Response properties of cones from the retina of the tiger salamander

The genomic evolution of visual opsin genes in amphibians

Among tetrapod (terrestrial) vertebrates, amphibians remain more closely tied to an amphibious lifestyle than amniotes, and their visual opsin genes may be adapted to this lifestyle. Previous studies have discussed physiological, morphological, and molecular changes in the evolution of amphibian vision. We predicted the locations of the visual opsin genes, their neighboring genes, and the tuning sites of the visual opsins, in 39 amphibian genomes. We found that all of the examined genomes lacked the Rh2 gene. The caecilian genomes have further lost the SWS1 and SWS2 genes; only the Rh1 and LWS genes were retained. The loss of the SWS1 and SWS2 genes in caecilians may be correlated with their cryptic lifestyles. The opsin gene syntenies were predicted to be highly similar to those of other bony vertebrates. Moreover, dual syntenies were identified in allotetraploid Xenopus laevis and X. borealis. Tuning site analysis showed that only some Caudata species might have UV vision. In addition, the S164A that occurred several times in LWS evolution might either functionally compensate for the Rh2 gene loss or fine-tuning visual adaptation. Our study provides the first genomic evidence for a caecilian LWS gene and a genomic viewpoint of visual opsin genes by reviewing the gains and losses of visual opsin genes, the rearrangement of syntenies, and the alteration of spectral tuning in the course of amphibians' evolution.

Distribution of intracellular Ca2+-ATPases in the mouse retina and their involvement in light-induced cone degeneration

Calcium signalling is involved in many processes in mammalian retina, from development to mature functions and neurodegeneration. Although proteins involved in Ca2+ entry in retinal cells have been well studied, less is known about Ca2+-clearance. Among the Ca2+ pumps, plasma membrane Ca2+-ATPases (PMCAs) have been identified as key proteins extruding Ca2+ across the plasma membrane with specific distribution in developing and adult retina. However, the two main isoforms of intracellular Ca2+-ATPases in the central nervous system, the sarco(endo)plasmic reticulum (ER) Ca2+-ATPase 2b (SERCA2b) and the secretory pathway Ca2+-ATPase 1 (SPCA1), which remove cytosolic Ca2+ into intracellular stores, have been less or not at all analysed, respectively. In this study, we described for the first time the SPCA1 localisation in adult mouse retina and we report differential distributions of SERCA2b and SPCA1 transporters within various classes of retinal neurons and distinct subcellular localisations. In addition, we studied the expression and localisation of both Ca2+ pumps in 661W cells, a cone photoreceptor-derived cell line. Since continuous exposure to high light intensity induces photodegeneration, we analysed the effect of LED light exposure on these cells and SERCA2b and SPCA1 distribution. We found that continuous mild LED-light exposure compromised cell survival and produced stress in the ER and Golgi, the Ca2+ stores where the two pumps are localised. These effects were reversed after halting light exposure and washing. This study demonstrates that Ca2+ signalling may be involved in light-induced photoreceptor cell damage and points to previously unrecognised functions of intracellular Ca2+-ATPases in retina physiology.

Standardization and Validation of the Flash Visual Evoked Potential-P2 Conversion Scores in the Diagnosis of Amnestic Mild Cognitive Impairment and Alzheimer's Dementia

Amnestic mild cognitive impairment (aMCI), which is characterized by normal daily activity, but a significant decline in episodic memory, is now widely accepted as a risk factor for the development of Alzheimer's dementia (AD). Research suggests that many of the same neuropathological changes associated with AD also occur in patients diagnosed with aMCI. A recent review of the literature revealed that the latency of the flash visual-evoked potential-P2 (FVEP-P2) may possess pathognomonic information that may assist in the early detection of aMCI. While standards exist for the recording of FVEP-P2, individual clinics often use recording parameters that may differ, resulting in latencies that may not generalize beyond the clinic that produced them. The present article illustrates the process by which the FVEP-P2 latency can be standardized across clinics using FVEP-P2 Conversion Scores. We then demonstrate the diagnostic accuracy of the newly developed scores. Method: In the present investigation, we used the previously unpublished data containing the FVEP-P2 latencies of 45 AD and 60 controls. Result: We were able to demonstrate the process by which individual clinics may first standardize FVEP-P2 latencies and then examine patient performance using FVEP-P2 Conversion Scores, providing clinicians with a richer context from which to examine the patient performance. Conclusion: Consistent with the findings of previous research, the findings of the present investigation support the use of the FVEP-P2 Conversion Scores in the diagnosis of AD. Future directions, including the modification of recording parameters associated with the FVEP-P2, are also discussed.

Mathematical analysis of phototransduction reaction parameters in rods and cones

Retinal photoreceptor cells, rods and cones, convert photons of light into chemical and electrical signals as the first step of the visual transduction cascade. Although the chemical processes in the phototransduction system are very similar to each other in these photoreceptors, the light sensitivity and time resolution of the photoresponse in rods are functionally different than those in the photoresponses of cones. To systematically investigate how photoresponses are divergently regulated in rods and cones, we have developed a detailed mathematical model on the basis of the Hamer model. The current model successfully reconstructed light intensity-, ATP- and GTP-dependent changes in concentrations of phosphorylated visual pigments (VPs), activated transducins (Tr*s) and phosphodiesterases (PDEs) in rods and cones. In comparison to rods, the lower light sensitivity of cones was attributed not only to the lower affinity of activated VPs for Trs but also to the faster desensitization of the VPs. The assumption of an intermediate inactive state, MIIⁱ, in the thermal decay of activated VPs was essential for inducing faster inactivation of VPs in rods, and possibly also in 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.

