Visual pigment phosphorylation but not transducin translocation can contribute to light adaptation in zebrafish cones

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.

Metabolism in the Zebrafish Retina

Retinal photoreceptors are amongst the most metabolically active cells in the body, consuming more glucose as a metabolic substrate than even the brain. This ensures that there is sufficient energy to establish and maintain photoreceptor functions during and after their differentiation. Such high dependence on glucose metabolism is conserved across vertebrates, including zebrafish from early larval through to adult retinal stages. As the zebrafish retina develops rapidly, reaching an adult-like structure by 72 hours post fertilisation, zebrafish larvae can be used to study metabolism not only during retinogenesis, but also in functionally mature retinae. The interplay between rod and cone photoreceptors and the neighbouring retinal pigment epithelium (RPE) cells establishes a metabolic ecosystem that provides essential control of their individual functions, overall maintaining healthy vision. The RPE facilitates efficient supply of glucose from the choroidal vasculature to the photoreceptors, which produce metabolic products that in turn fuel RPE metabolism. Many inherited retinal diseases (IRDs) result in photoreceptor degeneration, either directly arising from photoreceptor-specific mutations or secondary to RPE loss, leading to sight loss. Evidence from a number of vertebrate studies suggests that the imbalance of the metabolic ecosystem in the outer retina contributes to metabolic failure and disease pathogenesis. The use of larval zebrafish mutants with disease-specific mutations that mirror those seen in human patients allows us to uncover mechanisms of such dysregulation and disease pathology with progression from embryonic to adult stages, as well as providing a means of testing novel therapeutic approaches.

Biochemistry and physiology of zebrafish photoreceptors

All vertebrates share a canonical retina with light-sensitive photoreceptors in the outer retina. These photoreceptors are of two kinds: rods and cones, adapted to low and bright light conditions, respectively. They both show a peculiar morphology, with long outer segments, comprised of ordered stacks of disc-shaped membranes. These discs host numerous proteins, many of which contribute to the visual transduction cascade. This pathway converts the light stimulus into a biological signal, ultimately modulating synaptic transmission. Recently, the zebrafish (Danio rerio) has gained popularity for studying the function of vertebrate photoreceptors. In this review, we introduce this model system and its contribution to our understanding of photoreception with a focus on the cone visual transduction cascade.

Apo-Opsin and Its Dark Constitutive Activity across Retinal Cone Subtypes

Retinal rod and cone photoreceptors mediate vision in dim and bright light, respectively, by transducing absorbed photons into neural electrical signals. Their phototransduction mechanisms are essentially identical. However, one difference is that, whereas a rod visual pigment remains stable in darkness, a cone pigment has some tendency to dissociate spontaneously into apo-opsin and retinal (the chromophore) without isomerization. This cone-pigment property is long known but has mostly been overlooked. Importantly, because apo-opsin has weak constitutive activity, it triggers transduction to produce electrical noise even in darkness. Currently, the precise dark apo-opsin contents across cone subtypes are mostly unknown, as are their dark activities. We report here a study of goldfish red (L), green (M), and blue (S) cones, finding with microspectrophotometry widely different apo-opsin percentages in darkness, being ∼30% in L cones, ∼3% in M cones, and negligible in S cones. L and M cones also had higher dark apo-opsin noise than holo-pigment thermal isomerization activity. As such, given the most likely low signal amplification at the pigment-to-transducin/phosphodiesterase phototransduction step, especially in L cones, apo-opsin noise may not be easily distinguishable from light responses and thus may affect cone vision near threshold.

Whole-Cell Photobleaching Reveals Time-Dependent Compartmentalization of Soluble Proteins by the Axon Initial Segment

By limiting protein exchange between the soma and the axon, the axon initial segment (AIS) enables the segregation of specific proteins and hence the differentiation of the somatodendritic compartment and the axonal compartment. Electron microscopy and super-resolution fluorescence imaging have provided important insights in the ultrastructure of the AIS. Yet, the full extent of its filtering properties is not fully delineated. In particular, it is unclear whether and how the AIS opposes the free exchange of soluble proteins. Here we describe a robust framework to combine whole-cell photobleaching and retrospective high-resolution imaging in developing neurons. With this assay, we found that cytoplasmic soluble proteins that are not excluded from the axon upon expression over tens of hours exhibit a strong mobility reduction at the AIS – i.e., are indeed compartmentalized – when monitored over tens of minutes. This form of compartmentalization is developmentally regulated, requires intact F-actin and may be correlated with the composition and ultrastructure of the submembranous spectrin cytoskeleton. Using computational modeling, we provide evidence that both neuronal morphology and the AIS regulate this compartmentalization but act on distinct time scales, with the AIS having a more pronounced effect on fast exchanges. Our results thus suggest that the rate of protein accumulation in the soma may impact to what extent and over which timescales freely moving molecules can be segregated from the axon. This in turn has important implications for our understanding of compartment-specific signaling in neurons.

Phosphorylation at Serine 21 in G protein-coupled receptor kinase 1 (GRK1) is required for normal kinetics of dark adaption in rod but not cone photoreceptors

Timely recovery of the light response in photoreceptors requires efficient inactivation of photoactivated rhodopsin. This process is initiated by phosphorylation of its carboxyl terminus by G protein-coupled receptor kinase 1 (GRK1). Previously, we showed that GRK1 is phosphorylated in the dark at Ser21 in a cAMP-dependent manner and dephosphorylated in the light. Results in vitro indicate that dephosphorylation of Ser21 increases GRK1 activity, leading to increased phosphorylation of rhodopsin. This creates the possibility of light-dependent regulation of GRK1 activity and its efficiency in inactivating the visual pigment. To address the functional role of GRK1 phosphorylation in rods and cones in vivo, we generated mutant mice in which Ser21 is substituted with alanine (GRK1-S21A), preventing dark-dependent phosphorylation of GRK1. GRK1-S21A mice had normal retinal morphology, without evidence of degeneration. The function of dark-adapted GRK1-S21A rods and cones was also unaffected, as demonstrated by the normal amplitude and kinetics of their responses obtained by ex vivo and in vivo ERG recordings. In contrast, rod dark adaptation following exposure to bright bleaching light was significantly delayed in GRK1-S21A mice, suggesting that the higher activity of this kinase results in enhanced rhodopsin phosphorylation and therefore delays its regeneration. In contrast, dark adaptation of cones was unaffected by the S21A mutation. Taken together, these data suggest that rhodopsin phosphorylation/dephosphorylation modulates the recovery of rhodopsin to the ground state and rod dark adaptation. They also reveal a novel role for cAMP-dependent phosphorylation of GRK1 in regulating the dark adaptation of rod but not cone photoreceptors.

