The effect of recoverin-like calcium-binding proteins on the photoresponse of retinal rods

Molecular basis of rod and cone differences

In the vertebrate retina, rods and cones both detect light, but they are different in functional aspects such as light sensitivity and time resolution, for example, and in some of cell biological aspects. For functional aspects, both photoreceptors are known to share a common mechanism, phototransduction cascade, consisting of a series of enzyme reactions to convert a photon-capture signal to an electrical signal. To understand the mechanisms of the functional differences between rods and cones at the molecular level, we compared biochemically each of the reactions in the phototransduction cascade between rods and cones using the cells isolated and purified from carp retina. Although proteins in the cascade are functionally similar between rods and cones, their activities together with their expression levels are mostly different between these photoreceptors. In general, reactions to generate a response are slightly less effective, as a total, in cones than in rods, but each of the reactions for termination and recovery of a response are much more effective in cones. These findings explain lower light sensitivity and briefer light responses in cones than in rods. In addition, our considerations suggest that a Ca²⁺-binding protein, S-modulin or recoverin, has a currently unnoticed role in shaping light responses. With comparison of the expression levels of proteins and/or mRNAs using purified cells, several proteins were found to be specifically or predominantly expressed in cones. These proteins would be of interest for future studies on the difference between rods and cones.

The Binding Properties and Physiological Functions of Recoverin

Recoverin (Rcv) is a low molecular-weight, neuronal calcium sensor (NCS) primarily located in photoreceptor outer segments of the vertebrate retina. Calcium ions (Ca²⁺)-bound Rcv has been proposed to inhibit G-protein-coupled receptor kinase (GRKs) in darkness. During the light response, the Ca²⁺-free Rcv releases GRK, which in turn phosphorylates visual pigment, ultimately leading to the cessation of the visual transduction cascade. Technological advances over the last decade have contributed significantly to a deeper understanding of Rcv function. These include both biophysical and biochemical approaches that will be discussed in this review article. Furthermore, electrophysiological experiments uncovered additional functions of Rcv, such as regulation of the lifetime of Phosphodiesterase-Transducin complex. Recently, attention has been drawn to different roles in rod and cone photoreceptors.This review article focuses on Rcv binding properties to Ca²⁺, disc membrane and GRK, and its physiological functions in phototransduction and signal transmission.

Site-specific fluorescent labeling to visualize membrane translocation of a myristoyl switch protein

Fluorescence approaches have been widely used for elucidating the dynamics of protein-membrane interactions in cells and model systems. However, non-specific multi-site fluorescent labeling often results in a loss of native structure and function, and single cysteine labeling is not feasible when native cysteines are required to support a protein's folding or catalytic activity. Here, we develop a method using genetic incorporation of non-natural amino acids and bio-orthogonal chemistry to site-specifically label with a single fluorescent small molecule or protein the myristoyl-switch protein recoverin, which is involved in rhodopsin-mediated signaling in mammalian visual sensory neurons. We demonstrate reversible Ca(2+)-responsive translocation of labeled recoverin to membranes and show that recoverin favors membranes with negative curvature and high lipid fluidity in complex heterogeneous membranes, which confers spatio-temporal control over down-stream signaling events. The site-specific orthogonal labeling technique is promising for structural, dynamical, and functional studies of many lipid-anchored membrane protein switches.

Recoverin depletion accelerates cone photoresponse recovery

The neuronal Ca²⁺-binding protein Recoverin has been shown to regulate phototransduction termination in mammalian rods. Here we identify four recoverin genes in the zebrafish genome, rcv1a, rcv1b, rcv2a and rcv2b, and investigate their role in modulating the cone phototransduction cascade. While Recoverin-1b is only found in the adult retina, the other Recoverins are expressed throughout development in all four cone types, except Recoverin-1a, which is expressed only in rods and UV cones. Applying a double flash electroretinogram (ERG) paradigm, downregulation of Recoverin-2a or 2b accelerates cone photoresponse recovery, albeit at different light intensities. Exclusive recording from UV cones via spectral ERG reveals that knockdown of Recoverin-1a alone has no effect, but Recoverin-1a/2a double-knockdowns showed an even shorter recovery time than Recoverin-2a-deficient larvae. We also showed that UV cone photoresponse kinetics depend on Recoverin-2a function via cone-specific kinase Grk7a. This is the first in vivo study demonstrating that cone opsin deactivation kinetics determine overall photoresponse shut off kinetics.

Effective delivery of recombinant proteins to rod photoreceptors via lipid nanovesicles

The potential of liposomes to deliver functional proteins in retinal photoreceptors and modulate their physiological response was investigated by two experimental approaches. First, we treated isolated mouse retinas with liposomes encapsulating either recoverin, an important endogenous protein operating in visual phototransduction, or antibodies against recoverin. We then intravitrally injected in vivo liposomes encapsulating either rhodamin B or recoverin and we investigated the distribution in retina sections by confocal microscopy. The content of liposomes was found to be released in higher amount in the photoreceptor layer than in the other regions of the retina and the functional effects of the release were in line with the current model of phototransduction. Our study sets the basis for quantitative investigations aimed at assessing the potential of intraocular protein delivery via biocompatible nanovesicles, with promising implications for the treatment of retinal diseases affecting the photoreceptor layer.