Structural basis of the partially open central gate in the human CNGA1/CNGB1 channel explained by additional density for calmodulin in cryo-EM map

The recently reported structure of the human CNGA1/CNGB1 CNG channel in the open state (Xue et al., 2021a) shows that one CNGA1 and one CNGB1 subunit do not open the central hydrophobic gate completely upon cGMP binding. This is different from what has been reported for CNGA homomeric channels (Xue et al., 2021b; Zheng et al., 2020). In seeking to understand how this difference is due to the presence of the CNGB1 subunit, we find that the deposited density map (Xue et al., 2021a) (EMDB 24465) contains an additional density not reported in the images of the original publication. This additional density fits well the structure of calmodulin (CaM), and it unambiguously connects the newly identified D-helix of CNGB1 to one of the CNGA1 helices (A1_R) participating in the coiled-coil region. Interestingly, the CNGA1 subunit that engages in the interaction with this additional density is the one that, together with CNGB1, does not open completely the central gate. The sequence of the D-helix of CNGB1 contains a known CaM-binding site of exquisitely high affinity - named CaM2 (Weitz et al., 1998) -, and thus the presence of CaM in that region is not surprising. The mechanism through which CaM reduces currents across the membrane by acting on the native channel (Bauer, 1996; Hsu and Molday, 1993; Weitz et al., 1998) remains unclear. We suggest that the presence of CaM may explain the partially open central gate reported by Xue et al. (2021a). The structure of the open and closed states of the CNGA1/CNGB1 channel may be different with and without CaM present.

Light responses of mammalian cones

Cone photoreceptors provide the foundation of most of human visual experience, but because they are smaller and less numerous than rods in most mammalian retinas, much less is known about their physiology. We describe new techniques and approaches which are helping to provide a better understanding of cone function. We focus on several outstanding issues, including the identification of the features of the phototransduction cascade that are responsible for the more rapid kinetics and decreased sensitivity of the cone response, the roles of inner-segment voltage-gated and Ca²⁺-activated channels, the means by which cones remain responsive even in the brightest illumination, mechanisms of cone visual pigment regeneration in constant light, and energy consumption of cones in comparison to that of rods.

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.

A kinetic analysis of mouse rod and cone photoreceptor responses

KEY POINTS

Most vertebrate eyes have rods for dim-light vision and cones for brighter light and higher temporal sensitivity. Rods evolved from cone-like precursors through expression of different transduction genes or the same genes at different expression levels, but we do not know which molecular differences were most important. We approached this problem by analysing rod and cone responses with the same model but with different values for model parameters. We showed that, in addition to outer-segment volume, the most important differences between rods and cones are: (1) decreased transduction gain, reflecting smaller amplification in the G-protein cascade; (2) a faster rate of turnover of the second messenger cGMP in darkness; and (3) an accelerated rate of decay of the effector enzyme phosphodiesterase and perhaps also of activated visual pigment. We believe our analysis has identified the principal alterations during evolution responsible for the duplex retina.

ABSTRACT

Most vertebrates have rod and cone photoreceptors, which differ in their sensitivity and response kinetics. We know that rods evolved from cone-like precursors through the expression of different transduction genes or the same genes at different levels, but we do not know which molecular differences were most important. We have approached this problem in mouse retina by analysing the kinetic differences between rod flash responses and recent voltage-clamp recordings of cone flash responses, using a model incorporating the principal features of photoreceptor transduction. We apply a novel method of analysis using the log-transform of the current, and we ask which of the model's dynamic parameters need be changed to transform the flash response of a rod into that of a cone. The most important changes are a decrease in the gain of the response, reflecting a reduction in amplification of the transduction cascade; an increase in the rate of turnover of cGMP in darkness; and an increase in the rate of decay of activated phosphodiesterase, with perhaps also an increase in the rate of decay of light-activated visual pigment. Although we cannot exclude other differences, and in particular alterations in the Ca2+ economy of the photoreceptors, we believe that we have identified the kinetic parameters principally responsible for the differences in the flash responses of the two kinds of photoreceptors, which were likely during evolution to have resulted in the duplex retina.

Elevated energy requirement of cone photoreceptors

We have used recent measurements of mammalian cone light responses and voltage-gated currents to calculate cone ATP utilization and compare it to that of rods. The largest expenditure of ATP results from ion transport, particularly from removal of Na⁺ entering outer segment light-dependent channels and inner segment hyperpolarization-activated cyclic nucleotide-gated channels, and from ATP-dependent pumping of Ca²⁺ entering voltage-gated channels at the synaptic terminal. Single cones expend nearly twice as much energy as single rods in darkness, largely because they make more synapses with second-order retinal cells and thus must extrude more Ca²⁺ In daylight, cone ATP utilization per cell remains high because cones never remain saturated and must continue to export Na⁺ and synaptic Ca²⁺ even in bright illumination. In mouse and human retina, rods greatly outnumber cones and consume more energy overall even in background light. In primates, however, the high density of cones in the fovea produces a pronounced peak of ATP utilization, which becomes particularly prominent in daylight and may make this part of the retina especially sensitive to changes in energy availability.

Molecular components affecting ocular carotenoid and retinoid homeostasis

The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.

