All-trans-retinal is a closed-state inhibitor of rod cyclic nucleotide-gated ion channels

Assays for inverse agonists in the visual system

Visual pigment proteins belong to the superfamily of G protein-coupled receptors and are the light-sensitive molecules in rod and cone photoreceptor cells. The protein moiety is known as opsin and the ligand in the dark is 11-cis retinal, which serves as both the photon detector and an inverse agonist. While much is known about properties of the rod pigment rhodopsin, much less is understood about cone visual pigments. Being able to identify ligands that effect opsins give an insight into structure-activity relationships. The action of some ligands indicates that there are differences between not only rod and cone opsins but also among the different classes of cone opsins. Furthermore, inverse agonists of cone opsins may have potential therapeutic uses under conditions when the native 11-cis retinal ligand is absent. A method for determining the effects of ligands on rod and cone opsin activity is described.

Beta-ionone activates and bleaches visual pigment in salamander photoreceptors

Vision begins with photoisomerization of 11-cis retinal to the all-trans conformation within the chromophore-binding pocket of opsin, leading to activation of a biochemical cascade. Release of all-trans retinal from the binding pocket curtails but does not fully quench the ability of opsin to activate transducin. All-trans retinal and some other analogs, such as beta-ionone, enhance opsin's activity, presumably on binding the empty chromophore-binding pocket. By recording from isolated salamander photoreceptors and from patches of rod outer segment membrane, we now show that high concentrations of beta-ionone suppressed circulating current in dark-adapted green-sensitive rods by inhibiting the cyclic nucleotide-gated channels. There were also decreases in circulating current and flash sensitivity, and accelerated flash response kinetics in dark-adapted blue-sensitive (BS) rods and cones, and in ultraviolet-sensitive cones, at concentrations too low to inhibit the channels. These effects persisted in BS rods even after incubation with 9-cis retinal to ensure complete regeneration of their visual pigment. After long exposures to high concentrations of beta-ionone, recovery was incomplete unless 9-cis retinal was given, indicating that visual pigment had been bleached. Therefore, we propose that beta-ionone activates and bleaches some types of visual pigments, mimicking the effects of light.

Retinyl ester homeostasis in the adipose differentiation-related protein-deficient retina

The retinal pigmented epithelium (RPE) plays an essential role in vision, including storing and converting retinyl esters of the visual chromophore, 11-cis-retinal. Retinyl ester storage structures (RESTs), specialized lipid droplets within the RPE, take up retinyl esters synthesized in the endoplasmic reticulum. Here we report studies of mice lacking exons 2 and 3 of the gene encoding adipose differentiation-related protein (Adfp), a structural component of RESTs. We found that dark adaptation was slower in Adfp(Delta2-3/Delta2-3) than in Adfp+/+ mice and that Adfp(Delta2-3/Delta2-3) mice had consistently delayed clearances of all-trans-retinal and all-trans-retinol from rod photoreceptor cells. Two-photon microscopy revealed aberrant trafficking of all-trans-retinyl esters in the RPE of Adfp(Delta2-3/Delta2-3) mice, a problem caused by abnormal maintenance of RESTs in the dark-adapted state. Retinyl ester accumulation was also reduced in Adfp(Delta2-3/Delta2-3) as compared with Adfp+/+ mice. These observations suggest that Adfp plays a unique role in vision by maintaining proper storage and trafficking of retinoids within the eye.

Cyclic nucleotide-gated ion channels in rod photoreceptors are protected from retinoid inhibition

In vertebrate rods, photoisomerization of the 11-cis retinal chromophore of rhodopsin to the all-trans conformation initiates a biochemical cascade that closes cGMP-gated channels and hyperpolarizes the cell. All-trans retinal is reduced to retinol and then removed to the pigment epithelium. The pigment epithelium supplies fresh 11-cis retinal to regenerate rhodopsin. The recent discovery that tens of nanomolar retinal inhibits cloned cGMP-gated channels at low [cGMP] raised the question of whether retinoid traffic across the plasma membrane of the rod might participate in the signaling of light. Native channels in excised patches from rods were very sensitive to retinoid inhibition. Perfusion of intact rods with exogenous 9- or 11-cis retinal closed cGMP-gated channels but required higher than expected concentrations. Channels reopened after perfusing the rod with cellular retinoid binding protein II. PDE activity, flash response kinetics, and relative sensitivity were unchanged, ruling out pharmacological activation of the phototransduction cascade. Bleaching of rhodopsin to create all-trans retinal and retinol inside the rod did not produce any measurable channel inhibition. Exposure of a bleached rod to 9- or 11-cis retinal did not elicit channel inhibition during the period of rhodopsin regeneration. Microspectrophotometric measurements showed that exogenous 9- or 11-cis retinal rapidly cross the plasma membrane of bleached rods and regenerate their rhodopsin. Although dark-adapted rods could also take up large quantities of 9-cis retinal, which they converted to retinol, the time course was slow. Apparently cGMP-gated channels in intact rods are protected from the inhibitory effects of retinoids that cross the plasma membrane by a large-capacity buffer. Opsin, with its chromophore binding pocket occupied (rhodopsin) or vacant, may be an important component. Exceptionally high retinoid levels, e.g., associated with some retinal degenerations, could overcome the buffer, however, and impair sensitivity or delay the recovery after exposure to bright light.