Chemistry and biology of the initial steps in vision: the Friedenwald lecture

Visual transduction is the process in the eye whereby absorption of light in the retina is translated into electrical signals that ultimately reach the brain. The first challenge presented by visual transduction is to understand its molecular basis. We know that maintenance of vision is a continuous process requiring the activation and subsequent restoration of a vitamin A-derived chromophore through a series of chemical reactions catalyzed by enzymes in the retina and retinal pigment epithelium (RPE). Diverse biochemical approaches that identified key proteins and reactions were essential to achieve a mechanistic understanding of these visual processes. The three-dimensional arrangements of these enzymes' polypeptide chains provide invaluable insights into their mechanisms of action. A wealth of information has already been obtained by solving high-resolution crystal structures of both rhodopsin and the retinoid isomerase from pigment RPE (RPE65). Rhodopsin, which is activated by photoisomerization of its 11-cis-retinylidene chromophore, is a prototypical member of a large family of membrane-bound proteins called G protein-coupled receptors (GPCRs). RPE65 is a retinoid isomerase critical for regeneration of the chromophore. Electron microscopy (EM) and atomic force microscopy have provided insights into how certain proteins are assembled to form much larger structures such as rod photoreceptor cell outer segment membranes. A second challenge of visual transduction is to use this knowledge to devise therapeutic approaches that can prevent or reverse conditions leading to blindness. Imaging modalities like optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) applied to appropriate animal models as well as human retinal imaging have been employed to characterize blinding diseases, monitor their progression, and evaluate the success of therapeutic agents. Lately two-photon (2-PO) imaging, together with biochemical assays, are revealing functional aspects of vision at a new molecular level. These multidisciplinary approaches combined with suitable animal models and inbred mutant species can be especially helpful in translating provocative cell and tissue culture findings into therapeutic options for further development in animals and eventually in humans. A host of different approaches and techniques is required for substantial progress in understanding fundamental properties of the visual system.

Double electron-electron resonance probes Ca²⁺-induced conformational changes and dimerization of recoverin

Recoverin, a member of the neuronal calcium sensor (NCS) branch of the calmodulin superfamily, is expressed in retinal photoreceptor cells and serves as a calcium sensor in vision. Ca²⁺-induced conformational changes in recoverin cause extrusion of its covalently attached myristate (termed Ca²⁺-myristoyl switch) that promotes translocation of recoverin to disk membranes during phototransduction in retinal rod cells. Here we report double electron-electron resonance (DEER) experiments on recoverin that probe Ca²⁺-induced changes in distance as measured by the dipolar coupling between spin-labels strategically positioned at engineered cysteine residues on the protein surface. The DEER distance between nitroxide spin-labels attached at C39 and N120C is 2.5 ± 0.1 nm for Ca²⁺-free recoverin and 3.7 ± 0.1 nm for Ca²⁺-bound recoverin. An additional DEER distance (5-6 nm) observed for Ca²⁺-bound recoverin may represent an intermolecular distance between C39 and N120. ¹⁵N NMR relaxation analysis and CW-EPR experiments both confirm that Ca²⁺-bound recoverin forms a dimer at protein concentrations above 100 μM, whereas Ca²⁺-free recoverin is monomeric. We propose that Ca²⁺-induced dimerization of recoverin at the disk membrane surface may play a role in regulating Ca²⁺-dependent phosphorylation of dimeric rhodopsin. The DEER approach will be useful for elucidating dimeric structures of NCS proteins in general for which Ca²⁺-induced dimerization is functionally important but not well understood.

Exploring the rate-limiting steps in visual phototransduction recovery by bottom-up kinetic modeling

Background

Phototransduction in vertebrate photoreceptor cells represents a paradigm of signaling pathways mediated by G-protein-coupled receptors (GPCRs), which share common modules linking the initiation of the cascade to the final response of the cell. In this work, we focused on the recovery phase of the visual photoresponse, which is comprised of several interacting mechanisms.

Results

We employed current biochemical knowledge to investigate the response mechanisms of a comprehensive model of the visual phototransduction pathway. In particular, we have improved the model by implementing a more detailed representation of the recoverin (Rec)-mediated calcium feedback on rhodopsin kinase and including a dynamic arrestin (Arr) oligomerization mechanism. The model was successfully employed to investigate the rate limiting steps in the recovery of the rod photoreceptor cell after illumination. Simulation of experimental conditions in which the expression levels of rhodospin kinase (RK), of the regulator of the G-protein signaling (RGS), of Arr and of Rec were altered individually or in combination revealed severe kinetic constraints to the dynamics of the overall network.

Conclusions

Our simulations confirm that RGS-mediated effector shutdown is the rate-limiting step in the recovery of the photoreceptor and show that the dynamic formation and dissociation of Arr homodimers and homotetramers at different light intensities significantly affect the timing of rhodopsin shutdown. The transition of Arr from its oligomeric storage forms to its monomeric form serves to temper its availability in the functional state. Our results may explain the puzzling evidence that overexpressing RK does not influence the saturation time of rod cells at bright light stimuli. The approach presented here could be extended to the study of other GPCR signaling pathways.