Membrane conductances of mouse cone photoreceptors

Vertebrate photoreceptor cells respond to light through a closure of CNG channels located in the outer segment. Multiple voltage-sensitive channels in the photoreceptor inner segment serve to transform and transmit the light-induced outer-segment current response. Despite extensive studies in lower vertebrates, we do not know how these channels produce the photoresponse of mammalian photoreceptors. Here we examined these ionic conductances recorded from single mouse cones in unlabeled, dark-adapted retinal slices. First, we show measurements of the voltage dependence of the light response. After block of voltage-gated Ca2+ channels, the light-dependent current was nearly linear within the physiological range of voltages with constant chord conductance and a reversal potential similar to that previously determined in lower vertebrate photoreceptors. At a dark resting membrane potential of -45 mV, cones maintain a standing Ca2+ current (iCa) between 15 and 20 pA. We characterized the time and voltage dependence of iCa and a calcium-activated anion channel. After constitutive closure of the CNG channels by the nonhydrolysable analogue GTP-γ-S, we observed a light-dependent increase in iCa followed by a Ca2+-activated K+ current, both probably the result of feedback from horizontal cells. We also recorded the hyperpolarization-activated cyclic nucleotide-gated (HCN) conductance (ih) and measured its current-voltage relationship and reversal potential. With small hyperpolarizations, ih activated with a time constant of 25 ms; activation was speeded with larger hyperpolarizations. Finally, we characterized two voltage-gated K+-conductances (iK). Depolarizing steps beginning at -10 mV activated a transient, outwardly rectifying iK blocked by 4-AP and insensitive to TEA. A sustained iK isolated through subtraction was blocked by TEA but was insensitive to 4-AP. The sustained iK had a nearly linear voltage dependence throughout the physiological voltage range of the cone. Together these data constitute the first comprehensive study of the channel conductances of mouse photoreceptors.

Voltage-clamp recordings of light responses from wild-type and mutant mouse cone photoreceptors

We describe the first extensive study of voltage-clamp current responses of cone photoreceptors in unlabeled, dark-adapted mouse retina using only the position and appearance of cone somata as a guide. Identification was confirmed from morphology after dye filling. Photocurrents recorded from wild-type mouse cones were biphasic with a fast cone component and a slower rod component. The rod component could be eliminated with dim background light and was not present in mouse lines lacking the rod transducin-α subunit (Gnat1-/- ) or connexin 36 (Cx36-/- ). Cones from Gnat1-/- or Cx36-/- mice had resting membrane potentials between -45 and -55 mV, peak photocurrents of 20-25 picoamps (pA) at a membrane potential Vm = -50 mV, sensitivities 60-70 times smaller than rods, and a total membrane capacitance two to four times greater than rods. The rate of activation (amplification constant) was largely independent of the brightness of the flash and was 1-2 s-2, less than half that of rods. The role of Ca2+-dependent transduction modulation was investigated by recording from cones in mice lacking rod transducin (Gnat1), recoverin, and/or the guanylyl-cyclase-activating proteins (GCAPs). In confirmation of previous results, responses of Gnat1-/- ;Gcaps-/- cones and triple-mutant Gnat1-/- ;Gcaps-/- ;Rv-/- cones recovered more slowly both to light flashes and steps and were more sensitive than cones expressing the GCAPs. Cones from all four mouse lines showed significant recovery and escaped saturation even in bright background light. This recovery occurred too rapidly to be caused by pigment bleaching or metaII decay and appears to reflect some modulation of response inactivation in addition to those produced by recoverin and the GCAPs. Our experiments now make possible a more detailed understanding of the cellular physiology of mammalian cone photoreceptors and the role of conductances in the inner and outer segment in producing cone light responses.

Spectral Sensor Inspired by Cone Photoreceptors and Ion Channels without External Power

In this study, we focused the mimicking of the cone photoreceptor along with ion channel system, which is similar to real optical system. By realizing the ion channel and photoreceptor based on photoresponsive material and photoelectric film, we achieved the wavelength-selective sensor with self-power ability. For the first time, we combined the photoreceptor and ion channel system without external power. For this, we used the channel membrane with pores coated with light-responsive material. We measured the sensing signals without any external power, because photoelectric film assists the sensitive operation of the ion channel system. As a result, we demonstrated a spectral ion channel sensor triggered by the photoelectric effect. Mimicking the slow and fast responses typically found in cone photoreceptor, when induced light causes photovoltaic effect from the pyrrole-coated polyvinylidene fluoride film, this helps to normally operate the ion channel system with slow and fast responses to the light wavelengths. Consequently, this research opens the new scientific fields to realize the photosensor very similar to the real vision sensory organ.

The structured 'low temperature' phase of the retinal population code

Recent advances in experimental techniques have allowed the simultaneous recordings of populations of hundreds of neurons, fostering a debate about the nature of the collective structure of population neural activity. Much of this debate has focused on the empirical findings of a phase transition in the parameter space of maximum entropy models describing the measured neural probability distributions, interpreting this phase transition to indicate a critical tuning of the neural code. Here, we instead focus on the possibility that this is a first-order phase transition which provides evidence that the real neural population is in a 'structured', collective state. We show that this collective state is robust to changes in stimulus ensemble and adaptive state. We find that the pattern of pairwise correlations between neurons has a strength that is well within the strongly correlated regime and does not require fine tuning, suggesting that this state is generic for populations of 100+ neurons. We find a clear correspondence between the emergence of a phase transition, and the emergence of attractor-like structure in the inferred energy landscape. A collective state in the neural population, in which neural activity patterns naturally form clusters, provides a consistent interpretation for our results.

