr53, yielded similar results. We infer, from the finding that two distantly related BmGrs have an intracellular N-terminus and an odd number of transmembrane spans, that this is likely to be a general topology for all insect gustatory receptors. We also show that BmGr8 functions independently in Sf9 cells and responds in a concentration-dependent manner to the polyalcohols myo-inositol and epi-inositol but not to a range of mono- and di-saccharides. BmGr8 is the first chemoreceptor shown to respond specifically to inositol, an important or essential nutrient for some Lepidoptera. The selectivity of BmGr8 responses is consistent with the known responses of one of the gustatory receptor neurons in the lateral styloconic sensilla of B. mori, which responds to myo-inositol and epi-inositol but not to allo-inositol.

Dietary 9-cis-β,β-carotene fails to rescue vision in mouse models of leber congenital amaurosis

Synthetic 9-cis-stereoisomers of vitamin A (all-trans-retinol) are especially promising agents for the fight against blinding diseases. Several studies suggested that 9-cis-β,β-carotene (9-cis-BC), a natural and abundant β-carotene isomer in the diet, could be the precursor of 9-cis-retinoids and thus could have therapeutic applications. Here we showed that 9-cis-BC is metabolized both in vitro and in vivo by two types of mouse carotenoid oxygenases, β,β-Carotene monooxygenase 1 (BCMO1), and β,β-carotene dioxygenase 2 (BCDO2). In the symmetric oxidative cleavage reaction at C15,C15' position by BCMO1, part of the 9-cis-double bond was isomerized to the all-trans-stereoisomer, yielding all-trans-retinal and 9-cis-retinal in a molar ratio of 3:1. The asymmetric cleaving enzyme BCDO2 preferentially removed the 9-cis-ring site at the C9,C10 double bond from this substrate, providing an all-trans-β-10'-apocarotenal product that can be further metabolized to all-trans-retinal by BCMO1. Studies in knockout mouse models confirmed that each carotenoid oxygenase can metabolize 9-cis-BC. Therefore, treatment of mouse models of Leber congenital amaurosis with 9-cis-BC and 9-cis-retinyl-acetate, a well established 9-cis-retinal precursor, showed that the cis-carotenoid was far less effective than the cis-retinoid in rescuing vision. Thus, our in vitro and in vivo studies revealed that 9-cis-BC is not a major source for mouse 9-cis-retinoid production but is mainly converted to all-trans-retinoids to support canonical vitamin A action.

Thermal properties of rhodopsin: insight into the molecular mechanism of dim-light vision

Rhodopsin has developed mechanisms to optimize its sensitivity to light by suppressing dark noise and enhancing quantum yield. We propose that an intramolecular hydrogen-bonding network formed by ∼20 water molecules, the hydrophilic residues, and peptide backbones in the transmembrane region is essential to restrain thermal isomerization, the source of dark noise. We studied the thermal stability of rhodopsin at 55 °C with single point mutations (E181Q and S186A) that perturb the hydrogen-bonding network at the active site. We found that the rate of thermal isomerization increased by 1-2 orders of magnitude in the mutants. Our results illustrate the importance of the intact hydrogen-bonding network for dim-light detection, revealing the functional roles of water molecules in rhodopsin. We also show that thermal isomerization of 11-cis-retinal in solution can be catalyzed by wild-type opsin and that this catalytic property is not affected by the mutations. We characterize the catalytic effect and propose that it is due to steric interactions in the retinal-binding site and increases quantum yield by predetermining the trajectory of photoisomerization. Thus, our studies reveal a balancing act between dark noise and quantum yield, which have opposite effects on the thermal isomerization rate. The acquisition of the hydrogen-bonding network and the tuning of the steric interactions at the retinal-binding site are two important factors in the development of dim-light vision.

Optogenetic control of cells and circuits

The absorption of light by bound or diffusible chromophores causes conformational rearrangements in natural and artificial photoreceptor proteins. These rearrangements are coupled to the opening or closing of ion transport pathways, the association or dissociation of binding partners, the enhancement or suppression of catalytic activity, or the transcription or repression of genetic information. Illumination of cells, tissues, or organisms engineered genetically to express photoreceptor proteins can thus be used to perturb biochemical and electrical signaling with exquisite cellular and molecular specificity. First demonstrated in 2002, this principle of optogenetic control has had a profound impact on neuroscience, where it provides a direct and stringent means of probing the organization of neural circuits and of identifying the neural substrates of behavior. The impact of optogenetic control is also beginning to be felt in other areas of cell and organismal biology.