Conformational dynamics of recoverin's Ca2+-myristoyl switch probed by 15N NMR relaxation dispersion and chemical shift analysis

Recoverin, a member of the neuronal calcium sensor (NCS) branch of the calmodulin superfamily, serves as a calcium sensor in retinal rod cells. Ca(2+) -induced conformational changes in recoverin promote extrusion of its covalently attached myristate, known as the Ca(2+)-myristoyl switch. Here, we present nuclear magnetic resonance (NMR) relaxation dispersion and chemical shift analysis on (15) N-labeled recoverin to probe main chain conformational dynamics. (15) N NMR relaxation data suggest that Ca(2+)-free recoverin undergoes millisecond conformational dynamics at particular amide sites throughout the protein. The addition of trace Ca(2+) levels (0.05 equivalents) increases the number of residues that show detectable relaxation dispersion. The Ca(2+)-dependent chemical shifts and relaxation dispersion suggest that recoverin has an intermediate conformational state (I) between the sequestered apo state (T) and Ca(2+) saturated extruded state (R): T ↔ I ↔ R. The first step is a fast conformational equilibrium ([T]/[I] < 100) on the millisecond time scale (τ(ex) δω < 1). The final step (I ↔ R) is much slower (τ(ex) δω > 1). The main chain structure of I is similar in part to the structure of half-saturated E85Q recoverin with a sequestered myristoyl group. We propose that millisecond dynamics during T ↔ I may transiently increase the exposure of Ca(2+)-binding sites to initiate Ca(2+) binding that drives extrusion of the myristoyl group during I ↔ R.

Background light produces a recoverin-dependent modulation of activated-rhodopsin lifetime in mouse rods

The Ca(2+)-binding protein recoverin is thought to regulate rhodopsin kinase and to modulate the lifetime of the photoexcited state of rhodopsin (Rh*), the visual pigment of vertebrate rods. Recoverin has been postulated to inhibit the kinase in darkness, when Ca(2+) is high, and to be released from the disk membrane in light when Ca(2+) is low, accelerating rhodopsin phosphorylation and shortening the lifetime of Rh*. This proposal has remained controversial, in part because the normally rapid turnoff of Rh* has made Rh* modulation difficult to study in an intact rod. To circumvent this problem, we have made mice that underexpress rhodopsin kinase so that Rh* turnoff is rate limiting for the decay of the rod light response. We show that background light speeds the decay of Rh* turnoff, and that this no longer occurs in mice that have had recoverin knocked out. This is the first demonstration in an intact rod that light accelerates Rh* inactivation and that the Ca(2+)-binding protein recoverin may be required for the light-dependent modulation of Rh* lifetime.

Structural insights into membrane targeting by the flagellar calcium-binding protein (FCaBP), a myristoylated and palmitoylated calcium sensor in Trypanosoma cruzi

The flagellar calcium-binding protein (FCaBP) of the protozoan Trypanosoma cruzi is targeted to the flagellar membrane where it regulates flagellar function and assembly. As a first step toward understanding the Ca(2+)-induced conformational changes important for membrane-targeting, we report here the x-ray crystal structure of FCaBP in the Ca(2+)-free state determined at 2.2A resolution. The first 17 residues from the N terminus appear unstructured and solvent-exposed. Residues implicated in membrane targeting (Lys-19, Lys-22, and Lys-25) are flanked by an exposed N-terminal helix (residues 26-37), forming a patch of positive charge on the protein surface that may interact electrostatically with flagellar membrane targets. The four EF-hands in FCaBP each adopt a "closed conformation" similar to that seen in Ca(2+)-free calmodulin. The overall fold of FCaBP is closest to that of grancalcin and other members of the penta EF-hand superfamily. Unlike the dimeric penta EF-hand proteins, FCaBP lacks a fifth EF-hand and is monomeric. The unstructured N-terminal region of FCaBP suggests that its covalently attached myristoyl group at the N terminus may be solvent-exposed, in contrast to the highly sequestered myristoyl group seen in recoverin and GCAP1. NMR analysis demonstrates that the myristoyl group attached to FCaBP is indeed solvent-exposed in both the Ca(2+)-free and Ca(2+)-bound states, and myristoylation has no effect on protein structure and folding stability. We propose that exposed acyl groups at the N terminus may anchor FCaBP to the flagellar membrane and that Ca(2+)-induced conformational changes may control its binding to membrane-bound protein targets.

Determination of the contribution of the myristoyl group and hydrophobic amino acids of recoverin on its dynamics of binding to lipid monolayers

It has been postulated that myristoylation of peripheral proteins would facilitate their binding to membranes. However, the exact involvement of this lipid modification in membrane binding is still a matter of debate. Proteins containing a Ca(2+)-myristoyl switch where the extrusion of their myristoyl group is dependent on calcium binding is best illustrated by the Ca(2+)-binding recoverin, which is present in retinal rod cells. The parameters responsible for the modulation of the membrane binding of recoverin are still largely unknown. This study was thus performed to determine the involvement of different parameters on recoverin membrane binding. We have used surface pressure measurements and PM-IRRAS spectroscopy to monitor the adsorption of myristoylated and nonmyristoylated recoverin onto phospholipid monolayers in the presence and absence of calcium. The adsorption curves have shown that the myristoyl group and hydrophobic residues of myristoylated recoverin strongly accelerate membrane binding in the presence of calcium. In the case of nonmyristoylated recoverin in the presence of calcium, hydrophobic residues alone are responsible for its much faster monolayer binding than myristoylated and nonmyristoylated recoverin in the absence of calcium. The infrared spectra revealed that myristoylated and nonmyristoylated recoverin behave very different upon adsorption onto phospholipid monolayers. Indeed, PM-IRRAS spectra indicated that the myristoyl group allows a proper orientation and organization as well as faster and stronger binding of myristoylated recoverin to lipid monolayers compared to nonmyristoylated recoverin. Simulations of the spectra have allowed us to postulate that nonmyristoylated recoverin changes conformation and becomes hydrated at large extents of adsorption as well as to estimate the orientation of myristoylated recoverin with respect to the monolayer plane. In addition, adsorption measurements and electrophoresis of trypsin-treated myristoylated recoverin in the presence of zinc or calcium demonstrated that recoverin has a different conformation but a similar extent of monolayer binding in the presence of such ions.