Photoreception and vision in the ultraviolet

Ultraviolet (UV) light occupies the spectral range of wavelengths slightly shorter than those visible to humans. Because of its shorter wavelength, it is more energetic (and potentially more photodamaging) than ‘visible light’, and it is scattered more efficiently in air and water. Until 1990, only a few animals were recognized as being sensitive to UV light, but we now know that a great diversity, possibly even the majority, of animal species can visually detect and respond to it. Here, we discuss the history of research on biological UV photosensitivity and review current major research trends in this field. Some animals use their UV photoreceptors to control simple, innate behaviors, but most incorporate their UV receptors into their general sense of vision. They not only detect UV light but recognize it as a separate color in light fields, on natural objects or living organisms, or in signals displayed by conspecifics. UV visual pigments are based on opsins, the same family of proteins that are used to detect light in conventional photoreceptors. Despite some interesting exceptions, most animal species have a single photoreceptor class devoted to the UV. The roles of UV in vision are manifold, from guiding navigation and orientation behavior, to detecting food and potential predators, to supporting high-level tasks such as mate assessment and intraspecific communication. Our current understanding of UV vision is restricted almost entirely to two phyla: arthropods and chordates (specifically, vertebrates), so there is much comparative work to be done.

The Role of Motion Extrapolation in Amphibian Prey Capture

Sensorimotor delays decouple behaviors from the events that drive them. The brain compensates for these delays with predictive mechanisms, but the efficacy and timescale over which these mechanisms operate remain poorly understood. Here, we assess how prediction is used to compensate for prey movement that occurs during visuomotor processing. We obtained high-speed video records of freely moving, tongue-projecting salamanders catching walking prey, emulating natural foraging conditions. We found that tongue projections were preceded by a rapid head turn lasting ∼ 130 ms. This motor lag, combined with the ∼ 100 ms phototransduction delay at photopic light levels, gave a ∼ 230 ms visuomotor response delay during which prey typically moved approximately one body length. Tongue projections, however, did not significantly lag prey position but were highly accurate instead. Angular errors in tongue projection accuracy were consistent with a linear extrapolation model that predicted prey position at the time of tongue contact using the average prey motion during a ∼ 175 ms period one visual latency before the head movement. The model explained successful strikes where the tongue hit the fly, and unsuccessful strikes where the fly turned and the tongue hit a phantom location consistent with the fly's earlier trajectory. The model parameters, obtained from the data, agree with the temporal integration and latency of retinal responses proposed to contribute to motion extrapolation. These results show that the salamander predicts future prey position and that prediction significantly improves prey capture success over a broad range of prey speeds and light levels.

SIGNIFICANCE STATEMENT

Neural processing delays cause actions to lag behind the events that elicit them. To cope with these delays, the brain predicts what will happen in the future. While neural circuits in the retina and beyond have been suggested to participate in such predictions, few behaviors have been explored sufficiently to constrain circuit function. Here we show that salamanders aim their tongues by using extrapolation to estimate future prey position, thereby compensating for internal delays from both visual and motor processing. Predictions made just before a prey turn resulted in the tongue being projected to a position consistent with the prey's pre-turn trajectory. These results define the computations and operating regimen for neural circuits that predict target motion.

Recognizing Words and Reading Sentences with Microsecond Flash Displays

Strings of dots can be used to construct easily identifiable letters, and these in turn can be used to write words and sentences. Prior work found that respondents could identify individual letters when all the dots were simultaneously flashed for an ultra-brief duration. Four of the experiments reported here constructed five-letter words with these dot-letters and a fifth experiment used them to write complete sentences. Respondents were able to recognize individual words that were displayed with a single, simultaneous ultra-brief flash of all the letters. Further, sentences could be efficiently read with a sequence of simultaneous flashes at a frequency that produced perceptual fusion. One experiment determined the frequency range that would produce flicker-fusion. Two experiments established the relation of intensity to probability of recognition with single flashes and with fused-flicker frequencies. Another established the intensities at which flicker-fused and steady displays were judged to be equal in brightness. The final experiment used those flicker-fused and steady intensities to display sentences. The two display conditions were read with equal efficiency, even though the flicker-fused displays provided light stimulation only 0.003% of the time.

Cone Health and Retinoids

Cones are photoreceptor cells used for bright light and color vision. Retinoids are vitamin A derivatives, one of which is the 11-cis aldehyde form that serves as the chromophore for both cone and rod visual pigments. In the visual disease, Type 2 Leber congenital amaurosis (LCA2), 11-cis-retinal generation is inhibited or abolished. Work by others has shown that patients with LCA2 have symptoms consistent with degenerating cones. In mouse models for LCA2, early cone degeneration is readily apparent: cone opsins and other proteins associated with the outer segment are delocalized and cell numbers decline rapidly within the first month. Rods would appear normal morphologically and functionally, if not for the absence of chromophore. Supplementation of mouse models of LCA2 with cis-retinoids has been shown to slow loss of cone photoreceptor cells if mice were maintained in darkness. Thus, 11-cis-retinal appears not only to have a role in the light response reaction but also to promote proper trafficking of the cone opsins and maintain viable cones.

Activation and quenching of the phototransduction cascade in retinal cones as inferred from electrophysiology and mathematical modeling

PURPOSE

To experimentally identify and quantify factors responsible for the lower sensitivity of retinal cones compared to rods.

METHODS

Electrical responses of frog rods and fish (Carassius) cones to short flashes of light were recorded using the suction pipette technique. A fast solution changer was used to apply a solution that fixed intracellular Ca2+ concentration at the prestimulus level, thereby disabling Ca2+ feedback, to the outer segment (OS). The results were analyzed with a specially designed mathematical model of phototransduction. The model included all basic processes of activation and quenching of the phototransduction cascade but omitted unnecessary mechanistic details of each step.