Circadian regulation of phosphodiesterase 6 genes in zebrafish differs between cones and rods: Implications for photopic and scotopic vision

A correlation is known to exist between visual sensitivity and oscillations in red opsinand rhodopsin gene expression in zebrafish, both regulated by the clock gene. This indicates that an endogenous circadian clock regulates behavioural visual sensitivity, apart from the regulation exerted by the pineal organ. However, the specific mechanisms for cones (photopic vision) and rods (scotopic vision) are poorly understood. In this work, we performed gene expression, cosinor and immunohistochemical analyses to investigate other key genes involved in light perception, encoding the different subunits of phosphodiesterase pde6 and transducin Gα_T, in constant lighting conditions and compared to normal light-dark conditions. We found that cones display prominent circadian oscillations in mRNA levels for the inhibitory subunit gene pde6ha that could contribute to the regulation of photopic sensitivity by preventing overstimulation in photopic conditions. In rods, the mRNA levels of the inhibitory subunit gene pde6ga oscillate under normal conditions and dampen down in constant light but continue oscillating in constant darkness. There is an increase in total relative expression for pde6gb in constant conditions. These observations, together with previous data, suggest a complex regulation of the scotopic sensitivity involving endogenous and non-endogenous components, possibly present also in other teleost species. The Gα_T genes do not display mRNA oscillations and therefore may not be essential for the circadian regulation of photosensitivity. In summary, our results support different regulation for the zebrafish photopic and scotopic sensitivities and suggest circadian regulation of pde6ha as a key factor regulating photopic sensitivity, while the regulatory mechanisms in rods appear to be more complex.

Cul3-Klhl18 ubiquitin ligase modulates rod transducin translocation during light-dark adaptation

Adaptation is a general feature of sensory systems. In rod photoreceptors, light-dependent transducin translocation and Ca²⁺ homeostasis are involved in light/dark adaptation and prevention of cell damage by light. However, the underlying regulatory mechanisms remain unclear. Here, we identify mammalian Cul3-Klhl18 ubiquitin ligase as a transducin translocation modulator during light/dark adaptation. Under dark conditions, Klhl18^-/- mice exhibited decreased rod light responses and subcellular localization of the transducin α-subunit (Tα), similar to that observed in light-adapted Klhl18^+/+ mice. Cul3-Klhl18 promoted ubiquitination and degradation of Unc119, a rod Tα-interacting protein. Unc119 overexpression phenocopied Tα mislocalization observed in Klhl18^-/- mice. Klhl18 weakly recognized casein kinase-2-phosphorylated Unc119 protein, which is dephosphorylated by Ca²⁺ -dependent phosphatase calcineurin. Calcineurin inhibition increased Unc119 expression and Tα mislocalization in rods. These results suggest that Cul3-Klhl18 modulates rod Tα translocation during light/dark adaptation through Unc119 ubiquitination, which is affected by phosphorylation. Notably, inactivation of the Cul3-Klhl18 ligase and calcineurin inhibitors FK506 and cyclosporine A that are known immunosuppressant drugs repressed light-induced photoreceptor damage, suggesting potential therapeutic targets.

Ligand Binding Mechanisms in Human Cone Visual Pigments

Vertebrate vision starts with light absorption by visual pigments in rod and cone photoreceptor cells of the retina. Rhodopsin, in rod cells, responds to dim light, whereas three types of cone opsins (red, green, and blue) function under bright light and mediate color vision. Cone opsins regenerate with retinal much faster than rhodopsin, but the molecular mechanism of regeneration is still unclear. Recent advances in the area pinpoint transient intermediate opsin conformations, and a possible secondary retinal-binding site, as determinant factors for regeneration. In this Review, we compile previous and recent findings to discuss possible mechanisms of ligand entry in cone opsins, involving a secondary binding site, which may have relevant functional and evolutionary implications.

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.

Regulation of calcium homeostasis in the outer segments of rod and cone photoreceptors

Calcium plays important roles in the function and survival of rod and cone photoreceptor cells. Rapid regulation of calcium in the outer segments of photoreceptors is required for the modulation of phototransduction that drives the termination of the flash response as well as light adaptation in rods and cones. On a slower time scale, maintaining proper calcium homeostasis is critical for the health and survival of photoreceptors. Decades of work have established that the level of calcium in the outer segments of rods and cones is regulated by a dynamic equilibrium between influx via the transduction cGMP-gated channels and extrusion via rod- and cone-specific Na⁺/Ca²⁺, K⁺ exchangers (NCKXs). It had been widely accepted that the only mechanism for extrusion of calcium from rod outer segments is via the rod-specific NCKX1, while extrusion from cone outer segments is driven exclusively by the cone-specific NCKX2. However, recent evidence from mice lacking NCKX1 and NCKX2 have challenged that notion and have revealed a more complex picture, including a NCKX-independent mechanism in rods and two separate NCKX-dependent mechanisms in cones. This review will focus on recent findings on the molecular mechanisms of extrusion of calcium from the outer segments of rod and cone photoreceptors, and the functional and structural changes in photoreceptors when normal extrusion is disrupted.

Dephosphorylation by protein phosphatase 2A regulates visual pigment regeneration and the dark adaptation of mammalian photoreceptors

Resetting of G-protein-coupled receptors (GPCRs) from their active state back to their biologically inert ground state is an integral part of GPCR signaling. This "on-off" GPCR cycle is regulated by reversible phosphorylation. Retinal rod and cone photoreceptors arguably represent the best-understood example of such GPCR signaling. Their visual pigments (opsins) are activated by light, transduce the signal, and are then inactivated by a GPCR kinase and arrestin. Although pigment inactivation by phosphorylation is well understood, the enzyme(s) responsible for pigment dephosphorylation and the functional significance of this reaction remain unknown. Here, we show that protein phosphatase 2A (PP2A) acts as opsin phosphatase in both rods and cones. Elimination of PP2A substantially slows pigment dephosphorylation, visual chromophore recycling, and ultimately photoreceptor dark adaptation. These findings demonstrate that visual pigment dephosphorylation regulates the dark adaptation of photoreceptors and provide insights into the role of this reaction in GPCR signaling.

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.