Establishment of a novel small cell lung carcinoma cell line with specific recoverin expression from a patient with cancer-associated retinopathy

We analysed the biologic properties of a small cell lung carcinoma cell line (designated KK0206) established from a patient with SCLC who had cancer-associated retinopathy (CAR). Morphological and immunohistochemical studies showed that KK0206 cells have features of the classic type of SCLC. KK0206 cells grew in suspension, forming relatively small clumps of cells with a doubling time of 72 h. On light microscopy, the cells were relatively small with little cytoplasm. On immunohistochemistry using anti-bovine recoverin rabbit antibody, the cells were intensely positive for recoverin. In addition, they were positive for NSE, Ki-67, and TP53. They also expressed human recoverin, a photoreceptor protein, whose presence was confirmed by RT-PCR analysis with cDNA sequencing and Western blot analysis. The point mutation of their TP53 gene (exon 156) was detected as well. The present study demonstrates that human recoverin is expressed in SCLC cells cultured from an anti-recoverin antibody-negative patient with CAR. KK0206 might be important for further research on SCLC related retinopathy.

Structural basis for calcium-induced inhibition of rhodopsin kinase by recoverin

Recoverin, a member of the neuronal calcium sensor branch of the EF-hand superfamily, serves as a calcium sensor that regulates rhodopsin kinase (RK) activity in retinal rod cells. We report here the NMR structure of Ca(2+)-bound recoverin bound to a functional N-terminal fragment of rhodopsin kinase (residues 1-25, called RK25). The overall main-chain structure of recoverin in the complex is similar to structures of Ca(2+)-bound recoverin in the absence of target (<1.8A root-mean-square deviation). The first eight residues of recoverin at the N terminus are solvent-exposed, enabling the N-terminal myristoyl group to interact with target membranes, and Ca(2+) is bound at the second and third EF-hands of the protein. RK25 in the complex forms an amphipathic helix (residues 4-16). The hydrophobic face of the RK25 helix (Val-9, Val-10, Ala-11, Ala-14, and Phe-15) interacts with an exposed hydrophobic groove on the surface of recoverin lined by side-chain atoms of Trp-31, Phe-35, Phe-49, Ile-52, Tyr-53, Phe-56, Phe-57, Tyr-86, and Leu-90. Residues of recoverin that contact RK25 are highly conserved, suggesting a similar target binding site structure in all neuronal calcium sensor proteins. Site-specific mutagenesis and deletion analysis confirm that the hydrophobic residues at the interface are necessary and sufficient for binding. The recoverin-RK25 complex exhibits Ca(2+)-induced binding to rhodopsin immobilized on concanavalin-A resin. We propose that Ca(2+)-bound recoverin is bound between rhodopsin and RK in a ternary complex on rod outer segment disk membranes, thereby blocking RK interaction with rhodopsin at high Ca(2+).

Calcium-sensitive downregulation of the transduction chain in rod photoreceptors of the rat retina

In vertebrate rod outer segments phototransduction is suggested to be modulated by intracellular Ca. We aimed at verifying this hypothesis by recording saturated photosignals in the rat retina after single and double flashes of light and determining the time t(c) to the beginning of the signal recovery. The time course of Ca(i) after a flash was calculated from a change of the spatial Ca(2+) concentration profile recorded in the space between the rods. After single flashes t(c) increased linearly with the logarithm of flash intensity, confirming the assumption that t(c) is determined by deactivation of a single species X* in the phototransduction cascade. The photoresponse was shortened up to 45% if the test flash was preceded by a conditioning preflash. The shortening depended on the reduction of Ca(i) induced by the preflash. The data suggest that the phototransduction gain determining the amount of activated X* is regulated by a Ca(i)-dependent mechanism in a short time period (<800 ms) after the test flash. Lowering of Ca(i) by a preflash reduced the gain up to 20% compared to its value in a dark-adapted rod. The relation between phototransduction gain and Ca(i) revealed a K(1/2) value close to the dark level of Ca(i).

The vertebrate phototransduction cascade: amplification and termination mechanisms

The biochemical cascade which transduces light into a neuronal signal in retinal photoreceptors is a heterotrimeric GTP-binding protein (G protein) signaling pathway called phototransduction. Works from psychophysicists, electrophysiologists, biochemists, and geneticists over several decades have come together to shape our understanding of how photon absorption leads to photoreceptor membrane hyperpolarization. The insights of phototransduction provide the foundation for a mechanistic account of signaling from many other G protein-coupled receptors (GPCR) found throughout nature. The application of reverse genetic techniques has strengthened many historic findings and helped to describe this pathway at greater molecular details. However, many important questions remain to be answered.