RESULTS

Judging from the response versus intensity curves, Carassius cones were two to three orders of magnitude less sensitive than frog rods. There was a large scatter in sensitivity among individual cones, with red-sensitive cones being on average approximately two times less sensitive than green-sensitive ones. The scatter was mostly due to different signal amplification, since the kinetic parameters of the responses among cones were far less variable than sensitivity. We argue that the generally accepted definition of the biochemical amplification in phototransduction cannot be used for comparing amplification in rods and cones, since it depends on an irrelevant factor, that is, the cell's volume. We also show that the routinely used simplified parabolic curve fitting to an initial phase of the response leads to a few-fold underestimate of the amplification. We suggest a new definition of the amplification that only includes molecular parameters of the cascade activation, and show how it can be derived from experimental data. We found that the mathematical model with unrestrained parameters can yield an excellent fit to experimental responses. However, the fits with wildly different sets of parameters can be virtually indistinguishable, and therefore cannot provide meaningful data on underlying mechanisms. Based on results of Ca2+-clamp experiments, we developed an approach to strongly constrain the values of many key parameters that set the time course and sensitivity of the photoresponse (such as the dark turnover rate of cGMP, rates of turnoffs of the photoactivated visual pigment and phosphodiesterase, and kinetics of Ca2+ feedback). We show that applying these constraints to our mathematical model enables accurate determination of the biochemical amplification in phototransduction. It appeared that, contrary to many suggestions, maximum biochemical amplification derived for "best" Carassius cones was as high as in frog rods. On the other hand, all turnoff and recovery reactions in cones proceeded approximately 10 times faster than in rods.

CONCLUSIONS

The main cause of the differing sensitivity of rods and cones is cones' ability to terminate their photoresponse faster.

Single-photon sensitivity of lamprey rods with cone-like outer segments

Most vertebrates have a duplex retina containing rods for dim light vision and cones for bright lights and color detection. Photoreceptors like cones are present in many invertebrate phyla as well as in chordata, and rods evolved from cones, but the sequence of events is not well understood. Since duplex retinas are present in both agnatha and gnathostomata, which diverged more than 400 million years ago, some properties of ancestral rods may be inferred from a comparison of cells in these two groups. Lamprey have two kinds of photoreceptors, called "short" and "long", which seem to be rods and cones; however, the outer segments of both have an identical cone-like morphology of stacks of lamellae without a continuous surrounding plasma membrane. This observation and other aspects of the cellular and molecular biology of the photoreceptors have convinced several investigators that "the features of 'true' rod transduction in jawed vertebrates, which permit the reliable detection of single photons of light, evolved after the separation of gnathostomes from lampreys". To test this hypothesis, we recorded from photoreceptors of the sea lamprey Petromyzon marinus and show that their rods have a single-photon sensitivity similar to that of rods in other vertebrates. Thus, photoreceptors with many of the features of rods emerged before the split between agnatha and gnathostomata, and a rod-like outer segment with cytosolic disks surrounded by a plasma membrane is not necessary for high-sensitivity visual detection.

Cone-like rectification properties of cGMP-gated channels in transmutated retinal photoreceptors of nocturnal geckoes

Photoreceptors of nocturnal geckoes are scotopic, with rod-shaped outer segments, and sensitivities to light similar to the one of retinal rods from other species of lower vertebrates. However, these cells are not rods, but they originated from cones of ancestral diurnal geckoes with pure-cone retinas, after being forced to adapt to a nocturnal behavior. Several interesting adaptations of these rod-like cones have been studied to date; molecular biology and functional studies confirmed that several proteins of the phototransductive cascade display structural and functional properties that indicate their origin from cones rather than from rods. In this paper, we investigate, with whole cell voltage clamp in the photoreceptor detached outer segment preparation, the voltage rectification properties of cGMP-gated channels in three species, Gekko gecko, Tarentola mauritanica, and Hemidactylus frenatus. We show that the current-voltage properties in the physiological voltage range are reminiscent of the ones of cGMP-gated channels from cones rather than from rods of other cold-blooded vertebrates. The origin and the relevance of the mechanisms investigated are discussed.

The role of 11-cis-retinyl esters in vertebrate cone vision

A cycle of cis-to-trans isomerization of the chromophore is intrinsic to vertebrate vision where rod and cone photoreceptors mediate dim- and bright-light vision, respectively. Daylight illumination can greatly exceed the rate at which the photoproduct can be recycled back to the chromophore by the canonical visual cycle. Thus, an additional supply pathway(s) must exist to sustain cone-dependent vision. Two-photon microscopy revealed that the eyes of the zebrafish (Danio rerio) contain high levels of 11-cis-retinyl esters (11-REs) within the retinal pigment epithelium. HPLC analyses demonstrate that 11-REs are bleached by bright light and regenerated in the dark. Pharmacologic treatment with all-trans-retinylamine (Ret-NH2), a potent and specific inhibitor of the trans-to-cis reisomerization reaction of the canonical visual cycle, impeded the regeneration of 11-REs. Intervention with 11-cis-retinol restored the regeneration of 11-REs in the presence of all-trans-Ret-NH2. We used the XOPS:mCFP transgenic zebrafish line with a functional cone-only retina to directly demonstrate that this 11-RE cycle is critical to maintain vision under bright-light conditions. Thus, our analyses reveal that a dark-generated pool of 11-REs helps to supply photoreceptors with the chromophore under the varying light conditions present in natural environments.