Calpain and PARP activation during photoreceptor cell death in P23H and S334ter rhodopsin mutant rats

Retinitis pigmentosa (RP) is a heterogeneous group of inherited neurodegenerative diseases affecting photoreceptors and causing blindness. Many human cases are caused by mutations in the rhodopsin gene. An important question regarding RP pathology is whether different genetic defects trigger the same or different cell death mechanisms. To answer this question, we analysed photoreceptor degeneration in P23H and S334ter transgenic rats carrying rhodopsin mutations that affect protein folding and sorting respectively. We found strong activation of calpain and poly(ADP-ribose) polymerase (PARP) in both mutants, concomitant with calpastatin down-regulation, increased oxidative DNA damage and accumulation of PAR polymers. These parameters were strictly correlated with the temporal progression of photoreceptor degeneration, mirroring earlier findings in the phosphodiesterase-6 mutant rd1 mouse, and suggesting execution of non-apoptotic cell death mechanisms. Interestingly, activation of caspases-3 and -9 and cytochrome c leakage-key events in apoptotic cell death--were observed only in the S334ter mutant, which also showed increased expression of PARP-1. The identification of the same metabolic markers triggered by different mutations in two different species suggests the existence of common cell death mechanisms, which is a major consideration for any mutation independent treatment.

Membrane receptors and transporters involved in the function and transport of vitamin A and its derivatives

The eye is the human organ most sensitive to vitamin A deficiency because of vision's absolute and heavy dependence on vitamin A for light perception. Studies of the molecular basis of vision have provided important insights into the intricate mechanistic details of the function, transport and recycling of vitamin A and its derivatives (retinoid). This review focuses on retinoid-related membrane receptors and transporters. Three kinds of mammalian membrane receptors and transporters are discussed: opsins, best known as vitamin A-based light sensors in vision; ABCA4, an ATP-dependent transporter specializes in the transport of vitamin A derivative; and STRA6, a recently identified membrane receptor that mediates cellular uptake of vitamin A. The evolutionary driving forces for their existence and the wide spectrum of human diseases associated with these proteins are discussed. Lessons learned from the study of the visual system might be useful for understanding retinoid biology and retinoid-related diseases in other organ systems as well. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Rod outer segment retinol formation is independent of Abca4, arrestin, rhodopsin kinase, and rhodopsin palmitylation

PURPOSE

The reactive aldehyde all-trans retinal is released in rod photoreceptor outer segments by photoactivated rhodopsin and is eliminated through reduction to all-trans retinol. This study was undertaken to determine whether all-trans retinol formation depends on Abca4, arrestin, rhodopsin kinase, and the palmitylation of rhodopsin, all of which are factors that affect the release and sequestration of all-trans retinal.

METHODS

Experiments were performed in isolated retinas and single living rods derived from 129/sv wild-type mice and Abca4-, arrestin-, and rhodopsin kinase-deficient mice and in genetically modified mice containing unpalmitylated rhodopsin. Formation of all-trans retinol was measured by imaging its fluorescence and by HPLC of retina extracts. The release of all-trans retinal from photoactivated rhodopsin was measured in purified rod outer segment membranes according to the increase in tryptophan fluorescence. All experiments were performed at 37°C.

RESULTS

The kinetics of all-trans retinol formation in the different types of genetically modified mice are in reasonable agreement with those in wild-type animals. The kinetics of all-trans retinol formation in 129/sv mice are similar to those in C57BL/6, although the latter are known to regenerate rhodopsin much more slowly. The release of all-trans retinal from rhodopsin in purified membranes is significantly faster than the formation of all-trans retinol in intact cells and is independent of the presence of the palmitate groups.

CONCLUSIONS

The regeneration of rhodopsin and the recycling of its chromophore are not strongly coupled. Neither the activities of Abca4, rhodopsin kinase, and arrestin, nor the palmitylation of rhodopsin affects the formation of all-trans retinol.

Understanding functional residues of the cannabinoid CB1

The brain cannabinoid (CB(1)) receptor that mediates numerous physiological processes in response to marijuana and other psychoactive compounds is a G protein coupled receptor (GPCR) and shares common structural features with many rhodopsin class GPCRs. For the rational development of therapeutic agents targeting the CB(1) receptor, understanding of the ligand-specific CB(1) receptor interactions responsible for unique G protein signals is crucial. For a more than a decade, a combination of mutagenesis and computational modeling approaches has been successfully employed to study the ligand-specific CB(1) receptor interactions. In this review, after a brief discussion about recent advances in understanding of some structural and functional features of GPCRs commonly applicable to the CB(1) receptor, the CB(1) receptor functional residues reported from mutational studies are divided into three different types, ligand binding (B), receptor stabilization (S) and receptor activation (A) residues, to delineate the nature of the binding pockets of anandamide, CP55940, WIN55212-2 and SR141716A and to describe the molecular events of the ligand-specific CB(1) receptor activation from ligand binding to G protein signaling. Taken these CB(1) receptor functional residues, some of which are unique to the CB(1) receptor, together with the biophysical knowledge accumulated for the GPCR active state, it is possible to propose the early stages of the CB(1) receptor activation process that not only provide some insights into understanding molecular mechanisms of receptor activation but also are applicable for identifying new therapeutic agents by applying the validated structure-based approaches, such as virtual high throughput screening (HTS) and fragment-based approach (FBA).