Recoverin binds exclusively to an amphipathic peptide at the N terminus of rhodopsin kinase, inhibiting rhodopsin phosphorylation without affecting catalytic activity of the kinase

Recoverin is a calcium-dependent inhibitor of rhodopsin kinase. It prevents premature phosphorylation of rhodopsin until the opening of cGMP-gated ion channels causes a decrease in intracellular calcium levels, signaling completion of the light response. This calcium depletion causes release of recoverin from rhodopsin kinase, freeing the kinase to phosphorylate rhodopsin and to terminate the light response. Previous studies have shown that recoverin is able to bind to a region at the N terminus of rhodopsin kinase. In this study we map this interaction interface, showing that residues 1-15 of the kinase form the interaction site for recoverin binding. Mutation of hydrophobic residues in this region have the greatest effect on the interaction. The periodic nature of these residues suggests that they lie along one face of an amphipathic helix. We show that this region is essential for recoverin binding, as a catalytically active kinase lacking these residues is unable to bind recoverin. In addition, we show that neither the N-terminal deletion nor the presence of recoverin inhibits the overall catalytic activity of the kinase, as measured by light-independent autophosphorylation. Finally, we observe that a kinase mutant lacking the N-terminal recoverin binding site is unable to phosphorylate light-activated rhodopsin. Taken together, these data support a model in which recoverin prevents rhodopsin phosphorylation by sterically blocking a region of kinase essential for its interaction with rhodopsin, thereby preventing recognition of rhodopsin as a kinase substrate.

Beyond counting photons: trials and trends in vertebrate visual transduction

For over 30 years, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and G protein signaling. Recently, photoreceptors have become an equally attractive model for studying many facets of neuronal cell biology. The primary goal of this review is to illustrate this rapidly growing trend. We will highlight the areas of active research in photoreceptor biology that reveal how different specialized compartments of the cell cooperate in fulfilling its overall function: converting photon absorption into changes in neurotransmitter release. The same trend brings us closer to understanding how defects in photoreceptor signaling can lead to cell death and retinal degeneration.

Single-step purification of myristoylated and nonmyristoylated recoverin and substrate dependence of myristoylation level

Recoverin is cotranslationally modified by the covalent linkage of a myristoyl group to its N terminus. It is a member of a family of Ca(2+)-myristoyl switch proteins. Recombinant myristoylated revoverin is currently produced by the cotransformation of bacteria with recoverin and an enzyme that allows N-myristoylation and by supplementing the culture medium with myristic acid. A large variation in the myristoylation level of recoverin and in the amount of myristic acid supplied to the culture medium can be found in the literature. Moreover, although it is known to strongly affect bacterial growth, the amount of ethanol used to solubilize myristic acid is only scarcely mentioned. To improve our understanding of the parameters responsible for recombinant recoverin myristoylation, the effects of myristic acid and ethanol on recoverin myristoylation and expression levels have been systematically studied. In addition, a single-step purification procedure to produce purified myristoylated and nonmyristoylated recombinant recoverin has also been devised. Finally, sodium myristate has been used as an efficient alternative substrate to achieve high myristoylation and expression levels of recoverin. Given that a large number of proteins are myristoylated, these procedures could be applied to several other proteins in addition to recoverin.

Recoverin improves rod-mediated vision by enhancing signal transmission in the mouse retina

Vision in dim light requires that photons absorbed by rod photoreceptors evoke signals that reliably propagate through the retina. We investigated how a perturbation in rod physiology affects propagation of those signals in the retina and ultimately visual sensitivity. Recoverin is a protein in rods that prolongs phototransduction and enhances visual sensitivity. It is not present in neurons postsynaptic to rods, yet we found that light-evoked responses of rod bipolar and ganglion cells were shortened when measured in recoverin-deficient retinas. Unexpectedly, the effect of recoverin on postsynaptic signals could not be explained by its effect on phototransduction. Instead, it is an effect of recoverin downstream of phototransduction in rods that prolongs signal transmission and enhances visual sensitivity. An important implication of our findings is that the recovery phase of the rod photoresponse does not contribute significantly to visual sensitivity near absolute threshold.

Recoverin regulates light-dependent phosphodiesterase activity in retinal rods

The Ca²⁺-binding protein recoverin may regulate visual transduction in retinal rods and cones, but its functional role and mechanism of action remain controversial. We compared the photoresponses of rods from control mice and from mice in which the recoverin gene was knocked out. Our analysis indicates that Ca²⁺-recoverin prolongs the dark-adapted flash response and increases the rod's sensitivity to dim steady light. Knockout rods had faster Ca²⁺ dynamics, indicating that recoverin is a significant Ca²⁺ buffer in the outer segment, but incorporation of exogenous buffer did not restore wild-type behavior. We infer that Ca²⁺-recoverin potentiates light-triggered phosphodiesterase activity, probably by effectively prolonging the catalytic activity of photoexcited rhodopsin.

Cancer-related diseases of the eye: the role of calcium and calcium-binding proteins

The eye provides unique opportunities to study complex biochemical pathways and to describe how components of these pathways contribute to the molecular basis of disease. In this article, the role of calcium-binding proteins in cancer-related diseases of the eye is reviewed. First, paraneoplastic syndromes, or so-called remote effects of cancer, arise from damage to tissues distant from any tumor or its metastases. Many of these syndromes are believed to be immune-mediated. Cancer-associated retinopathy (CAR), a blinding disease due to the degeneration of retinal photoreceptor cells, is one of the best characterized of the paraneoplastic syndromes. The CAR autoantigen has been identified as recoverin, a calcium-binding protein of the EF-hand superfamily. Its features as a calcium-binding protein, along with its function in photoreceptor cells and its role as the CAR autoantigen, are discussed. Next, unlike visual symptoms instigated by a distant tumor, ocular melanoma is the primary malignancy originating in the eye. ALG-2 encodes a pro-apoptotic calcium-binding protein that is down-regulated in ocular melanoma, thus providing these tumor cells with a selective advantage. In addition to background discussion of ALG-2, data describing the expression, cellular localization, and dimerization characteristics of ALG-2 in melanoma cells are presented. Biochemical studies of ALG-2 and its interactions with its target Alix/AIP1 also are presented. Finally, the function of ALG-2 in calcium-induced cell death is discussed. Additional calcium-binding proteins in retina and in ocular tumors are described in relation to different disease entities. Such proteins and their expression in the eye provide valuable examples bridging studies of protein chemistry, cellular function, and human disease.