Nanoparticle-assisted targeted delivery of eye-specific genes to eyes significantly improves the vision of blind mice in vivo

Application of viruses as a carrier, though not safe, to deliver genes to eye tissue was successful. However, a safer, nonviral, biocompatible lipid-based nanoparticle has never been tested to treat blinding eye diseases. We created an artificial virus using a nanoparticle, liposome-protamine-DNA complex (LPD), modified with a cell permeable peptide and a nuclear localization signaling (NLS) peptide, to deliver a functional gene for eye disease treatment. In the eye, a photochemical, 11-cis-retinal, allows the visual pigment rhodopsin to absorb light in the visible range. Without the photochemical, we lose the ability to see light. Retinal pigment epithelium protein 65 (Rpe65) is the key enzyme in regulating the availability of photochemical; deficiency of this gene results in a blinding eye disease. Here we show for the first time that LPD promotes efficient delivery in a cell specific-manner, and a long-term expression of Rpe65 gene to mice lacking Rpe65 gene, leading to in vivo correction of blindness. Thus, LPD nanoparticles could provide a promising, efficient, nonviral method of gene delivery with clinical applications in eye disease treatment.

Membrane guanylyl cyclase complexes shape the photoresponses of retinal rods and cones

In vertebrate rods and cones, photon capture by rhodopsin leads to the destruction of cyclic GMP (cGMP) and the subsequent closure of cyclic nucleotide gated ion channels in the outer segment plasma membrane. Replenishment of cGMP and reopening of the channels limit the growth of the photon response and are requisite for its recovery. In different vertebrate retinas, there may be as many as four types of membrane guanylyl cyclases (GCs) for cGMP synthesis. Ten neuronal Ca²⁺ sensor proteins could potentially modulate their activities. The mouse is proving to be an effective model for characterizing the roles of individual components because its relative simplicity can be reduced further by genetic engineering. There are two types of GC activating proteins (GCAPs) and two types of GCs in mouse rods, whereas cones express one type of GCAP and one type of GC. Mutant mouse rods and cones bereft of both GCAPs have large, long lasting photon responses. Thus, GCAPs normally mediate negative feedback tied to the light-induced decline in intracellular Ca²⁺ that accelerates GC activity to curtail the growth and duration of the photon response. Rods from other mutant mice that express a single GCAP type reveal how the two GCAPs normally work together as a team. Because of its lower Ca²⁺ affinity, GCAP1 is the first responder that senses the initial decrease in Ca²⁺ following photon absorption and acts to limit response amplitude. GCAP2, with a higher Ca²⁺ affinity, is recruited later during the course of the photon response as Ca²⁺ levels continue to decline further. The main role of GCAP2 is to provide for a timely response recovery and it is particularly important after exposure to very bright light. The multiplicity of GC isozymes and GCAP homologs in the retinas of other vertebrates confers greater flexibility in shaping the photon responses in order to tune visual sensitivity, dynamic range and frequency response.

Light responses of primate and other mammalian cones

Retinal cones are photoreceptors for daylight vision. For lower vertebrates, cones are known to give monophasic, hyperpolarizing responses to light flashes. For primate cones, however, they have been reported to give strongly biphasic flash responses, with an initial hyperpolarization followed by a depolarization beyond the dark level, now a textbook dogma. We have reexamined this primate-cone observation and, surprisingly, found predominantly monophasic cone responses. Correspondingly, we found that primate cones began to adapt to steady light at much lower intensities than previously reported, explainable by a larger steady response to background light for a monophasic than for a biphasic response. Similarly, we have found a monophasic cone response for several other mammalian species. Thus, a monophasic flash response may in fact be the norm for primate and other mammalian cones as for lower-vertebrate cones. This revised information is important for ultimately understanding human retinal signal processing and correlating with psychophysical data.

Ultraviolet vision in lacertid lizards: evidence from retinal structure, eye transmittance, SWS1 visual pigment genes, and behaviour

Ultraviolet (UV) vision and UV colour patches have been reported in a wide range of taxa and are increasingly appreciated as an integral part of vertebrate visual perception and communication systems. Previous studies with Lacertidae, a lizard family with diverse and complex coloration, have revealed the existence of UV-reflecting patches that may function as social signals. However, confirmation of the signalling role of UV coloration requires demonstrating that the lizards are capable of vision in the UV waveband. Here we use a multidisciplinary approach to characterize the visual sensitivity of a diverse sample of lacertid species. Spectral transmission measurements of the ocular media show that wavelengths down to 300 nm are transmitted in all the species sampled. Four retinal oil droplet types can be identified in the lacertid retina. Two types are pigmented and two are colourless. Fluorescence microscopy reveals that a type of colourless droplet is UV-transmitting and may thus be associated with UV-sensitive cones. DNA sequencing shows that lacertids have a functional SWS1 opsin, very similar at 13 critical sites to that in the presumed ancestral vertebrate (which was UV-sensitive) and other UV-sensitive lizards. Finally, males of Podarcis muralis are capable of discriminating between two views of the same stimulus that differ only in the presence/absence of UV radiance. Taken together, these results provide convergent evidence of UV vision in lacertids, very likely by means of an independent photopigment. Moreover, the presence of four oil droplet types suggests that lacertids have a four-cone colour vision system.

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.

Adaptive colour contrast coding in the salamander retina efficiently matches natural scene statistics

The visual system continually adjusts its sensitivity to the statistical properties of the environment through an adaptation process that starts in the retina. Colour perception and processing is commonly thought to occur mainly in high visual areas, and indeed most evidence for chromatic colour contrast adaptation comes from cortical studies. We show that colour contrast adaptation starts in the retina where ganglion cells adjust their responses to the spectral properties of the environment. We demonstrate that the ganglion cells match their responses to red-blue stimulus combinations according to the relative contrast of each of the input channels by rotating their functional response properties in colour space. Using measurements of the chromatic statistics of natural environments, we show that the retina balances inputs from the two (red and blue) stimulated colour channels, as would be expected from theoretical optimal behaviour. Our results suggest that colour is encoded in the retina based on the efficient processing of spectral information that matches spectral combinations in natural scenes on the colour processing level.