Dephosphorylation during bleach and regeneration of visual pigment in carp rod and cone membranes

On absorption of light by vertebrate visual pigment, the chromophore, 11-cis retinal, is isomerized to all-trans retinal to activate the phototransduction cascade, which leads to a hyperpolarizing light response. Activated pigment is inactivated by phosphorylation on the protein moiety, opsin. Isomerized all-trans retinal is ultimately released from opsin, and the pigment is regenerated by binding to 11-cis retinal. In this pigment regeneration cycle, the phosphates incorporated should be removed in order that the pigment regains the capability of activating the phototransduction cascade. However, it is not clear yet how pigment dephosphorylation takes place in the regeneration cycle. First in this study, we tried to estimate the dephosphorylation activity in living carp rods and cones and found that the activity, which is present mainly in the cytoplasm in both rods and cones, is three times higher in cones than in rods. Second, we examined at which stage the dephosphorylation takes place; before or after the release of all-trans retinal, during pigment regeneration, or after pigment regeneration. For this purpose we prepared three types of phosphorylated substrates in purified carp rod and cone membranes: phosphorylated bleaching intermediate, phosphorylated opsin, and phosphorylated and regenerated pigment. We also examined the effect of pigment regeneration on the dephosphorylation. The results showed that the dephosphorylation does not show substrate preference in the regeneration cycle and suggested that the dephosphorylation takes place constantly. The results also suggest that, under bright light, some of the regenerated visual pigment remains phosphorylated to reduce the light sensitivity in cones.

Phosphorylation-independent suppression of light-activated visual pigment by arrestin in carp rods and cones

Visual pigment in photoreceptors is activated by light. Activated visual pigment (R*) is believed to be inactivated by phosphorylation of R* with subsequent binding of arrestin. There are two types of photoreceptors, rods and cones, in the vertebrate retina, and they express different subtypes of arrestin, rod and cone type. To understand the difference in the function between rod- and cone-type arrestin, we first identified the subtype of arrestins expressed in rods and cones in carp retina. We found that two rod-type arrestins, rArr1 and rArr2, are co-expressed in a rod and that a cone-type arrestin, cArr1, is expressed in blue- and UV-sensitive cones; the other cone-type arrestin, cArr2, is expressed in red- and green-sensitive cones. We quantified each arrestin subtype and estimated its concentration in the outer segment of a rod or a cone in the dark; they were ∼0.25 mm (rArr1 plus rArr2) in a rod and 0.6-0.8 mm (cArr1 or cArr2) in a cone. The effect of each arrestin was examined. In contrast to previous studies, both rod and cone arrestins suppressed the activation of transducin in the absence of visual pigment phosphorylation, and all of the arrestins examined (rArr1, rArr2, and cArr2) bound transiently to most probably nonphosphorylated R*. One rod arrestin, rArr2, bound firmly to phosphorylated pigment, and the other two, rArr1 and cArr2, once bound to phosphorylated R* but dissociated from it during incubation. Our results suggested a novel mechanism of arrestin effect on the suppression of the R* activity in both rods and cones.

Regulation of mammalian cone phototransduction by recoverin and rhodopsin kinase

Cone photoreceptors function under daylight conditions and are essential for color perception and vision with high temporal and spatial resolution. A remarkable feature of cones is that, unlike rods, they remain responsive in bright light. In rods, light triggers a decline in intracellular calcium, which exerts a well studied negative feedback on phototransduction that includes calcium-dependent inhibition of rhodopsin kinase (GRK1) by recoverin. Rods and cones share the same isoforms of recoverin and GRK1, and photoactivation also triggers a calcium decline in cones. However, the molecular mechanisms by which calcium exerts negative feedback on cone phototransduction through recoverin and GRK1 are not well understood. Here, we examined this question using mice expressing various levels of GRK1 or lacking recoverin. We show that although GRK1 is required for the timely inactivation of mouse cone photoresponse, gradually increasing its expression progressively delays the cone response recovery. This surprising result is in contrast with the known effect of increasing GRK1 expression in rods. Notably, the kinetics of cone responses converge and become independent of GRK1 levels for flashes activating more than ∼1% of cone pigment. Thus, mouse cone response recovery in bright light is independent of pigment phosphorylation and likely reflects the spontaneous decay of photoactivated visual pigment. We also find that recoverin potentiates the sensitivity of cones in dim light conditions but does not contribute to their capacity to function in bright light.

Different phosphorylation rates among vertebrate cone visual pigments with different spectral sensitivities

Cone photoreceptor subtypes having different spectral sensitivities exhibit different recovery kinetics in their photoresponses in some vertebrates. Phosphorylation by G protein-coupled receptor kinase (GRK) is essential for the rapid inactivation of light-activated visual pigment, which is the rate-limiting step of the cone photoresponse recovery in salamander. In this study we compared the rate of light-dependent phosphorylation by GRK7 of carp green- and blue-sensitive cone visual pigments. Blue pigment was phosphorylated significantly less effectively than green pigment, suggesting that the difference in the pigment phosphorylation rate is responsible for the difference in photoresponse kinetics among cone photoreceptor subtypes.

Protein sorting, targeting and trafficking in photoreceptor cells

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

Origin and control of the dominant time constant of salamander cone photoreceptors

Recovery of the light response in vertebrate photoreceptors requires the shutoff of both active intermediates in the phototransduction cascade: the visual pigment and the transducin–phosphodiesterase complex. Whichever intermediate quenches more slowly will dominate photoresponse recovery. In suction pipette recordings from isolated salamander ultraviolet- and blue-sensitive cones, response recovery was delayed, and the dominant time constant slowed when internal [Ca²⁺] was prevented from changing after a bright flash by exposure to 0Ca²⁺/0Na⁺ solution. Taken together with a similar prior observation in salamander red-sensitive cones, these observations indicate that the dominance of response recovery by a Ca²⁺-sensitive process is a general feature of amphibian cone phototransduction. Moreover, changes in the external pH also influenced the dominant time constant of red-sensitive cones even when changes in internal [Ca²⁺] were prevented. Because the cone photopigment is, uniquely, exposed to the external solution, this may represent a direct effect of protons on the equilibrium between its inactive Meta I and active Meta II forms, consistent with the notion that the process dominating recovery of the bright flash response represents quenching of the active Meta II form of the cone photopigment.