Calmodulin and Ca2+-binding proteins (CaBPs): variations on a theme

Ca2+ is a ubiquitous second messenger that frequently exerts its effects through Ca2+-binding proteins. In response to changes in the intracellular [Ca2+], Ca2+-binding proteins modulate the cellular activities of enzymes, channels and structural proteins. Multiple Ca2+-binding proteins are expressed in the retina and, in most cases, in a unique cellular and sub-cellular manner. CaBPs are retinal Ca2+-binding proteins displaying a high similarity to calmodulin (CaM). CaBPs are able to mimic some of the interactions of CaM with effector enzymes, although their physiological role has not yet been resolved. CaBPs could be cell-type specific proteins that play a key role in the Ca2+ signaling of specialized retinal neurons.

Recoverin and rhodopsin kinase

The majority of proteins involved in vertebrate phototransduction are expressed specifically in photoreceptors. Recoverin and rhodopsin kinase are expressed primarily in retinal photoreceptors and they interact with each other in a Ca2+-dependent manner. This Ca2+-dependent interaction has been studied extensively in vitro. Experiments utilizing animal models and electrophysiological approaches have started to provide important insight regarding its invivo function. Recoverin can be viewed as a negative regulator of rhodopsin kinase in vertebrate phototransduction. This interaction imparts a negative feedback loop at the receptor level and may play an important role in light adaptation and in recovery.

Structure and membrane-targeting mechanism of retinal Ca2+-binding proteins, recoverin and GCAP-2

Recoverin and the guanylate cyclase activating proteins (GCAPs) are calcium-sensing proteins in retinal rod and cone cells that belong to the EF-hand superfamily and serve as important calcium sensors in vision. Recoverin and the OCAP proteins are myristoylated at their amino-terminus and are targeted to retinal disc membranes by a myristoyl switch. Here, we present the three-dimensional, atomic-resolution structures of recombinant myristoylated recoverin containing 0, 1 and 2 calcium ions (Ca2+) bound and unmyristoylated GCAP-2 with 3 Ca2+ bound as determined by nuclear magnetic resonance. The Ca2+-induced structural changes in these proteins are important for elucidating their membrane-targeting mechanisms and for understanding the molecular mechanism of Ca2+-sensitive regulation of phototransduction.

Ca2+-dependent control of rhodopsin phosphorylation: recoverin and rhodopsin kinase

Over many years until the middle of the 1980s, the main problem in vision research had been the mechanism of transducing the visual signal from photobleached rhodopsin to the cationic channels in the plasma membrane of a photoreceptor to trigger the electrophysiological response of the cell. After cGMP was proven to be the secondary messenger, the main intriguing question has become the mechanisms of negative feedback in photoreceptors to modulate their response to varying conditions of illumination. Although the mechanisms of light-adaptation are not completely understood, it is obvious that Ca2+ plays a crucial role in these mechanisms and that the effects of Ca2+ can be mediated by several Ca2+-binding proteins. One of them is recoverin. The leading candidate for the role of an intracellular target for recoverin is believed to be rhodopsin kinase, a member of a family of G-protein-coupled receptor kinases. The present review considers recoverin, rhodopsin kinase and their interrelationships in the in vitro as well as in vivo contexts.

S-modulin

S-Modulin is a Ca2+-binding protein found in frog rod photoreceptors (1,2) and its bovine homologue is known as recoverin (3,4). In the Ca2+-bound form, S-modulin inhibits rhodopsin phosphorylation5 through inhibition of rhodopsin kinase. (6-9) Because rhodopsin phosphorylation is the quench mechanism of light-activated rhodopsin (R*), (10,11) the inhibition of the phosphorylation by S-modulin probably contributes to increase the lifetime of R* to result in sustained hydrolysis of cGMP5. The Ca2+ concentration decreases in the light in vertebrate photoreceptors, (12-14) and this decrease is essential for light-adaptation. (15,16) Thus, S-modulin is expected to regulate the lifetime of R* and thereby regulate the extent and the time course of hydrolysis of cGMP depending on the intensity of background light. With this mechanism, S-modulin is believed to regulate the waveform of a photoresponse and the efficiency of the light in the generation of a photoresponse.

Measurement of membrane binding between recoverin, a calcium-myristoyl switch protein, and lipid bilayers by AFM-based force spectroscopy

Myristoyl switch is a feature of several peripheral membrane proteins involved in signal transduction pathways. This unique molecular property is best illustrated by the "Ca(2+)-myristoyl switch" of recoverin, which is a Ca(2+)-binding protein present in retinal rod cells of vertebrates. In this transduction pathway, the Ca(2+)-myristoyl switch acts as a calcium sensor involved in cell recovery from photoactivation. Ca(2+) binding by recoverin induces the extrusion of its myristoyl group to the solvent, which leads to its translocation from cytosol to rod disk membranes. Force spectroscopy, based on atomic force microscope (AFM) technology, was used to determine the extent of membrane binding of recoverin in the absence and presence of calcium, and to quantify this force of binding. An adhesion force of 48 +/- 5 pN was measured between recoverin and supported phospholipid bilayers in the presence of Ca(2+). However, no binding was observed in the absence of Ca(2+). Experiments with nonmyristoylated recoverin confirmed these observations. Our results are consistent with previously measured extraction forces of lipids from membranes.