New perspectives in cyclic nucleotide-mediated functions in the CNS: the emerging role of cyclic nucleotide-gated (CNG) channels

Cyclic nucleotides play fundamental roles in the central nervous system (CNS) under both physiological and pathological conditions. The impact of cAMP and cGMP signaling on neuronal and glial cell functions has been thoroughly characterized. Most of their effects have been related to cyclic nucleotide-dependent protein kinase activity. However, cyclic nucleotide-gated (CNG) channels, first described as key mediators of sensory transduction in retinal and olfactory receptors, have been receiving increasing attention as possible targets of cyclic nucleotides in the CNS. In the last 15 years, consistent evidence has emerged for their expression in neurons and astrocytes of the rodent brain. Far less is known, however, about the functional role of CNG channels in these cells, although several of their features, such as Ca(2+) permeability and prolonged activation in the presence of cyclic nucleotides, make them ideal candidates for mediators of physiological functions in the CNS. Here, we review literature suggesting the involvement of CNG channels in a number of CNS cellular functions (e.g., regulation of membrane potential, neuronal excitability, and neurotransmitter release) as well as in more complex phenomena, like brain plasticity, adult neurogenesis, and pain sensitivity. The emerging picture is that functional and dysfunctional cyclic nucleotide signaling in the CNS has to be reconsidered including CNG channels among possible targets. However, concerted efforts and multidisciplinary approaches are still needed to get more in-depth knowledge in this field.

Cone visual pigments

Cone visual pigments are visual opsins that are present in vertebrate cone photoreceptor cells and act as photoreceptor molecules responsible for photopic vision. Like the rod visual pigment rhodopsin, which is responsible for scotopic vision, cone visual pigments contain the chromophore 11-cis-retinal, which undergoes cis-trans isomerization resulting in the induction of conformational changes of the protein moiety to form a G protein-activating state. There are multiple types of cone visual pigments with different absorption maxima, which are the molecular basis of color discrimination in animals. Cone visual pigments form a phylogenetic sister group with non-visual opsin groups such as pinopsin, VA opsin, parapinopsin and parietopsin groups. Cone visual pigments diverged into four groups with different absorption maxima, and the rhodopsin group diverged from one of the four groups of cone visual pigments. The photochemical behavior of cone visual pigments is similar to that of pinopsin but considerably different from those of other non-visual opsins. G protein activation efficiency of cone visual pigments is also comparable to that of pinopsin but higher than that of the other non-visual opsins. Recent measurements with sufficient time-resolution demonstrated that G protein activation efficiency of cone visual pigments is lower than that of rhodopsin, which is one of the molecular bases for the lower amplification of cones compared to rods. In this review, the uniqueness of cone visual pigments is shown by comparison of their molecular properties with those of non-visual opsins and rhodopsin. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.

New insights into retinoid metabolism and cycling within the retina

The retinoid cycle is a series of biochemical reactions within the eye that is responsible for synthesizing the chromophore, 11-cis retinal, for visual function. The chromophore is bound to G-protein coupled receptors, opsins, within rod and cone photoreceptor cells forming the photosensitive visual pigments. Integral to the sustained function of photoreceptors is the continuous generation of chromophore by the retinoid cycle through two separate processes, one that supplies both rods and cones and another that exclusively supplies cones. Recent findings such as RPE65 localization within cones and the pattern of distribution of retinoid metabolites within mouse and human retinas have challenged previous proposed schemes. This review will focus on recent findings regarding the transport of retinoids, the mechanisms by which chromophore is supplied to both rods and cones, and the metabolism of retinoids within the posterior segment of the eye.

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.

Rapid release of retinal from a cone visual pigment following photoactivation

As part of the visual cycle, the retinal chromophore in both rod and cone visual pigments undergoes reversible Schiff base hydrolysis and dissociation following photobleaching. We characterized light-activated release of retinal from a short-wavelength-sensitive cone pigment (VCOP) in 0.1% dodecyl maltoside using fluorescence spectroscopy. The half-time (t(1/2)) of release of retinal from VCOP was 7.1 s, 250-fold faster than that of rhodopsin. VCOP exhibited pH-dependent release kinetics, with the t(1/2) decreasing from 23 to 4 s with the pH decreasing from 4.1 to 8, respectively. However, the Arrhenius activation energy (E(a)) for VCOP derived from kinetic measurements between 4 and 20 °C was 17.4 kcal/mol, similar to the value of 18.5 kcal/mol for rhodopsin. There was a small kinetic isotope (D(2)O) effect in VCOP, but this effect was smaller than that observed in rhodopsin. Mutation of the primary Schiff base counterion (VCOP(D108A)) produced a pigment with an unprotonated chromophore (λ(max) = 360 nm) and dramatically slowed (t(1/2) ~ 6.8 min) light-dependent retinal release. Using homology modeling, a VCOP mutant with two substitutions (S85D and D108A) was designed to move the counterion one α-helical turn into the transmembrane region from the native position. This double mutant had a UV-visible absorption spectrum consistent with a protonated Schiff base (λ(max) = 420 nm). Moreover, the VCOP(S85D/D108A) mutant had retinal release kinetics (t(1/2) = 7 s) and an E(a) (18 kcal/mol) similar to those of the native pigment exhibiting no pH dependence. By contrast, the single mutant VCOP(S85D) had an ~3-fold decreased retinal release rate compared to that of the native pigment. Photoactivated VCOP(D108A) had kinetics comparable to those of a rhodopsin counterion mutant, Rho(E113Q), both requiring hydroxylamine to fully release retinal. These results demonstrate that the primary counterion of cone visual pigments is necessary for efficient Schiff base hydrolysis. We discuss how the large differences in retinal release rates between rod and cone visual pigments arise, not from inherent differences in the rate of Schiff base hydrolysis but rather from differences in the properties of noncovalent binding of the retinal chromophore to the protein.

