Biochemical characterization of the retinoid isomerase system of the eye

We have previously shown that membranes from the retinal pigment epithelium can transform added all-trans-retinol into a mixture of 11-cis-retinoids, demonstrating the "missing reaction" in the visual cycle for the first time (Bernstein, P. S., Law, W. C., and Rando, R. R. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1849-1853). In this article, this isomerase activity is further characterized. Double-label experiments with [15-3H]- and [15-14C]all-trans-retinol as the substrate show that the tritium label is retained in the 11-cis-retinol and 11-cis-retinyl palmitate products. This requires that isomerization occur at the alcohol level of oxidation. All-trans-retinyl esters, such as the palmitate, acetate, butyrate, and hexanoate esters, are not directly transformed into their 11-cis counterparts by the membranes. The data are consistent with the presence of an all-trans-retinol isomerase enzyme system or enzyme complex, which produces 11-cis-retinol. Other isomeric retinols were tested for substrate activity. Neither 9-cis-retinol(al) nor 13-cis-retinol were processed by the isomerase. Since the membranes containing the isomerase possess other retinol metabolizing activities, such as retinyl ester synthetase and dehydrogenase activities, further purification was attempted. Appreciable quantities of all detergents tested led to the disappearance of isomerase activity, and high salt or EDTA did not dissociate isomerase activity from the membranes. However, extensive sonication of the membranes did produce a 100,000 x g supernatant fraction of light membranes depleted of other all-trans-retinol processing activities. The isomerase activity in these membranes was saturable with all-trans-retinol, as required for a biologically significant process, and showed a Vmax of 5 pmol/h/mg of protein, a KM of 0.8 microM, and a pH optimum of 8. The isomerase was destroyed by proteinase K, by phospholipase C, by heating, or by ethanol at concentrations greater than 1%. The addition of high energy compounds, such as MgATP, MgGTP, or palmitoyl-CoA, did not appear to stimulate isomerase activity in the 100,000 x g supernatant.     

Inhibitors of retinyl ester formation also prevent the biosynthesis of 11-cis-retinol

Lecithin retinol acyl transferase (LRAT) from the retinyl pigment epithelium is potently inhibited by all-trans-retinyl alpha-bromoacetate in the micromolar range. The inhibition is competitive and reversible. The retinyl pigment epithelium also contains an enzymatic activity capable of converting added all-trans-retinol into 11-cis-retinol. This isomerization is likely to require the intermediate formation of all-trans-retinyl esters, which are themselves produced by LRAT action. Here this possibility is directly tested by studying the effect of all-trans-retinyl alpha-bromoacetate on the isomerization reaction. When pigment epithelium membranes are preincubated with all-trans-retinyl alpha-bromoacetate, they form neither retinyl esters nor 11-cis-retinol from added all-trans-retinol. However, if the pigment epithelium membranes are first allowed to form all-trans-retinyl esters from all-trans-retinol before the addition of all-trans-retinyl alpha-bromoacetate, then 11-cis-retinol formation proceeds at close to the rate found in the absence of inhibitor. In addition, 11-cis-retinyl esters are not formed under these conditions, eliminating the possibility of a direct ester-ester isomerization route. Therefore, all-trans-retinyl esters are obligate intermediates in the biosynthesis of 11-cis-retinol.     

Solubilization and partial purification of retinyl ester synthetase and retinoid isomerase from bovine ocular pigment epithelium

Studies reported previously from this laboratory have demonstrated that membranes from the pigment epithelium of the vertebrate eye can transform free all-trans-retinol to 11-cis-retinol as well as 11-cis- and all trans-retinyl esters (Bernstein, P. S., Law, W. C., and Rando, R. R. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1849-1853; Bernstein, P. S., Law, W. C., and Rando, R. R. (1987) J. Biol. Chem. 262, 16848-16857; Fulton, B. S., and Rando, R. R. (1987) Biochemistry 26, 7938-7945). The congeneric retinals are also formed under conditions where retinol redox activity is present. Here we report the successful solubilization of both the retinyl ester synthetase and isomerase activities from the pigment epithelium membranes of the bovine eye. The zwitterionic detergent Zwittergent 3-14(N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate; cmc 0.012%) gave optimal solubilization of both activities. Three initial criteria for successful solubilization were used. First, high speed centrifugation (greater than 150,000 x g) left the activities in the supernatant. Second, the solubilized enzymatic activities were found in the included volume upon gel filtration. Finally, the solubilized activities were quantitatively passed through a 0.22-microns filter. Employing anion exchange and gel filtration chromatography results in a partial purification of the retinyl ester synthetase (approximately 189-fold). The solubilized retinoid isomerase is also partially purified (approximately 10-14-fold) following anion exchange chromatography. It is also shown that the membrane-bound and solubilized ester synthetase catalyzes the esterification of retinol using added lecithins as exogenous acyl donors. In addition, evidence is provided indicating that there is a positional selectivity for the acyl group transfer from the lecithin to retinol. The transfer occurs largely, if not entirely, from the 1-position of the lecithin.     