Speed, adaptation, and stability of the response to light in cone photoreceptors: the functional role of Ca-dependent modulation of ligand sensitivity in cGMP-gated ion channels

The response of cone photoreceptors to light is stable and reproducible because of the exceptional regulation of the cascade of enzymatic reactions that link visual pigment (VP) excitation to the gating of cyclic GMP (cGMP)-gated ion channels (cyclic nucleotide–gated [CNG]) in the outer segment plasma membrane. Regulation is achieved in part through negative feedback control of some of these reactions by cytoplasmic free Ca²⁺. As part of the control process, Ca²⁺ regulates the phosphorylation of excited VP, the activity of guanylate cyclase, and the ligand sensitivity of the CNG ion channels. We measured photocurrents elicited by stimuli in the form of flashes, steps, and flashes superimposed on steps in voltage-clamped single bass cones isolated from striped bass retina. We also developed a computational model that comprises all the known molecular events of cone phototransduction, including all Ca-dependent controls. Constrained by available experimental data in bass cones and cone transduction biochemistry, we achieved an excellent match between experimental photocurrents and those simulated by the model. We used the model to explore the physiological role of CNG ion channel modulation. Control of CNG channel activity by both cGMP and Ca²⁺ causes the time course of the light-dependent currents to be faster than if only cGMP controlled their activity. Channel modulation also plays a critical role in the regulation of the light sensitivity and light adaptation of the cone photoresponse. In the absence of ion channel modulation, cone photocurrents would be unstable, oscillating during and at the offset of light stimuli.

Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models

The light responses of rod and cone photoreceptors in the vertebrate retina are quantitatively different, yet extremely stable and reproducible because of the extraordinary regulation of the cascade of enzymatic reactions that link photon absorption and visual pigment excitation to the gating of cGMP-gated ion channels in the outer segment plasma membrane. While the molecular scheme of the phototransduction pathway is essentially the same in rods and cones, the enzymes and protein regulators that constitute the pathway are distinct. These enzymes and regulators can differ in the quantitative features of their functions or in concentration if their functions are similar or both can be true. The molecular identity and distinct function of the molecules of the transduction cascade in rods and cones are summarized. The functional significance of these molecular differences is examined with a mathematical model of the signal-transducing enzymatic cascade. Constrained by available electrophysiological, biochemical and biophysical data, the model simulates photocurrents that match well the electrical photoresponses measured in both rods and cones. Using simulation computed with the mathematical model, the time course of light-dependent changes in enzymatic activities and second messenger concentrations in non-mammalian rods and cones are compared side by side.

Operation profile of zebrafish guanylate cyclase-activating protein 3

The expression pattern and property profile of the neuronal Ca(2+) sensor guanylate cyclase-activating protein 3 (zGCAP3) was studied by immunochemical approaches, biophysical methods and enzymatic assays. Using affinity purified antibodies immunoreactivity towards zGCAP3 was weakly detected in the outer and strongly in the inner segments of cone cells as well as in the outer plexiform layer, to a lesser degree also in the inner plexiform and ganglion cell layer of the zebrafish retina. This cellular distribution was independent of a dark/light cycle. Some neuronal Ca(2+) sensors are acylated (mainly myristoylated) at the amino-terminus. Probing larval and adult stages of the developing zebrafish retina indicated that zGCAP3 was first expressed in a non-myristoylated form, but was finally present in the adult retina as a myristoylated protein. While zGCAP3 did not undergo a classical Ca(2+) -myristoyl switch as investigated by surface plasmon resonance spectroscopy, myristoylation had two main other consequences: it enhanced the Ca(2+) -sensitivity of the Ca(2+) -induced conformational change and it stabilized the protein conformation. Differences between myristoylated and non-myristoylated zGCAP3 were also observed in modulating the kinetic and catalytic parameters of the GCAP-target, a membrane bound guanylate cyclase. Thus, the stabilizing effect of the myristoyl group is apparently less important in the larval than in the adult fish.

Comparative analysis of cone and rod transducins using chimeric Gα subunits

The molecular nature of transducin-α subunits (Gα(t)) may contribute to the distinct physiology of cone and rod photoreceptors. Biochemical properties of mammalian cone Gα(t2) subunits and their differences with rod Gα(t1) are largely unknown. Here, we examined properties of chimeric Gα(t2) in comparison with its rod counterpart. The key biochemical difference between the rod- and cone-like Gα(t) was ~10-fold higher intrinsic nucleotide exchange on the chimeric Gα(t2). Presented mutational analysis suggests that weaker interdomain interactions between the GTPase (Ras-like) domain and the helical domain in Gα(t2) are in part responsible for its increased spontaneous nucleotide exchange. However, the rates of R*-dependent nucleotide exchange of chimeric Gα(t2) and Gα(t1) were equivalent. Furthermore, chimeric Gα(t2) and Gα(t1) exhibited similar rates of intrinsic GTPase activity as well as similar acceleration of GTP hydrolysis by the RGS domain of RGS9. Our results suggest that the activation and inactivation properties of cone and rod Gα(t) subunits in an in vitro reconstituted system are comparable.

Photoreceptor signaling: supporting vision across a wide range of light intensities

For decades, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and biochemistry and for understanding many facets of neuronal cell biology. In recent years, new knowledge of the kinetics of signaling and the large-scale movements of proteins underlying signaling has led to a deeper appreciation of the photoreceptor's unique challenge in mediating the first steps in vision over a wide range of light intensities.

Amino acid sequences of two immune-dominant epitopes of recoverin are involved in Ca2+/recoverin-dependent inhibition of phosphorylation of rhodopsin

Antibodies AB(60-72) and AB(80-92) against two immune-dominant epitopes of photoreceptor Ca(2+)-binding protein recoverin, 60-DPKAYAQHVFRSF-72 and 80-LDFKEYVIALHMT-92, which can be exposed in a Ca(2+)-dependent manner, were obtained. The presence of AB(60-72) or AB(80-92) results in a slight increase in Ca(2+)-affinity of recoverin and does not affect significantly a Ca(2+)-myristoyl switch mechanism of the protein. However in the presence of AB(60-72) or AB(80-92) recoverin loses its ability to interact with rhodopsin kinase and consequently to perform a function of Ca(2+)-sensitive inhibitor of rhodopsin phosphorylation in photoreceptor cells.