The clinical spectrum and pathogenesis of paraneoplastic disorders of the central nervous system

Paraneoplastic neurologic disorders (PND) refer to neurologic disorders of unknown cause that occur at higher frequency in patients with cancer than in the general population. There is increasing evidence that many of these disorders are immune mediated and associated with cytotoxic antitumor immunity. The identification of the immune responses in these patients facilitates the diagnosis of the PND and has led to the cloning and characterization of the target antigens in the nervous system and tumor.

Calcium-sensitive regions of GCAP1 as observed by chemical modifications, fluorescence, and EPR spectroscopies

Guanylyl cyclase-activating proteins are EF-hand Ca(2+)-binding proteins that belong to the calmodulin superfamily. They are involved in the regulation of photoreceptor membrane-associated guanylyl cyclases that produce cGMP, a second messenger of vertebrate vision. Here, we investigated changes in GCAP1 structure using mutagenesis, chemical modifications, and spectroscopic methods. Two Cys residues of GCAP1 situated in spatially distinct regions of the N-terminal domain (positions 18 and 29) and two Cys residues located within the C-terminal lobe (positions 106 and 125) were employed to detect conformational changes upon Ca(2+) binding. GCAP1 mutants with only a single Cys residue at each of these positions, modified with N,N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)ethylenediamine, an environmentally sensitive fluorophore, and with (1-oxy-2,2,5,5-tetramethylpyrroline-3-methyl)methanethiosulfonate, a spin label reagent, were studied using fluorescence and EPR spectroscopy, respectively. Only minor structural changes around Cys(18), Cys(29), Cys(106), and Cys(125) were observed as a function of Ca(2+) concentration. No Ca(2+)-dependent oligomerization of GCAP1 was observed at physiologically relevant Ca(2+) concentrations, in contrast to the observation reported by others for GCAP2. Based on these results and previous studies, we propose a photoreceptor activation model that assumes changes within the flexible central helix upon Ca(2+) dissociation, causing relative reorientation of two structural domains containing a pair of EF-hand motifs and thus switching its partner, guanylyl cyclase, from an inactive (or low activity) to an active conformation.

Activation, deactivation, and adaptation in vertebrate photoreceptor cells

Visual transduction captures widespread interest because its G-protein signaling motif recurs throughout nature yet is uniquely accessible for study in the photoreceptor cells. The light-activated currents generated at the photoreceptor outer segment provide an easily observed real-time measure of the output of the signaling cascade, and the ease of obtaining pure samples of outer segments in reasonable quantity facilitates biochemical experiments. A quiet revolution in the study of the mechanism has occurred during the past decade with the advent of gene-targeting techniques. These have made it possible to observe how transduction is perturbed by the deletion, overexpression, or mutation of specific components of the transduction apparatus.

Intracellular neuronal calcium sensor (NCS) protein VILIP-1 modulates cGMP signalling pathways in transfected neural cells and cerebellar granule neurones

The family of intracellular neuronal calcium-sensors (NCS) belongs to the superfamily of EF-hand proteins. Family members have been shown by in vitro assays to regulate signal cascades in retinal photoreceptor cells. To study the functions of NCS proteins not expressed in photoreceptor cells we examined Visinin-like protein-1 (VILIP-1) effects on signalling pathways in living neural cells. Visinin-like protein-1 expression increased cGMP levels in transfected C6 and PC12 cells. Interestingly, in transfected PC12 cells stimulation was dependent on the subcellular localization of VILIP-1. In cells transfected with membrane-associated wild-type VILIP-1 particulate guanylyl cyclase (GC) was stimulated more strongly than soluble GC. In contrast, deletion of the N-terminal myristoylation site resulted in cytosolic localization of VILIP-1 and enhanced stimulation of soluble GC. To study the molecular mechanisms underlying GC stimulation VILIP-1 was examined to see if it can physically interact with GCs. A direct physical interaction of VILIP-1 with the recombinant catalytic domain of particulate GCs-A, B and with native GCs enriched from rat brain was observed in GST pull-down as well as in surface plasmon resonance interaction studies. Furthermore, following trituration of recombinant VILIP-1 protein into cerebellar granule cells the protein influenced only signalling by GC-B. Together with the observed colocalization of GC-B, but not GC-A, with VILIP-1 in cerebellar granule cells, these results suggest that VILIP-1 may be a physiological regulator of GC-B.

In vitro inhibition of antirecoverin immunoglobulin-mediated death of mammalian photoreceptor cells

Cancer-associated retinopathy (CAR) is a blinding disease, which can be mediated by autoimmune reactions with a specific calcium-binding retinal protein, recoverin. A number of recent studies demonstrate that agents that mobilize intracellular calcium can protect neurons from apoptotic death induced by a variety of insults. In this study, we investigated the effect of one such agent, potassium, on the survival of mammalian rod photoreceptors exposed to antirecoverin IgG. Primary cell cultures of rat retinal neurons were grown in a chemically defined medium, and cells were exposed to antirecoverin IgG for 72 hr in various concentrations of potassium and the surviving cells counted. Rod photoreceptors were quantitated using antirhodopsin immunofluorescence microscopy, and total cell numbers were determined by 4',6-diamidino-2-phenylindole (DAPI) staining of nuclei. Apoptosis was evaluated by TdT-mediated biotin-dUTP nick-end labeling (TUNEL), cell death-detection ELISA, and DNA laddering. The present study shows that elevated extracellular K+ ([K+](o)) protects retinal neurons from antirecoverin antibody-mediated cell death. The protective effects of ([K+](o)) were shown to be time- and dose-dependent. The inhibition of antirecoverin IgG-mediated death of photoreceptors by elevated ([K+](o)) suggests that the mobilization of internal calcium stores rescues the cells by interfering with apoptotic signal transduction pathways. These data also suggest that the death of photoreceptor cells occurring in CAR possibly can be prevented by reagents and/or environmental changes that mobilize intracellular calcium.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Effects of mutations in the calcium-binding sites of recoverin on its calcium affinity: evidence for successive filling of the calcium binding sites