Heterogeneous N-terminal acylation of retinal proteins results from the retina's unusual lipid metabolism

Protein N-myristoylation occurs by a covalent attachment of a C14:0 fatty acid to the N-terminal Gly residue. This reaction is catalyzed by a N-myristoyltransferase that uses myristoyl-coenzyme A as substrate. But proteins in the retina also undergo heterogeneous N-acylation with C14:2, C14:1, and C12:0 fatty acids. The basis and the role of this retina-specific phenomenon are poorly understood. We studied guanylate cyclase-activating protein 1 (GCAP1) as an example of retina-specific heterogeneously N-acylated protein. The types and the abundance of fatty acids bound to bovine retinal GCAP1 were C14:2, 37.0%; C14:0, 32.4%; C14:1, 22.3%; and C12:0, 8.3% as quantified by liquid chromatography coupled mass spectrometry. We also devised a method for N-acylating proteins in vitro and used it to modify GCAP1 with acyl moieties of different lengths. Analysis of these GCAPs both confirmed that N-terminal acylation of GCAP1 is critical for its high activity and proper Ca(2+)-dependent response and revealed comparable functionality for GCAP1 with acyl moieties of various lengths. We also tested the hypothesis that retinal heterogeneous N-acylation results from retinal enrichment of unusual N-myristoyltransferase substrates. Thus, acyl-coenzyme A esters were purified from both bovine retina and brain and analyzed by liquid chromatography coupled mass spectrometry. Substantial differences in acyl-coenzyme A profiles between the retina and brain were detected. Importantly, the ratios of uncommon N-acylation substrates--C14:2- and C14:1-coenyzme A to C14:0-coenzyme A--were higher in the retina than in the brain. Thus, our results suggest that heterogeneous N-acylation, responsible for expansion of retinal proteome, reflects the unique character of retinal lipid metabolism. Additionally, we propose a new hypothesis explaining the physiological relevance of elevated retinal ratios of C14:2- and C14:1-coenzyme A to C14:0-coenzyme A.

Key enzymes of the retinoid (visual) cycle in vertebrate retina

A major goal in vision research over the past few decades has been to understand the molecular details of retinoid processing within the retinoid (visual) cycle. This includes the consequences of side reactions that result from delayed all-trans-retinal clearance and condensation with phospholipids that characterize a variety of serious retinal diseases. Knowledge of the basic retinoid biochemistry involved in these diseases is essential for development of effective therapeutics. Photoisomerization of the 11-cis-retinal chromophore of rhodopsin triggers a complex set of metabolic transformations collectively termed phototransduction that ultimately lead to light perception. Continuity of vision depends on continuous conversion of all-trans-retinal back to the 11-cis-retinal isomer. This process takes place in a series of reactions known as the retinoid cycle, which occur in photoreceptor and RPE cells. All-trans-retinal, the initial substrate of this cycle, is a chemically reactive aldehyde that can form toxic conjugates with proteins and lipids. Therefore, much experimental effort has been devoted to elucidate molecular mechanisms of the retinoid cycle and all-trans-retinal-mediated retinal degeneration, resulting in delineation of many key steps involved in regenerating 11-cis-retinal. Three particularly important reactions are catalyzed by enzymes broadly classified as acyltransferases, short-chain dehydrogenases/reductases and carotenoid/retinoid isomerases/oxygenases. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Role of bulk water in hydrolysis of the rhodopsin chromophore

Rhodopsin (Rho) is a prototypical G protein-coupled receptor that changes from an inactive conformational state to a G protein-activating state as a consequence of its retinal chromophore isomerization, 11-cis-retinal → all-trans-retinal. The photoisomerized chromophore covalently linked to Lys(296) by a Schiff base is subsequently hydrolyzed, but little is known about this reaction. Recent research indicates a significant role for tightly bound transmembrane water molecules in the Rho activation process. Atomic structures of Rho and hydroxyl radical footprinting reveal ordered waters within Rho transmembrane helices that are located close to highly conserved and functionally important receptor residues, forming a hydrogen bond network. Using (18)O-labeled H(2)O, we now report that water from bulk solvent, but not tightly bound water, is involved in the hydrolytic release of chromophore upon Rho activation by light. Moreover, small molecules (and presumably, water) enter the Rho structure from the cytoplasmic side of the membrane. Thus, this work indicates two distinct origins of water vital for Rho function.

Cyclopropyl and isopropyl derivatives of 11-cis and 9-cis retinals at C-9 and C-13: subtle steric differences with major effects on ligand efficacy in rhodopsin

Retinal is the natural ligand (chromophore) of the vertebrate rod visual pigment. It occurs in either the 11-cis (rhodopsin) or the 9-cis (isorhodopsin) configuration. In its evolution to a G protein coupled photoreceptor, rhodopsin has acquired exceptional photochemical properties. Illumination isomerizes the chromophore to the all-trans isomer, which acts as a full agonist. This process is extremely efficient, and there is abundant evidence that the C-9 and C-13 methyl groups of retinal play a pivotal role in this process. To examine the steric limits of the C-9 and C-13 methyl binding pocket of the binding site, we have prepared C-9 and C-13 cyclopropyl and isopropyl derivatives of its native ligands and of α-retinal at C-9. Most isopropyl analogues show very poor binding, except for 9-cis-13-isopropylretinal. Most cyclopropyl derivatives exhibit intermediate binding activity, except for 9-cis-13-cyclopropylretinal, which presents good binding activity. In general, the binding site shows preference for the 9-cis analogues over the 11-cis analogues. In fact, 13-isopropyl-9-cis-retinal acts as a superagonist after illumination. Another surprising finding was that 9-cyclopropylisorhodopsin is more like native rhodopsin with respect to spectral and photochemical properties, whereas 9-cyclopropylrhodopsin behaves more like native isorhodopsin in these aspects.