Cone arrestin confers cone vision of high temporal resolution in zebrafish larvae

Vision of high temporal resolution depends on careful regulation of photoresponse kinetics, beginning with the lifetime of activated photopigment. The activity of rhodopsin is quenched by high-affinity binding of arrestin to photoexcited phosphorylated photopigment, which effectively terminates the visual transduction cascade. This regulation mechanism is well established for rod photoreceptors, yet its role for cone vision is still controversial. In this study we therefore analyzed arrestin function in the cone-dominated vision of larval zebrafish. For both rod (arrS ) and cone (arr3 ) arrestin we isolated two paralogs, each expressed in the respective subset of photoreceptors. Labeling with paralog-specific antibodies revealed subfunctionalized expression of Arr3a in M- and L-cones, and Arr3b in S- and UV-cones. The inactivation of arr3a by morpholino knockdown technology resulted in a severe delay in photoresponse recovery which, under bright light conditions, was rate-limiting. Comparison to opsin phosphorylation-deficient animals confirmed the role of cone arrestin in late cone response recovery. Arr3a activity partially overlapped with the function of the cone-specific kinase Grk7a involved in initial response recovery. Behavioral measurements further revealed Arr3a deficiency to be sufficient to reduce temporal contrast sensitivity, providing evidence for the importance of arrestin in cone vision of high temporal resolution.

Mechanistic basis for the failure of cone transducin to translocate: why cones are never blinded by light

The remarkable ability of our vision to function under ever-changing conditions of ambient illumination is mediated by multiple molecular mechanisms regulating the light sensitivity of rods and cones. One such mechanism involves massive translocation of signaling proteins, including the G-protein transducin, into and out of the light-sensitive photoreceptor outer segment compartment. Transducin translocation extends the operating range of rods, but in cones transducin never translocates, which is puzzling because cones typically function in much brighter light than rods. Using genetically manipulated mice in which the rates of transducin activation and inactivation were altered, we demonstrate that, like in rods, transducin translocation in cones can be triggered when transducin activation exceeds a critical level, essentially saturating the photoresponse. However, this level is never achieved in wild-type cones: their superior ability to tightly control the rates of transducin activation and inactivation, responsible for avoiding saturation by light, also accounts for the prevention of transducin translocation at any light intensity.

Photopigment quenching is Ca2+ dependent and controls response duration in salamander L-cone photoreceptors

The time scale of the photoresponse in photoreceptor cells is set by the slowest of the steps that quench the light-induced activity of the phototransduction cascade. In vertebrate photoreceptor cells, this rate-limiting reaction is thought to be either shutoff of catalytic activity in the photopigment or shutoff of the pigment's effector, the transducin-GTP–phosphodiesterase complex. In suction pipette recordings from isolated salamander L-cones, we found that preventing changes in internal [Ca²⁺] delayed the recovery of the light response and prolonged the dominant time constant for recovery. Evidence that the Ca²⁺-sensitive step involved the pigment itself was provided by the observation that removal of Cl⁻ from the pigment's anion-binding site accelerated the dominant time constant for response recovery. Collectively, these observations indicate that in L-cones, unlike amphibian rods where the dominant time constant is insensitive to [Ca²⁺], pigment quenching rate limits recovery and provides an additional mechanism for modulating the cone response during light adaptation.

Retrograde intraflagellar transport by cytoplasmic dynein-2 is required for outer segment extension in vertebrate photoreceptors but not arrestin translocation

PURPOSE

Anterograde intraflagellar transport (IFT) is essential for photoreceptor outer segment formation and maintenance, as well as for opsin trafficking. However, the role of retrograde IFT in vertebrate photoreceptors remains unclear. The purpose of this study was to evaluate zebrafish photoreceptors lacking the retrograde IFT motor, cytoplasmic dynein-2.

METHODS

Morpholino oligonucleotides against the heavy chain (dync2-h1), light intermediate chain (dync2-li1), and intermediate chain (dync2-i1) subunits of cytoplasmic dynein-2 were injected into zebrafish embryos. Retinas and ciliated cells of these zebrafish morphants were analyzed by immunohistochemistry and transmission electron microscopy. Whole-field electroretinograms (ERGs) were performed on dynein morphants at 5 to 6 days after fertilization (dpf).

RESULTS

Zebrafish lacking cytoplasmic dynein-2 function exhibited small eyes, kidney cysts, and short photoreceptor outer segments, some of which were disorganized with accumulated vesicles. Morphant photoreceptor connecting cilia were swollen, but neither opsin nor arrestin was mislocalized, although IFT88 accumulated in the distal region of the connecting cilium. Nasal cilia were shortened and displayed cytoplasmic swelling along the axoneme. Loss of cytoplasmic dynein-2 function resulted in a significant reduction in the amplitude of ERG a-, b-, and d-waves but no change in threshold response.

CONCLUSIONS

Retrograde IFT is essential for outer segment extension and IFT protein recycling in vertebrate photoreceptors. The results show, for the first time, that the dync2-i1 subunit of cytoplasmic dynein-2 is necessary for retrograde IFT. In addition, arrestin translocation does not require retrograde IFT. Finally, the ERG results indicate that loss of cytoplasmic dynein-2 reduces the photoreceptor light response.

Phosphorylation of GRK7 by PKA in cone photoreceptor cells is regulated by light

The retina-specific G protein-coupled receptor kinases, GRK1 and GRK7, have been implicated in the shutoff of the photoresponse and adaptation to changing light conditions via rod and cone opsin phosphorylation. Recently, we have defined sites of phosphorylation by cAMP-dependent protein kinase (PKA) in the amino termini of both GRK1 and GRK7 in vitro. To determine the conditions under which GRK7 is phosphorylated in vivo, we have generated an antibody that recognizes GRK7 phosphorylated on Ser36, the PKA phosphorylation site. Using this phospho-specific antibody, we have shown that GRK7 is phosphorylated in vivo and is located in the cone inner and outer segments of mammalian, amphibian and fish retinas. Using Xenopus laevis as a model, GRK7 is phosphorylated under dark-adapted conditions, but becomes dephosphorylated when the animals are exposed to light. The conservation of phosphorylation at Ser36 in GRK7 in these different species (which span a 400 million-year evolutionary period), and its light-dependent regulation, indicates that phosphorylation plays an important role in the function of GRK7. Our work demonstrates for the first time that cAMP can regulate proteins involved in the photoresponse in cones and introduces a novel mode of regulation for the retinal GRKs by PKA.

Unique transducins expressed in long and short photoreceptors of lamprey Petromyzon marinus

Lampreys represent the most primitive vertebrate class of jawless fish and serve as an evolutionary model of the vertebrate visual system. Transducin-alpha (G alpha(t)) subunits were investigated in lamprey Petromyzon marinus in order to understand the molecular origins of rod and cone photoreceptor G proteins. Two G alpha(t) subunits, G alpha(tL) and G alpha(tS), were identified in the P. marinus retina. G alpha(tL) is equally distant from cone and rod G proteins and is expressed in the lamprey's long photoreceptors. The short photoreceptor G alpha(tS) is a rod-like transducin-alpha that retains several unique features of cone transducins. Thus, the duplication of the ancestral transducin gene giving rise to rod transducins has already occurred in the last common ancestor of the jawed and jawless vertebrates.