Retinol dehydrogenase (RDH12) protects photoreceptors from light-induced degeneration in mice

RDH12 has been suggested to be one of the retinol dehydrogenases (RDH) involved in the vitamin A recycling system (visual cycle) in the eye. Loss of function mutations in the RDH12 gene were recently reported to be associated with autosomal recessive childhood-onset severe retinal dystrophy. Here we show that RDH12 localizes to the photoreceptor inner segments and that deletion of this gene in mice slows the kinetics of all-trans-retinal reduction, delaying dark adaptation. However, accelerated 11-cis-retinal production and increased susceptibility to light-induced photoreceptor apoptosis were also observed in Rdh12(-/-) mice, suggesting that RDH12 plays a unique, nonredundant role in the photoreceptor inner segments to regulate the flow of retinoids in the eye. Thus, severe visual impairments of individuals with null mutations in RDH12 may likely be caused by light damage(1).

Visual cycle and its metabolic support in gecko photoreceptors

Photoreceptors of nocturnal geckos are transmuted cones that acquired rod morphological and physiological properties but retained cone-type phototransduction proteins. We have used microspectrophotometry and microfluorometry of solitary isolated green-sensitive photoreceptors of Tokay gecko to study the initial stages of the visual cycle within these cells. These stages are the photolysis of the visual pigment, the reduction of all-trans retinal to all-trans retinol, and the clearance of all-trans retinol from the outer segment (OS) into the interphotoreceptor space. We show that the rates of decay of metaproducts (all-trans retinal release) and retinal-to-retinol reduction are intermediate between those of typical rods and cones. Clearance of retinol from the OS proceeds at a rate that is typical of rods and is greatly accelerated by exposure to interphotoreceptor retinoid-binding protein, IRBP. The rate of retinal release from metaproducts is independent of the position within the OS, while its conversion to retinol is strongly spatially non-uniform, being the fastest at the OS base and slowest at the tip. This spatial gradient of retinol production is abolished by dialysis of saponin-permeabilized OSs with exogenous NADPH or substrates for its production by the hexose monophosphate pathway (NADP+glucose-6-phosphate or 6-phosphogluconate, glucose-6-phosphate alone). Following dialysis by these agents, retinol production is accelerated by several-fold compared to the fastest rates observed in intact cells in standard Ringer solution. We propose that the speed of retinol production is set by the availability of NADPH which in turn depends on ATP supply within the outer segment. We also suggest that principal source of this ATP is from mitochondria located within the ellipsoid region of the inner segment.

Visual cycle: Dependence of retinol production and removal on photoproduct decay and cell morphology

The visual cycle is a chain of biochemical reactions that regenerate visual pigment following exposure to light. Initial steps, the liberation of all-trans retinal and its reduction to all-trans retinol by retinol dehydrogenase (RDH), take place in photoreceptors. We performed comparative microspectrophotometric and microfluorometric measurements on a variety of rod and cone photoreceptors isolated from salamander retinae to correlate the rates of photoproduct decay and retinol production. Metapigment decay rate was spatially uniform within outer segments and 50-70 times faster in the cells that contained cone-type pigment (SWS2 and M/LWS) compared to cells with rod-type pigment (RH1). Retinol production rate was strongly position dependent, fastest at the base of outer segments. Retinol production rate was 10-40 times faster in cones with cone pigments (SWS2 and M/LWS) than in the basal OS of rods containing rod pigment (RH1). Production rate was approximately five times faster in rods containing cone pigment (SWS2) than the rate in basal OS of rods containing the rod pigment (RH1). We show that retinol production is defined either by metapigment decay rate or RDH reaction rate, depending on cell type or outer segment region, whereas retinol removal is defined by the surface-to-volume ratio of the outer segment and the availability of retinoid binding protein (IRBP). The more rapid rates of retinol production in cones compared to rods are consistent with the more rapid operation of the visual cycle in these cells.