Retinyl ester storage particles (retinosomes) from the retinal pigmented epithelium resemble lipid droplets in other tissues

Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.

Coupling efficiency of rhodopsin and transducin in bicelles

G protein coupled receptors (GPCRs) can be activated by various extracellular stimuli, including hormones, peptides, odorants, neurotransmitters, nucleotides, or light. After activation, receptors interact with heterotrimeric G proteins and catalyze GDP release from the Gα subunit, the rate limiting step in G protein activation, to form a high affinity nucleotide-free GPCR-G protein complex. In vivo, subsequent GTP binding reduces affinity of the Gα protein for the activated receptor. In this study, we investigated the biochemical and structural characteristics of the prototypical GPCR, rhodopsin, and its signaling partner, transducin (G(t)), in bicelles to better understand the effects of membrane composition on high affinity complex formation, stability, and receptor mediated nucleotide release. Our results demonstrate that the high-affinity complex (rhodopsin-G(t)(empty)) forms more readily and has dramatically increased stability when rhodopsin is integrated into bicelles of a defined composition. We increased the half-life of functional complex to 1 week in the presence of negatively charged phospholipids. These data suggest that a membrane-like structure is an important contributor to the formation and stability of functional receptor-G protein complexes and can extend the range of studies that investigate properties of these complexes.

Role of membrane integrity on G protein-coupled receptors: Rhodopsin stability and function

Rhodopsin is a prototypical G protein-coupled receptor (GPCR) - a member of the superfamily that shares a similar structural architecture consisting of seven-transmembrane helices and propagates various signals across biological membranes. Rhodopsin is embedded in the lipid bilayer of specialized disk membranes in the outer segments of retinal rod photoreceptor cells where it transmits a light-stimulated signal. Photoactivated rhodopsin then activates a visual signaling cascade through its cognate G protein, transducin or Gt, that results in a neuronal response in the brain. Interestingly, the lipid composition of ROS membranes not only differs from that of the photoreceptor plasma membrane but is critical for visual transduction. Specifically, lipids can modulate structural changes in rhodopsin that occur after photoactivation and influence binding of transducin. Thus, altering the lipid organization of ROS membranes can result in visual dysfunction and blindness.

Crystal structure of metarhodopsin II

G-protein-coupled receptors (GPCRs) are seven transmembrane helix (TM) proteins that transduce signals into living cells by binding extracellular ligands and coupling to intracellular heterotrimeric G proteins (Gαβγ). The photoreceptor rhodopsin couples to transducin and bears its ligand 11-cis-retinal covalently bound via a protonated Schiff base to the opsin apoprotein. Absorption of a photon causes retinal cis/trans isomerization and generates the agonist all-trans-retinal in situ. After early photoproducts, the active G-protein-binding intermediate metarhodopsin II (Meta II) is formed, in which the retinal Schiff base is still intact but deprotonated. Dissociation of the proton from the Schiff base breaks a major constraint in the protein and enables further activating steps, including an outward tilt of TM6 and formation of a large cytoplasmic crevice for uptake of the interacting C terminus of the Gα subunit. Owing to Schiff base hydrolysis, Meta II is short-lived and notoriously difficult to crystallize. We therefore soaked opsin crystals with all-trans-retinal to form Meta II, presuming that the crystal's high concentration of opsin in an active conformation (Ops*) may facilitate all-trans-retinal uptake and Schiff base formation. Here we present the 3.0 Å and 2.85 Å crystal structures, respectively, of Meta II alone or in complex with an 11-amino-acid C-terminal fragment derived from Gα (GαCT2). GαCT2 binds in a large crevice at the cytoplasmic side, akin to the binding of a similar Gα-derived peptide to Ops* (ref. 7). In the Meta II structures, the electron density from the retinal ligand seamlessly continues into the Lys 296 side chain, reflecting proper formation of the Schiff base linkage. The retinal is in a relaxed conformation and almost undistorted compared with pure crystalline all-trans-retinal. By comparison with early photoproducts we propose how retinal translocation and rotation induce the gross conformational changes characteristic for Meta II. The structures can now serve as models for the large GPCR family.