Mechanism of light-induced translocation of arrestin and transducin in photoreceptors: interaction-restricted diffusion

Many signaling proteins change their location within cells in response to external stimuli. In photoreceptors, this phenomenon is remarkably robust. The G protein of rod photoreceptors and rod transducin concentrates in the outer segments (OS) of these neurons in darkness. Within approximately 30 minutes after illumination, rod transducin redistributes throughout all of the outer and inner compartments of the cell. Visual arrestin concurrently relocalises from the inner compartments to become sequestered primarily within the OS. In the past several years, the question of whether these proteins are actively moved by molecular motors or whether they are redistributed by simple diffusion has been extensively debated. This review focuses on the most essential works in the area and concludes that the basic principle driving this protein movement is diffusion. The directionality and light dependence of this movement is achieved by the interactions of arrestin and transducin with their spatially restricted binding partners.

Signal transducing membrane complexes of photoreceptor outer segments

Signal transduction in outer segments of vertebrate photoreceptors is mediated by a series of reactions among multiple polypeptides that form protein-protein complexes within or on the surface of the disk and plasma membranes. The individual components in the activation reactions include the photon receptor rhodopsin and the products of its absorption of light, the three subunits of the G protein, transducin, the four subunits of the cGMP phosphodiesterase, PDE6 and the four subunits of the cGMP-gated cation channel. Recovery involves membrane complexes with additional polypeptides including the Na(+)/Ca(2+), K(+) exchanger, NCKX2, rhodopsin kinases RK1 and RK7, arrestin, guanylate cyclases, guanylate cyclase activating proteins, GCAP1 and GCAP2, and the GTPase accelerating complex of RGS9-1, G(beta5L), and membrane anchor R9AP. Modes of membrane binding by these polypeptides include transmembrane helices, fatty acyl or isoprenyl modifications, polar interactions with lipid head groups, non-polar interactions of hydrophobic side chains with lipid hydrocarbon phase, and both polar and non-polar protein-protein interactions. In the course of signal transduction, complexes among these polypeptides form and dissociate, and undergo structural rearrangements that are coupled to their interactions with and catalysis of reactions by small molecules and ions, including guanine nucleotides, ATP, Ca(2+), Mg(2+), and lipids. The substantial progress that has been made in understanding the composition and function of these complexes is reviewed, along with the more preliminary state of our understanding of the structures of these complexes and the challenges and opportunities that present themselves for deepening our understanding of these complexes, and how they work together to convert a light signal into an electrical signal.

Light-dependent compartmentalization of transducin in rod photoreceptors

Three major visual signaling proteins, transducin, arrestin, and recoverin undergo bidirectional translocations between the outer segment and inner compartments of rod photoreceptors in a light-dependent manner. The light-dependent translocation of proteins is believed to contribute to adaptation and neuroprotection of photoreceptor cells. The potential physiological significance and mechanisms of light-controlled protein translocations are at the center of current discussion. In this paper, I outline the latest advances in understanding the mechanisms of bidirectional translocation of transducin and determinants of its steady-state distribution in dark- and light-adapted photoreceptor cells.

Quantitation of the effect of hydroxylamine on rhodopsin palmitylation

Rhodopsin (the photosensitive rod visual pigment) has been a model for photobiologic studies of the opsins as well as a structural model for G-protein-coupled receptors. The two palmitate groups attached to cysteines 322 and 323 are thought to serve as membrane anchors for the rhodopsin C-terminus, but the absence of the palmitates does not alter membrane localization. However, removal of the palmitates affects rhodopsin function. Therefore, it is important to quantitate the stability of rhodopsin palmitates to hydroxylamine, which is a widely utilized reagent in biochemical preparations of the apoprotein. We have developed a mass spectrometric method to quantitate the resulting opsin palmitylation. Our data show that both of the bovine rhodopsin palmitates are labile to hydroxylamine, with significant depalmitylation occurring at concentrations of >or=100 mM, with an EC(50) of 220 mM L(-1). The palmitate at position 322 is the more stable to hydroxylamine. Samples prepared in the presence of >50 mM should therefore be considered to be at least partially depalmitylated and the results interpreted accordingly.

Rpe65-/- and Lrat-/- mice: comparable models of leber congenital amaurosis

PURPOSE

The Rpe65-/- mouse, used as a model for Leber congenital amaurosis, has slow rod degeneration and rapid cone loss, presumably because of the mistrafficking of cone opsins. This animal does not generate 11-cis retinal, and both cone loss and rod response are restored by 11-cis retinal administration. Similarly, the Lrat-/- mouse does not produce 11-cis retinal. The authors sought to determine whether the same effects on rod and cone opsins in the Rpe65-/- mouse are also present in the Lrat-/- mouse, thereby establishing that these changes can be attributed to the lack of 11-cis retinal rather than to some unknown function of RPE65.

METHODS

Rod and cone opsins were localized by immunohistochemical methods. Functional opsin levels were determined by regeneration with 11-cis retinal. Isorhodopsin levels were determined from pigment extraction. Opsin phosphorylation was determined by mass spectrometry.

RESULTS

Rods in both models degenerated slowly. Regenerable rod opsin levels were similar over the 6-month time course investigated, rod opsin was phosphorylated at a low level (approximately 10%), and minimal 9-cis retinal was generated by a nonphotic process, giving a trace light response. In both models, S-opsin and M/L-opsin failed to traffic to the cone outer segments appropriately, and rapid cone degeneration occurred. Cone opsin mistrafficking in both models was arrested on 11-cis retinal administration.

CONCLUSIONS

These data show that the Lrat-/- and Rpe65-/- mice are comparable models for studies of Leber congenital amaurosis and that the destructive cone opsin mistrafficking is caused by the lack of 11-cis retinal.