Male attractiveness is influenced by UV wavelengths in a newt species but not in its close relative

BACKGROUND

Functional communication in the UV range has been reported in Invertebrates and all major groups of Vertebrates but Amphibians. Although perception in this wavelength range has been shown in a few species, UV signalling has not been demonstrated in this group. One reason may be that in lentic freshwater habitats, litter decomposition generates dissolved organic carbon that absorbs UV radiation and thus hinders its use for visual signalling. We tested the effect of male UV characteristics on female sexual preference in two newt species that experience contrasting levels of UV water transmission when breeding.

METHODOLOGY/PRINCIPAL FINDINGS

We analysed water spectral characteristics of a sample of breeding ponds in both species. We quantified male ventral coloration and measured male attractiveness under two lighting conditions (UV present, UV absent) using a no-choice female preference design. UV transmission was higher in Lissotriton vulgaris breeding sites. Male UV patterns also differed between experimental males of the two species. We observed a first common peak around 333 nm, higher in L. vulgaris, and a second peak around 397 nm, more frequent and higher in L. helveticus. Male attractiveness was significantly reduced in L. vulgaris when UV was not available but not in L. helveticus. Male attractiveness depended on the hue of the first UV peak in L. vulgaris.

CONCLUSION/SIGNIFICANCE

Our study is the first report of functional UV-based communication in Amphibians. Interestingly, male spectral characteristics and female preferences were consistent with the differences in habitat observed between the two species as L. helveticus often breeds in ponds containing more UV blocking compounds. We discuss the three hypotheses proposed so far for UV signalling in animals (enhanced signal detectability, private communication channel, indicator of individual quality).

Increased cone sensitivity to ABCA4 deficiency provides insight into macular vision loss in Stargardt's dystrophy

Autosomal recessive Stargardt macular dystrophy is caused by mutations in the photoreceptor disc rim protein ABCA4/ABCR. Key clinical features of Stargardt disease include relatively mild rod defects such as delayed dark adaptation, coupled with severe cone defects reflected in macular atrophy and central vision loss. In spite of this clinical divergence, there has been no biochemical study of the effects of ABCA4 deficiency on cones vs. rods. Here we utilize the cone-dominant Abca4(-/-)/Nrl(-/-) double knockout mouse to study this issue. We show that as early as post-natal day (P) 30, Abca4(-/-)/Nrl(-/-) retinas have significantly fewer rosettes than Abca4(+/+)/Nrl(-/-) retinas, a phenotype often associated with accelerated degeneration. Abca4-deficient mice in both the wild-type and cone-dominant background accumulate more of the toxic bisretinoid A2E than their ABCA4-competent counterparts, but Abca4(-/-)/Nrl(-/-) eyes generate significantly more A2E per mole of 11-cis-retinal (11-cisRAL) than Abca4(-/-) eyes. At P120, Abca4(-/-)/Nrl(-/-) produced 340 ± 121 pmoles A2E/nmol 11-cisRAL while Abca4(-/-) produced 50.4 ± 8.05 pmoles A2E/nmol 11-cisRAL. Nevertheless, the retinal pigment epithelium (RPE) of Abca4(-/-)/Nrl(-/-) eyes exhibits fewer lipofuscin granules than the RPE of Abca4(-/-) eyes; at P120: Abca4(-/-)/Nrl(-/-) exhibit 0.045 ± 0.013 lipofuscingranules/μm² of RPE vs. Abca4(-/-) 0.17 ± 0.030 lipofuscingranules/μm² of RPE. These data indicate that ABCA4-deficient cones simultaneously generate more A2E than rods and are less able to effectively clear it, and suggest that primary cone toxicity may contribute to Stargardt's-associated macular vision loss in addition to cone death secondary to RPE atrophy.

Regeneration of photopigment is enhanced in mouse cone photoreceptors expressing RPE65 protein

As cone photoreceptors mediate vision in bright light, their photopigments are bleached at a rapid rate and require substantial recycling of the chromophore 11-cis-retinal (RAL) for continued function. The retinal pigment epithelium (RPE) supplies 11-cis-RAL to both rod and cone photoreceptors; however, stringent demands imposed by the function of cones in bright light exceed the output from this source. Recent evidence has suggested that cones may be able to satisfy this demand through privileged access to an additional source of chromophore located within the inner retina. In this study, we demonstrate that the protein RPE65, previously identified in RPE as the isomerohydrolase of the RPE-retinal visual cycle, is found within cones of the rod-dominant mouse retina, and the level of RPE65 in cones is inversely related to the level in the RPE. The light sensitivity of cone ERGs of BALB/c mice, which had an undetectable level of cone RPE65, was enhanced by approximately threefold with administration of exogenous chromophore, indicating that the cones of these animals are chromophore deficient. This enhancement with chromophore administration was not observed in C57BL/6 mice, whose cones contain RPE65. These results demonstrate that RPE65 within cones may be essential for the efficient regeneration of cone photopigments under bright-light conditions.