Toll-like receptor 3 is required for development of retinopathy caused by impaired all-trans-retinal clearance in mice

Chronic inflammation is an important component that contributes to many age-related neurodegenerative diseases, including macular degeneration. Here, we report a role for toll-like receptor 3 (TLR3) in cone-rod dystrophy (CORD) of mice lacking ATP-binding cassette transporter 4 (ABCA4) and retinol dehydrogenase 8 (RDH8), proteins critical for all-trans-retinal clearance in the retina. Increased expression of toll-like receptor-signaling elements and inflammatory changes were observed in Rdh8(-/-)Abca4(-/-) eyes by RNA expression analysis. Unlike 3-month-old Rdh8(-/-)Abca4(-/-) mice that developed CORD, 6-month-old Tlr3(-/-)Rdh8(-/-)Abca4(-/-) mice did not evidence an abnormal retinal phenotype. Light-induced retinal degeneration in Tlr3(-/-)Rdh8(-/-)Abca4(-/-) mice was milder than that in Rdh8(-/-)Abca4(-/-) mice, and a 2-fold increased TLR3 expression was detected in light-illuminated retinas of Rdh8(-/-)Abca4(-/-) mice compared with nonilluminated retinas. Poly(I-C), a TLR3 ligand, caused caspase-8-independent cellular apoptosis. Whereas poly(I-C) induced retinal cell death in Rdh8(-/-)Abca4(-/-) and WT mice both in vivo and ex vivo, this was not seen in mice lacking Tlr3. Far fewer invasive macrophage/microglial cells in the subretinal space and weaker activation of Muller glial cells were exhibited by Tlr3(-/-)Rdh8(-/-) Abca4(-/-) mice compared with Rdh8(-/-)Abca4(-/-) mice at 3 and 6 months of age, indicating that loss of TLR3 inhibits local inflammation in the retina. Both poly(I-C) and endogenous products emanating from dying/dead retinal cells induced NF-κB and IRF3 activation. These findings demonstrate that endogenous products from degenerating retina stimulate TLR3 that causes cellular apoptosis and retinal inflammation and that loss of TLR3 protects mice from CORD.

Photoresponse in the heterotrophic marine dinoflagellate Oxyrrhis marina

Expressed rhodopsins were detected by proteomic analysis in an investigation of potential signal receptors in the cell membrane of the marine heterotrophic dinoflagellate Oxyrrhis marina (CCMP604). We inferred these to be sensory rhodopsins, a type of G-protein-coupled receptor trans-membrane signaling molecule. Because phototactic behavior based on sensory rhodopsins has been reported in other protists, we investigated the photosensory response of O. marina. This dinoflagellate exhibited strongest positive phototaxis at low levels (2-3 μE/m(2)/s) of white light when the cells were previously light adapted and well fed. Positive phototaxis was also found for blue (450 nm), green (525 nm), and red (680 nm) wavelengths. In a further test, O. marina showed significantly greater phototaxis toward concentrated algal food illuminated by blue light to stimulate red chlorophyll-a autofluorescence in the prey, compared with using bleached algae as prey. Concentration of a cytoplasmic downstream messenger molecule, cyclic adenosine monophosphate, a component of the signaling pathway of G-protein-coupled receptor molecules, rapidly increased in O. marina cells after exposure to white light. In addition, treatment with hydroxylamine, a rhodopsin signaling inhibitor, significantly decreased their phototactic response. Our results demonstrate that a heterotrophic marine dinoflagellate can orient to light based on rhodopsins present in the outer cell membrane and may be able to use photosensory response to detect algal prey based on chlorophyll autofluorescence.

Opsin is a phospholipid flippase

Polar lipids must flip-flop rapidly across biological membranes to sustain cellular life [1, 2], but flipping is energetically costly [3] and its intrinsic rate is low. To overcome this problem, cells have membrane proteins that function as lipid transporters (flippases) to accelerate flipping to a physiologically relevant rate. Flippases that operate at the plasma membrane of eukaryotes, coupling ATP hydrolysis to unidirectional lipid flipping, have been defined at a molecular level [2]. On the other hand, ATP-independent bidirectional flippases that translocate lipids in biogenic compartments, e.g., the endoplasmic reticulum, and specialized membranes, e.g., photoreceptor discs [4, 5], have not been identified even though their activity has been recognized for more than 30 years [1]. Here, we demonstrate that opsin is the ATP-independent phospholipid flippase of photoreceptor discs. We show that reconstitution of opsin into large unilamellar vesicles promotes rapid (τ<10 s) flipping of phospholipid probes across the vesicle membrane. This is the first molecular identification of an ATP-independent phospholipid flippase in any system. It reveals an unexpected activity for opsin and, in conjunction with recently available structural information on this G protein-coupled receptor [6, 7], significantly advances our understanding of the mechanism of ATP-independent lipid flip-flop.

Probing mechanisms of photoreceptor degeneration in a new mouse model of the common form of autosomal dominant retinitis pigmentosa due to P23H opsin mutations

Rhodopsin, the visual pigment mediating vision under dim light, is composed of the apoprotein opsin and the chromophore ligand 11-cis-retinal. A P23H mutation in the opsin gene is one of the most prevalent causes of the human blinding disease, autosomal dominant retinitis pigmentosa. Although P23H cultured cell and transgenic animal models have been developed, there remains controversy over whether they fully mimic the human phenotype; and the exact mechanism by which this mutation leads to photoreceptor cell degeneration remains unknown. By generating P23H opsin knock-in mice, we found that the P23H protein was inadequately glycosylated with levels 1-10% that of wild type opsin. Moreover, the P23H protein failed to accumulate in rod photoreceptor cell endoplasmic reticulum but instead disrupted rod photoreceptor disks. Genetically engineered P23H mice lacking the chromophore showed accelerated photoreceptor cell degeneration. These results indicate that most synthesized P23H protein is degraded, and its retinal cytotoxicity is enhanced by lack of the 11-cis-retinal chromophore during rod outer segment development.