Regulation of arrestin binding by rhodopsin phosphorylation level

Arrestins ensure the timely termination of receptor signaling. The role of rhodopsin phosphorylation in visual arrestin binding was established more than 20 years ago, but the effects of the number of receptor-attached phosphates on this interaction remain controversial. Here we use purified rhodopsin fractions with carefully quantified content of individual phosphorylated rhodopsin species to elucidate the impact of phosphorylation level on arrestin interaction with three biologically relevant functional forms of rhodopsin: light-activated and dark phosphorhodopsin and phospho-opsin. We found that a single receptor-attached phosphate does not facilitate arrestin binding, two are necessary to induce high affinity interaction, and three phosphates fully activate arrestin. Higher phosphorylation levels do not increase the stability of arrestin complex with light-activated rhodopsin but enhance its binding to the dark phosphorhodopsin and phospho-opsin. The complex of arrestin with hyperphosphorylated light-activated rhodopsin is less sensitive to high salt and appears to release retinal faster. These data suggest that arrestin likely quenches rhodopsin signaling after the third phosphate is added by rhodopsin kinase. The complex of arrestin with heavily phosphorylated rhodopsin, which appears to form in certain disease states, has distinct characteristics that may contribute to the phenotype of these visual disorders.

Modulation of locomotor activity in larval zebrafish during light adaptation

The neural basis of behavioral choice in vertebrates remains largely unknown. Zebrafish larvae have a defined locomotor repertoire as well as a simple nervous system and are therefore an attractive vertebrate system in which to study this process. Here we describe a high-throughput system for quantifying the kinematics of motor events in zebrafish larvae in order to measure the initiation frequency of different maneuvers. We use this system to analyze responses to photic stimuli and find that larvae respond to changes in illumination with both acute responses and extended behavioral programs. Reductions in illumination elicit large angle turns, distinct from startle responses, which orient larvae toward the source of light. In continuing darkness, larvae are transiently hyperactive before adopting a quiescent state. Indeed, locomotor activity is controlled by the state of light or dark adaptation similar to masking phenomena in higher vertebrates where light directly regulates motor activity. We propose that regulation of motor activity by photic stimuli in zebrafish larvae serves a behavioral goal of maximizing exposure to well lit environments optimal for feeding.

Subunit dissociation and diffusion determine the subcellular localization of rod and cone transducins

Activation of rod photoreceptors by light induces a massive redistribution of the heterotrimeric G-protein transducin. In darkness, transducin is sequestered within the membrane-enriched outer segments of the rod cell. In light, it disperses throughout the entire neuron. We show here that redistribution of rod transducin by light requires activation, but it does not require ATP. This observation rules out participation of molecular motors in the redistribution process. In contrast to the light-stimulated redistribution of rod transducin in rods, cone transducin in cones does not redistribute during activation. Remarkably, when cone transducin is expressed in rods, it does undergo light-stimulated redistribution. We show here that the difference in subcellular localization of activated rod and cone G-proteins correlates with their affinity for membranes. Activated rod transducin releases from membranes, whereas activated cone transducin remains bound to membranes. A synthetic peptide that dissociates G-protein complexes independently of activation facilitates dispersion of both rod and cone transducins within the cells. This peptide also facilitates detachment of both G-proteins from the membranes. Together, these results show that it is the dissociation state of transducin that determines its localization in photoreceptors. When rod transducin is stimulated, its subunits dissociate, leave outer segment membranes, and equilibrate throughout the cell. Cone transducin subunits do not dissociate during activation and remain sequestered within the outer segment. These findings indicate that the subunits of some heterotrimeric G-proteins remain associated during activation in their native environments.

Transducin translocation in rods is triggered by saturation of the GTPase-activating complex

Light causes massive translocation of G-protein transducin from the light-sensitive outer segment compartment of the rod photoreceptor cell. Remarkably, significant translocation is observed only when the light intensity exceeds a critical threshold level. We addressed the nature of this threshold using a series of mutant mice and found that the threshold can be shifted to either a lower or higher light intensity, dependent on whether the ability of the GTPase-activating complex to inactivate GTP-bound transducin is decreased or increased. We also demonstrated that the threshold is not dependent on cellular signaling downstream from transducin. Finally, we showed that the extent of transducin alpha subunit translocation is affected by the hydrophobicity of its acyl modification. This implies that interactions with membranes impose a limitation on transducin translocation. Our data suggest that transducin translocation is triggered when the cell exhausts its capacity to activate transducin GTPase, and a portion of transducin remains active for a sufficient time to dissociate from membranes and to escape from the outer segment. Overall, the threshold marks the switch of the rod from the highly light-sensitive mode of operation required under limited lighting conditions to the less-sensitive energy-saving mode beneficial in bright light, when vision is dominated by cones.

Phototransduction in mouse rods and cones

Phototransduction is the process by which light triggers an electrical signal in a photoreceptor cell. Image-forming vision in vertebrates is mediated by two types of photoreceptors: the rods and the cones. In this review, we provide a summary of the success in which the mouse has served as a vertebrate model for studying rod phototransduction, with respect to both the activation and termination steps. Cones are still not as well-understood as rods partly because it is difficult to work with mouse cones due to their scarcity and fragility. The situation may change, however.

Turning cones off: the role of the 9-methyl group of retinal in red cones

Our ability to see in bright light depends critically on the rapid rate at which cone photoreceptors detect and adapt to changes in illumination. This is achieved, in part, by their rapid response termination. In this study, we investigate the hypothesis that this rapid termination of the response in red cones is dependent on interactions between the 9-methyl group of retinal and red cone opsin, which are required for timely metarhodopsin (Meta) II decay. We used single-cell electrical recordings of flash responses to assess the kinetics of response termination and to calculate guanylyl cyclase (GC) rates in salamander red cones containing native visual pigment as well as visual pigment regenerated with 11-cis 9-demethyl retinal, an analogue of retinal in which the 9-methyl group is missing. After exposure to bright light that photoactivated more than ∼0.2% of the pigment, red cones containing the analogue pigment had a slower recovery of both flash response amplitudes and GC rates (up to 10 times slower at high bleaches) than red cones containing 11-cis retinal. This finding is consistent with previously published biochemical data demonstrating that red cone opsin regenerated in vitro with 11-cis 9-demethyl retinal exhibited prolonged activation as a result of slowed Meta II decay. Our results suggest that two different mechanisms regulate the recovery of responsiveness in red cones after exposure to light. We propose a model in which the response recovery in red cones can be regulated (particularly at high light intensities) by the Meta II decay rate if that rate has been inhibited. In red cones, the interaction of the 9-methyl group of retinal with opsin promotes efficient Meta II decay and, thus, the rapid rate of recovery.

Light-driven translocation of signaling proteins in vertebrate photoreceptors

The dynamic localization of proteins within cells is often determined by environmental stimuli. In retinal photoreceptors, light exposure results in the massive translocation of three key signal transduction proteins, transducin, arrestin and recoverin, into and out of the outer segment compartment where phototransduction takes place. This phenomenon has rapidly taken the center stage of photoreceptor cell biology, thanks to the introduction of new quantitative and transgenic approaches. Here, we discuss evidence that intracellular protein translocation contributes to adaptation of photoreceptors to diurnal changes in ambient light intensity and summarize the current debate on whether it is driven by diffusion or molecular motors.