A molecule of the photoreceptor Ca(2+)-binding protein recoverin contains four potential EF-hand Ca(2+)-binding sites, of which only two, the second and the third, are capable of binding calcium ions. We have studied the effects of substitutions in the second, third and fourth EF-hand sites of recoverin on its Ca(2+)-binding properties and some other characteristics, using intrinsic fluorescence, circular dichroism spectroscopy and differential scanning microcalorimetry. The interaction of the two operating binding sites of wild-type recoverin with calcium increases the protein's thermal stability, but makes the environment around the tryptophan residues more flexible. The amino acid substitution in the EF-hand 3 (E121Q) totally abolishes the high calcium affinity of recoverin, while the mutation in the EF-hand 2 (E85Q) causes only a moderate decrease in calcium binding. Based on this evidence, we suggest that the binding of calcium ions to recoverin is a sequential process with the EF-hand 3 being filled first. Estimation of Ca(2+)-binding constants according to the sequential binding scheme gave the values 3.7 x 10(6) and 3.1 x 10(5) M(-1) for third and second EF-hands, respectively. The substitutions in the EF-hand 2 or 3 (or in both the sites simultaneously) do not disturb significantly either tertiary or secondary structure of the apo-protein. Amino acid substitutions, which have been designed to restore the calcium affinity of the EF-hand 4 (G160D, K161E, K162N, D165G and K166Q), increase the calcium capacity and affinity of recoverin but also perturb the protein structure and decrease the thermostability of its apo-form.

Recoverin negative photoreceptor cells

Recoverin, a calcium-binding protein, is unique with respect to its cellular regulation. It is present in retinal rods, cones, cone bipolar cells, and in a rare population of cells in the ganglion cell layer. Inappropriate turn-on or turn-off of recoverin expression has been reported both in small cell lung carcinoma cells from patients with cancer-associated retinopathy (Matsusara et al. [1996] Br. J. Cancer 74:1419-1422; Adamus et al. [1998] J. Autoimmun. 11: 523-533; Ohguro et al. [1999] Invest. Ophthalmol. Vis. Sci. 40:82-89) and in cultured retinal neurons (McGinnis et al. [1999] J. Neursci. Res. 55:252-260). In a recent report using double labeling immunofluorescence microscopy methods with antibodies against either rhodopsin and recoverin or arrestin and recoverin, two unique cell phenotypes, rhodopsin-positive and recoverin-negative, and arrestin-positive, and recoverin-negative were observed in vitro. These two unique cell types could be nonphotoreceptor cells in which rhodopsin and arrestin are inappropriately turned on or they are photoreceptor cells in which the recoverin gene is inappropriately turned off. In this study, multiple antibodies were used to study, on a single-cell basis, whether the photoreceptor cell-specific marker, rhodopsin, is inappropriately expressed in nonphotoreceptor cells in our retinal neuronal culture system. We also examined the hypothesis that the two unique cell phenotypes represent the same population of cells. A triple labeling method has been established to visualize recoverin, rhodopsin, and arrestin protein expression simultaneously in cultured retinal neurons. Our data clearly and directly demonstrate that the previously described unique cell phenotypes are the same population of cells, rod photoreceptors. The existence of recoverin-negative photoreceptors demonstrates that the recoverin gene can be regulated independently of other photoreceptor cell-specific proteins and suggests that this primary cell culture may be useful as a model system for investigating the illicit expression of the recoverin gene in cancer associated retinopathy.

Calcium-dependent regulation of rhodopsin phosphorylation

Depending on ambient light conditions, a rod photoreceptor cell adapts to a light stimulus. For example, when it is kept in the light, its light sensitivity decreases because of light adaptation. The adaptational state is regulated by the Ca2+ concentration in the cytoplasm ([Ca2+]i). The [Ca2+]i is high in the dark and becomes low when the cell is light-adapted. The change in [Ca2+]i is detected by several Ca(2+)-binding proteins that change their conformations by binding Ca2+. S-modulin, found in frog rods, or its bovine homologue recoverin, is a 23 kDa Ca(2+)-binding protein that inhibits rhodopsin phosphorylation at high Ca2+ concentrations by inhibiting rhodopsin kinase. Since rhodopsin phosphorylation is an inactivating mechanism for light-activated rhodopsin (R*), the inhibition of this reaction will prolong the lifetime of R*. In this way, S-modulin is expected to increase the efficiency of phototransduction and therefore the light-sensitivity of rods in the dark. When rods are light-adapted, [Ca2+]i decreases so that the lifetime of the R* is expected to reduce, resulting in a decrease in light sensitivity. Even though it is generally agreed that S-modulin inhibits rhodopsin phosphorylation, its physiological function is not yet well understood.