Homology modeling of human CCR5 and analysis of its binding properties through molecular docking and molecular dynamics simulation

In this study, homology modeling, molecular docking and molecular dynamics simulation were performed to explore structural features and binding mechanism of some inhibitors of chemokine receptor type 5 (CCR5), and to construct a model for designing new CCR5 inhibitors for preventing HIV attachment to the host cell. A homology modeling procedure was employed to construct a 3D model of CCR5. For this procedure, the X-ray crystal structure of bovine rhodopsin (1F88A) at 2.80Å resolution was used as template. After inserting the constructed model into a hydrated lipid bilayer, a 20ns molecular dynamics (MD) simulation was performed on the whole system. After reaching the equilibrium, twenty-four CCR5 inhibitors were docked in the active site of the obtained model. The binding models of the investigated antagonists indicate the mechanism of binding of the studied compounds to the CCR5 obviously. Moreover, 3D pictures of inhibitor-protein complex provided precious data regarding the binding orientation of each antagonist into the active site of this protein. One additional 20 ns MD simulation was performed on the initial structure of the CCR5-ligand 21 complex, resulted from the previous docking calculations, embedded in a hydrated POPE bilayer to explore the effects of the presence of lipid bilayer in the vicinity of CCR5-ligand complex. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

The cone-specific visual cycle

Cone photoreceptors mediate our daytime vision and function under bright and rapidly-changing light conditions. As their visual pigment is destroyed in the process of photoactivation, the continuous function of cones imposes the need for rapid recycling of their chromophore and regeneration of their pigment. The canonical retinoid visual cycle through the retinal pigment epithelium cells recycles chromophore and supplies it to both rods and cones. However, shortcomings of this pathway, including its slow rate and competition with rods for chromophore, have led to the suggestion that cones might use a separate mechanism for recycling of chromophore. In the past four decades biochemical studies have identified enzymatic activities consistent with recycling chromophore in the retinas of cone-dominant animals, such as chicken and ground squirrel. These studies have led to the hypothesis of a cone-specific retina visual cycle. The physiological relevance of these studies was controversial for a long time and evidence for the function of this visual cycle emerged only in very recent studies and will be the focus of this review. The retina visual cycle supplies chromophore and promotes pigment regeneration only in cones but not in rods. This pathway is independent of the pigment epithelium and instead involves the Müller cells in the retina, where chromophore is recycled and supplied selectively to cones. The rapid supply of chromophore through the retina visual cycle is critical for extending the dynamic range of cones to bright light and for their rapid dark adaptation following exposure to light. The importance of the retina visual cycle is emphasized also by its preservation through evolution as its function has now been demonstrated in species ranging from salamander to zebrafish, mouse, primate, and human.

Analysis of disease-linked rhodopsin mutations based on structure, function, and protein stability calculations

Retinitis pigmentosa (RP) refers to a heterogeneous group of inherited diseases that result in progressive retinal degeneration, characterized by visual field constriction and night blindness. A total of 103 mutations in rhodopsin are linked to RP to date, and the phenotypes range from severe to asymptomatic. To study the relation between phenotype and rhodopsin stability in disease mutants, we used a structure-based approach. For 12 of the mutants located at the protein-lipid interphase, we used the von Heijne water-membrane transfer scale, and we find that 9 of the mutations could affect membrane insertion. For 91 mutants, we used the protein design algorithm FoldX. The 3 asymptomatic mutations had no significant reduced stability, 2 were unsuitable for FoldX analysis since the structure was incorrect in this region, 63 mutations had a significant change in protein stability (>1.6 kcal/mol), and 23 mutations had energy change values under the prediction error threshold (<1.6 kcal/mol). Out of these 23, the disease-causing effect could be explained by the involvement in other functions (e.g., glycosylation motifs, the interface with arrestin and transducin, and the cilia-binding motif) for 19 mutants. The remaining 4 mutants were probably incorrectly associated with RP or have functionalities not discovered yet. For destabilizing mutations where clinical data were available, we found a highly significant correlation between FoldX energy changes and the average age of night blindness and between FoldX energy changes and daytime vision loss onset. Our detailed structural, functional, and energetic analysis provides a complete picture of the rhodopsin mutations and can guide mutation-specific therapies.

Conformational changes in the g protein-coupled receptor rhodopsin revealed by histidine hydrogen-deuterium exchange

G protein-coupled receptors (GPCRs) are activated by ligand binding, allowing extracellular signals to be efficiently transmitted through the membrane to the G protein recognition site, 40 Å away. Utilizing His residues found spaced throughout the GPCR, rhodopsin, we used His hydrogen-deuterium exchange (His-HDX) to monitor long-time scale structural rearrangements previously inaccessible by other means. The half-lives of His-HDX indicate clear differences in the solvent accessibility of three His residues in rhodopsin/opsin and Zn2+-dependent changes in the pKa for His195. These results indicate the utility of His-HDX in examining structural rearrangements in native source and membrane proteins without requiring structural modification.