Origin of electroretinogram amplitude growth during light adaptation in pigmented rats

We assessed the growth of the rat photopic electroretinogram (ERG) during light adaptation and the mechanisms underlying this process. Full field ERG responses were recorded from anesthetized adult Brown-Norway rats at each minute for 20 min of light adaptation (backgrounds: 1.8, 2.1, 2.4 log scotopic cd m(-2)). The rat photopic b-wave amplitude increased with duration of light adaptation and its width at 33% maximal amplitude narrowed (by approximately 40 ms). These effects peaked 12-15 min after background onset. The narrowing of the b-wave reflected steepening of the b-wave recovery phase, with little change in the rising phase. OP amplitudes grew in proportion to the b-wave. Inhibition of inner retinal responses using TTX resulted in a greater relative growth of b-wave and OP amplitude compared with fellow control eyes, and delayed the change in recovery phase by approximately 5 min. Inhibition of all ionotropic glutamate receptors with CNQX/D-AP7 delayed both rising and recovery phases equally (approximately 12 ms) without altering b-wave width or the time course of adaptation changes. These outcomes suggest that inner retinal light responses are not directly responsible for b-wave amplitude growth, but may contribute to the change in its recovery phase during adaptation. A TTX-sensitive mechanism may help to hasten this process. The cone a-wave was isolated using PDA/L-AP4 or CNQX/L-AP4. A-wave amplitude (35 ms after stimulus onset) also increased with time during light adaptation and reached a maximum (130 +/- 29% above baseline) 12-15 min after background onset. B-wave amplitude growth in fellow control eyes closely followed the course and relative magnitude of cone a-wave amplitude growth. Hence, the increase of the cone response during light adaptation is sufficient to explain b-wave amplitude growth.

GRK1 and GRK7: unique cellular distribution and widely different activities of opsin phosphorylation in the zebrafish rods and cones

Retinal cone cells exhibit distinctive photoresponse with a more restrained sensitivity to light and a more rapid shutoff kinetics than those of rods. To understand the molecular basis for these characteristics of cone responses, we focused on the opsin deactivation process initiated by G protein-coupled receptor kinase (GRK) 1 and GRK7 in the zebrafish, an animal model suitable for studies on retinal physiology and biochemistry. Screening of the ocular cDNAs identified two homologs for each of GRK1 (1A and 1B) and GRK7 (7-1 and 7-2), and they were classified into three GRK subfamilies, 1 A, 1B and 7 by phylogenetic analysis. In situ hybridization and immunohistochemical studies localized both GRK1B and GRK7-1 in the cone outer segments and GRK1A in the rod outer segments. The opsin/GRKs molar ratio was estimated to be 569 in the rod and 153 in the cone. The recombinant GRKs phosphorylated light-activated rhodopsin, and the Vmax value of the major cone subtype, GRK7-1, was 32-fold higher than that of the rod kinase, GRK1A. The reinforced activity of the cone kinase should provide a strengthened shutoff mechanism of the light-signaling in the cone and contribute to the characteristics of the cone responses by reducing signal amplification efficiency.

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.

Crystal structure of cone arrestin at 2.3A: evolution of receptor specificity

Arrestins play a fundamental role in the regulation and signal transduction of G protein-coupled receptors. Here we describe the crystal structure of cone arrestin at 2.3A resolution. The overall structure of cone visual arrestin is similar to the crystal structures of rod visual and the non-visual arrestin-2, consisting of two domains, each containing ten beta-sheets. However, at the tertiary structure level, there are two major differences, in particular on the concave surfaces of the two domains implicated in receptor binding and in the loop between beta-strands I and II. Functional analysis shows that cone arrestin, in sharp contrast to its rod counterpart, bound cone pigments and non-visual receptors. Conversely, non-visual arrestin-2 bound cone pigments, suggesting that it may also regulate phototransduction and/or photopigment trafficking in cone photoreceptors. These findings indicate that cone arrestin displays structural and functional features intermediate between the specialized rod arrestin and the non-visual arrestins, which have broad receptor specificity. A unique functional feature of cone arrestin was the low affinity for its cognate receptor, resulting in an unusually rapid dissociation of the complex. Transient arrestin binding to the photopigment in cones may be responsible for the extremely rapid regeneration and reuse of the photopigment that is essential for cone function at high levels of illumination.

Farnesylation of retinal transducin underlies its translocation during light adaptation

G proteins are posttranslationally modified by isoprenylation: either farnesylation or geranylgeranylation. The gamma subunit of retinal transducin (Talpha/Tbetagamma) is selectively farnesylated, and the farnesylation is required for light signaling mediated by transducin in rod cells. However, whether and how this selective isoprenylation regulates cellular functions remain poorly understood. Here we report that knockin mice expressing geranylgeranylated Tgamma showed normal rod responses to dim flashes under dark-adapted conditions but exhibited impaired properties in light adaptation. Of note, geranylgeranylation of Tgamma suppressed light-induced transition of Tbetagamma from membrane to cytosol, and also attenuated its light-dependent translocation from the outer segment to the inner region, an event contributing to retinal light adaptation. These results indicate that, while the farnesylation of transducin is interchangeable with the geranylgeranylation in terms of the light signaling, the selective farnesylation is important for visual sensitivity regulation by providing sufficient but not excessive membrane anchoring of Tbetagamma.

Knockdown of cone-specific kinase GRK7 in larval zebrafish leads to impaired cone response recovery and delayed dark adaptation

Phosphorylation of rhodopsin by rhodopsin kinase GRK1 is an important desensitization mechanism in scotopic vision. For cone vision GRK1 is not essential. However, cone opsin is phosphorylated following light stimulation. In cone-dominant animals as well as in humans, but not in rodents, GRK7, a cone-specific homolog of GRK1, has been identified in cone outer segments. To investigate the function of GRK7 in vivo, we cloned two orthologs of grk7 in zebrafish and knocked down gene expression of grk7a in zebrafish larvae by morpholino antisense nucleotides. Photoresponse recovery in Grk7a-deficient larvae was delayed in electroretinographic measurements, and temporal contrast sensitivity was reduced, particularly under bright-light conditions. These results show that function of a cone-specific kinase is essential for cone vision in the zebrafish retina and argue that pigment bleaching and spontaneous decay alone are not sufficient for light adaptation and rapid cone response inactivation.