Dark adaptation of human rod bipolar cells measured from the b-wave of the scotopic electroretinogram

To examine the dark adaptation of human rod bipolar cells in vivo, we recorded ganzfeld ERGs to (a) a family of flashes of increasing intensity, (b) dim test flashes presented on a range of background intensities, and (c) dim test flashes presented before, and up to 40 min after, exposure to intense illumination eliciting bleaches from a few per cent to near total. The dim flash ERG was characterized by a prominent b-wave response generated principally by rod bipolar cells. In the presence of background illumination the response reached peak earlier and desensitized according to Weber's Law. Following bleaching exposures, the response was initially greatly desensitized, but thereafter recovered slowly with time. For small bleaches, the desensitization was accompanied by acceleration, in much the same way as for real light. Following a near-total bleach, the response was unrecordable for >10 min, but after approximately 23 min half-maximal sensitivity was reached, and full sensitivity was restored between approximately 35 and 40 min. With smaller bleaches, recovery commenced earlier. We converted the post-bleach measurements of desensitization into 'equivalent background intensities' using a Crawford transformation. Across the range of bleaching levels, the results were described by a prominent 'S2' component (0.24 decades min(-1)) together with a smaller and slower 'S3' component (0.06 decades min(-1)), as is found for dark adaptation of the scotopic visual system. We attribute the S2 component to the presence of unregenerated opsin, and we speculate that the S3 component results from ion channel closure by all-trans retinal.

Defining the retinoid binding site in the rod cyclic nucleotide-gated channel

Rod vision is initiated when 11-cis-retinal, bound within rhodopsin, absorbs a photon and isomerizes to all-trans-retinal (ATR). This triggers an enzyme cascade that lowers cGMP, thereby closing cyclic nucleotide-gated (CNG) channels. ATR then dissociates from rhodopsin, with bright light releasing millimolar levels of ATR. We have recently shown that ATR is a potent closed-state inhibitor of the rod CNG channel, and that it requires access to the cytosolic face of the channel (McCabe, S.L., D.M. Pelosi, M. Tetreault, A. Miri, W. Nguitragool, P. Kovithvathanaphong, R. Mahajan, and A.L. Zimmerman. 2004. J. Gen. Physiol. 123:521–531). However, the details of the interaction between the channel and ATR have not been resolved. Here, we explore the nature of this interaction by taking advantage of specific retinoids and retinoid analogues, namely, β-ionone, all-trans-C15 aldehyde, all-trans-C17 aldehyde, all-trans-C22 aldehyde, all-trans-retinol, all-trans-retinoic acid, and all-trans-retinylidene-n-butylamine. These retinoids differ in polyene chain length, chemical functionality, and charge. Results obtained from patch clamp and NMR studies have allowed us to better define the characteristics of the site of retinoid–channel interaction. We propose that the cytoplasmic face of the channel contains a retinoid binding site. This binding site likely contains a hydrophobic region that allows the ionone ring and polyene tail to sit in an optimal position to promote interaction of the terminal functional group with residues ∼15 Å away from the ionone ring. Based on our functional data with retinoids possessing either a positive or a negative charge, we speculate that these amino acid residues may be polar and/or aromatic.

Functional Characterization of Mouse RDH11 as a Retinol Dehydrogenase Involved in Dark Adaptation in Vivo

We previously cloned mouse RDH11 (mRDH11) as a gene regulated by the transcription factor sterol regulatory element-binding proteins and showed that it is a retinol dehydrogenase expressed in non-ocular tissues such as the liver and testis and in the retina (Kasus-Jacobi, A., Ou, J., Bashmakov, Y. K., Shelton, J. M., Richardson, J. A., Goldstein, J. L., and Brown, M. S. (2003) J. Biol. Chem. 278, 32380-32389). It was proposed to function in the recycling of the visual chromophore 11-cis-retinal after photoisomerization by a bleaching light, a pathway referred to as the visual cycle. In this work, we describe our studies on the ocular function of mRDH11. We created a knockout mouse by replacing the mrdh11 coding sequence with the lacZ reporter gene for expression profiling. 5-Bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining demonstrated active transcription of this gene in photoreceptor cells. We show by immunoblot analysis that mRDH11 is associated with retinal membranes purified from a non-outer segment fraction of the retina. No obvious retinal defect was found during development and aging of RDH11-deficient mice. The functional consequences of mRDH11 disruption were investigated by electroretinography. Dark adaptation was delayed by a factor of 2.5-3 compared with wild-type mice. However, the kinetics of 11-cis-retinal recycling during dark adaptation was not affected, suggesting that mRDH11 is not involved in the visual cycle. We propose that mRDH11 disruption affects retinoid metabolism in photoreceptor inner segments and delays the kinetics of dark adaptation through modulation of calcium homeostasis.