Colors with functions: elucidating the biochemical and molecular basis of carotenoid metabolism

Carotenoids affect a rich variety of physiological functions in nature and are beneficial for human health, serving as antioxidants in lipophilic environments and blue light filters in the macula of human retina. These dietary compounds also serve as precursors of a unique set of apo-carotenoid cleavage products, including retinoids. Although knowledge about retinoid biology has tremendously increased, the metabolism of retinoids' parent precursors remains poorly understood. Recently, molecular players in carotenoid metabolism have been identified and biochemically characterized. Moreover, mutations in their corresponding genes impair carotenoid metabolism and induce various pathologies in animal models. Polymorphisms in these genes alter carotenoid and retinoid homeostasis in humans as well. This review summarizes our current knowledge about the molecular/biochemical basis of carotenoid metabolism and particularly the physiological role of carotenoids in retinoid-dependent physiological processes.

Retinoids for treatment of retinal diseases

Knowledge about retinal photoreceptor signal transduction and the visual cycle required for normal eyesight has increased exponentially over the past decade. Substantial progress in human genetics has facilitated the identification of candidate genes and complex networks underlying inherited retinal diseases. Natural mutations in animal models that mimic human diseases have been characterized and advanced genetic manipulation can now be used to generate small mammalian models of human retinal diseases. Pharmacological repair of defective visual processes in animal models not only validates their involvement in vision, but also provides great promise for the development of improved therapies for millions who are progressing towards blindness or are almost completely robbed of their eyesight.

Rapid formation of all-trans retinol after bleaching in frog and mouse rod photoreceptor outer segments

All-trans retinol is formed in the outer segments of vertebrate rod photoreceptors from the reduction of the all-trans retinal released by photoactivated rhodopsin. The reduction requires NADPH and is therefore dependent on metabolic input. In metabolically intact photoreceptors, a large increase in rod outer segment fluorescence, attributed to the fluorescence of all-trans retinol, follows rhodopsin photoactivation. The fluorescence increase is biphasic, including a rapid and a slow component. In metabolically compromised cells, there is a much smaller fluorescence increase following rhodopsin photoactivation, but it too contains a rapid component. We have measured the fluorescence signal in single living frog and mouse rod photoreceptors, and have characterized its dependence on the wavelengths of light selected for excitation and for collecting emission. We find that in metabolically intact cells, the excitation and emission properties of both the rapid and slow components of the fluorescence signal are in close agreement with those of all-trans retinol fluorescence. In metabolically compromised cells, however, the signal can only partially be due to all-trans retinol, and most of it is consistent with all-trans retinal. The results suggest that in the outer segments of living rod photoreceptors there is rapid release of all-trans retinal, which in metabolically intact cells is accompanied by rapid conversion to all-trans retinol.

Role of aggregation in rhodopsin signal transduction

Many G protein-coupled receptors (GPCRs) are known to form dimers or even oligomers, and these aggregated states have been proposed as functional units responsible for signal transduction and G protein activation. However, the nature of their involvement has remained elusive. Here, we have investigated the role of aggregation in the signal transduction for dimeric forms of the prototypical GPCR rhodopsin using molecular dynamics simulations. The early steps after photoexcitation are characterized by a tandem mechanism in which one monomer is responsible for light detection while the other serves as the G protein activation site. Dimerization ensures efficient cross-talk between the two units within a few tens of nanoseconds following photoexcitation. This interface-mediated pathway suggests oligomerization-aided signal transduction as a crucial biological strategy to enhance activation efficiency across the entire family of GPCRs.

The polarization sense in human vision

Unlike humans, numerous animals are differentially sensitive to the vector orientation of linearly polarized light. However as early as 1844 Haidinger noted that weak blue-yellow brushes appear, centered on the fovea, when the sky is observed through a slowly rotating polarizer. Different models have been proposed to try to understand this phenomenon, but the precise mechanism remains unknown and the polarization unexploited. We suggest that when Fresnel's laws are applied to the unguided oblique rays, that the cylindrical geometry of the blue cones in the fovea along with their distribution induces an extrinsic dichroism and could explain why the human eye is sensitive to polarization. We have constructed an artificial eye model system using the same laws and were able to photograph the appearance of entoptic-like blue-dark brushes, confirming the observations and our mathematical simulations. Moreover, our in vivo and in vitro tests show that in addition to the usual 3s fading time measured using a stationary stimulus, there exists for this entoptic image a short extra creating and erasing time of about 0.1s, using a dynamical stimulus. We have also found that, surprisingly, the rotating pattern is more regular and symmetrical with one of our two eyes around a more circular blue cone-free area, the dominant eye. Our results suggest that the polarization sense can provide important information in many areas that remain to be explored.

Electrostatic compensation restores trafficking of the autosomal recessive retinitis pigmentosa E150K opsin mutant to the plasma membrane

Rhodopsin is the rod photoreceptor G protein-coupled receptor responsible for capturing light. Mutations in the gene encoding this protein can lead to a blinding disease called retinitis pigmentosa, which is inherited frequently in an autosomal dominant manner. The E150K opsin mutant associated with rarely occurring autosomal recessive retinitis pigmentosa localizes to trans-Golgi network membranes rather than to plasma membranes of rod photoreceptor cells. We investigated the molecular mechanisms underlying opsin retention in the Golgi apparatus. Electrostatic calculations reveal that the E150K mutant features an overall accumulation of positive charges between helices H-IV and H-II. Human E150K and several other closely related opsin mutants were then expressed in HEK-293 cells. Spectral characteristics and functional biochemistry of each mutant were analyzed after reconstitution with the cis-retinoid chromophore. UV-visible spectra and rhodopsin/transducin activation assays revealed only minor differences between the purified wild type control and rhodopsin mutants. However, partial restoration of the surface electrostatic charge in the compensatory R69E/E150K double mutant rescues the plasma membrane localization of opsin. These findings emphasize the fundamental importance of electrostatic interactions for appropriate membrane trafficking of opsin and advance our understanding of the pathophysiology of autosomal recessive retinitis pigmentosa due to the E150K mutation.

An acyl-covalent enzyme intermediate of lecithin:retinol acyltransferase

Synthesis of fatty acid retinyl esters determines systemic vitamin A levels and provides substrate for production of visual chromophore (11-cis-retinal) in vertebrates. Lecithin:retinol acyltransferase (LRAT), the main enzyme responsible for retinyl ester formation, catalyzes the transfer of an acyl group from the sn-1 position of phosphatidylcholine to retinol. To delineate the catalytic mechanism of this reaction, we expressed and purified a fully active, soluble form of this enzyme and used it to examine the possible formation of a transient acyl-enzyme intermediate. Detailed mass spectrometry analyses revealed that LRAT undergoes spontaneous, covalent modification upon incubation with a variety of phosphatidylcholine substrates. The addition of an acyl chain occurs at the Cys(161) residue, indicating formation of a thioester intermediate. This observation provides the first direct experimental evidence of thioester intermediate formation that constitutes the initial step in the proposed LRAT catalytic reaction. Additionally, we examined the effect of increasing fatty acyl side chain length in phosphatidylcholine on substrate accessibility in this reaction, which provided insights into the function of the single membrane-spanning domain of LRAT. These observations are critical to understanding the catalytic mechanism of LRAT protein family members as well as other lecithin:acyltransferases wherein Cys residues are required for catalysis.

Structure and activation of the visual pigment rhodopsin

Rhodopsin is a specialized G protein-coupled receptor (GPCR) found in vertebrate rod cells. Absorption of light by its 11-cis retinal chromophore leads to rapid photochemical isomerization and receptor activation. Recent results from protein crystallography and NMR spectroscopy show how structural changes on the extracellular side of rhodopsin induced by retinal isomerization are coupled to the motion of membrane-spanning helices to create a G protein binding pocket on the intracellular side of the receptor. The signaling pathway provides a comprehensive explanation for the conservation of specific amino acids and structural motifs across the class A family of GPCRs, as well as for the conservation of selected residues within the visual receptor subfamily. The emerging model of activation indicates that, rather than being unique, the visual receptors provide a basis for understanding the common structural and dynamic elements in the class A GPCRs.

2-Hydroxypropyl-beta-cyclodextrin removes all-trans retinol from frog rod photoreceptors in a concentration-dependent manner

PURPOSE

To determine whether a nonprotein lipophilic carrier, 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD), can remove all-trans retinol from rod photoreceptor outer segments. All-trans retinol is generated in rod outer segments after light exposure. It is highly insoluble, and its efficient transport across extra- and intracellular aqueous space requires specialized carriers.

METHODS

Experiments were carried out with isolated frog rod photoreceptor cells. The removal of all-trans retinol by different concentrations of this carrier was measured by imaging its fluorescence in single-rod photoreceptors.

RESULTS

HP-beta-CD concentrations >0.3 mM significantly increased the rate of all-trans retinol removal. The rate of removal increased linearly with carrier concentration, with a slope of 0.0058 min(-1)/mM.

CONCLUSIONS

The effectiveness of HP-beta-CD shows that a specialized interaction with the cell membrane is not necessary for the efficient transfer of all-trans retinol between the cell membrane and the carrier. The transfer occurs through a collision-based mechanism, as indicated by the linear increase of the rate of removal with the carrier concentration.

Lighting up multiprotein complexes: lessons from GPCR oligomerization

Spatiotemporal characterization of protein-protein interactions (PPIs) is essential in determining the molecular mechanisms of intracellular signaling processes. In this review, we discuss how new methodological strategies derived from non-invasive fluorescence- and luminescence-based approaches (FRET, BRET, BiFC and BiLC), when applied to the study of G protein-coupled receptor (GPCR) oligomerization, can be used to detect specific PPIs in live cells. These technologies alone or in concert with complementary methods (SRET, BRET or BiFC, and SNAP-tag or TR-FRET) can be extremely powerful approaches for PPI visualization, even between more than two proteins. Here we provide a comprehensive update on all the biotechnological aspects, including the strengths and weaknesses, of new fluorescence- and luminescence-based methodologies, with a specific focus on their application for studying PPIs.