Hydrolysis of 11-cis- and all-trans-retinyl palmitate by retinal pigment epithelium microsomes.

A partial characterization of the enzymatic hydrolysis of 11-cis- and all-trans-retinyl palmitate by bovine retinal pigment epithelium microsomes was carried out using a micro-radiometric method to quantitate liberated palmitic acid. Retinyl ester hydrolase (REH) activity was examined in the absence of detergent. Hydrolysis of 11-cis- and all-trans-retinyl palmitate was protein- and time-dependent. Optimal enzyme activity occurred at slightly alkaline pH (8-9). Apparent kinetic constants (Vmax and Km) for the 11-cis-REH were 2.1 nmol/min/mg protein and 66 microM, respectively. All-trans-REH demonstrated a lower maximum velocity of 0.3 nmol/min/mg protein and a slightly higher substrate affinity of 27 microM. Further characterization of 11-cis-retinyl palmitate hydrolysis involved monitoring formation of reaction products, 11-cis retinol and palmitic acid, which were found to be released in essentially a 1:1 stoichiometry. Addition of all-trans retinyl bromoacetate, a known inhibitor of lecithin:retinol acyltransferase reduced both 11-cis and all-trans-REH activities but to significantly different degrees (50 and 76%, respectively). Although the microsomal preparation exhibited LRAT activity, acyl transfer was not readily reversible as labeled palmitic acid was not transferred to added acyl acceptor compounds. These findings suggest that hydrolysis of 11-cis-retinyl palmitate by bovine retinal pigment epithelium microsomes may occur at a catalytic site distinct from that for the all-trans isomer and that this hydrolysis is not representative of a reverse transesterification reaction.     

Rpe65 is a retinyl ester binding protein that presents insoluble substrate to the isomerase in retinal pigment epithelial cells

Photon capture by a rhodopsin pigment molecule induces 11-cis to all-trans isomerization of its retinaldehyde chromophore. To restore light sensitivity, the all-trans-retinaldehyde must be chemically re-isomerized by an enzyme pathway called the visual cycle. Rpe65, an abundant protein in retinal pigment epithelial (RPE) cells and a homolog of beta-carotene dioxygenase, appears to play a role in this pathway. Rpe65-/- knockout mice massively accumulate all-trans-retinyl esters but lack 11-cis-retinoids and rhodopsin visual pigment in their retinas. Mutations in the human RPE65 gene cause a severe recessive blinding disease called Leber's congenital amaurosis. The function of Rpe65, however, is unknown. Here we show that Rpe65 specifically binds all-trans-retinyl palmitate but not 11-cis-retinyl palmitate by a spectral-shift assay, by co-elution during gel filtration, and by co-immunoprecipitation. Using a novel fluorescent resonance energy transfer (FRET) binding assay in liposomes, we demonstrate that Rpe65 extracts all-trans-retinyl esters from phospholipid membranes. Assays of isomerase activity reveal that Rpe65 strongly stimulates the enzymatic conversion of all-trans-retinyl palmitate to 11-cis-retinol in microsomes from bovine RPE cells. Moreover, we show that addition of Rpe65 to membranes from rpe65-/- mice, which possess no detectable isomerase activity, restores isomerase activity to wild-type levels. Rpe65 by itself, however, has no intrinsic isomerase activity. These observations suggest that Rpe65 presents retinyl esters as substrate to the isomerase for synthesis of visual chromophore. This proposed function explains the phenotype in mice and humans lacking Rpe65.     

All-trans-retinyl esters are the substrates for isomerization in the vertebrate visual cycle

The identification of the critical enzyme(s) that carries out the trans to cis isomerization producing 11-cis-retinol during the operation of the visual cycle remains elusive. Confusion exists in the literature as to the exact nature of the isomerization substrate. At issue is whether it is an all-trans-retinyl ester or all-trans-retinol (vitamin A). As both putative substrates interconvert rapidly in retinal pigment epithelial membranes, the choice of substrate can be ambiguous. The two enzymes that effect interconversion of all-trans-retinol and all-trans-retinyl esters are lecithin retinol acyl transferase (LRAT) and retinyl ester hydrolase (REH). The retinyl ester or all-trans-retinol pools are radioactively labeled separately in the presence of inhibitors of LRAT and REH, effectively preventing their interconversion. Pulse-chase experiments unambiguously demonstrate that all-trans-retinyl esters, and not all-trans-retinol, are the precursors of 11-cis-retinol. When the all-trans-retinyl ester pool is radioactively labeled, the resulting 11-cis-retinol is labeled with the same specific activity as the precursor ester. The converse is true with vitamin A. These data unambiguously establish all-trans-retinyl esters as the precursors of 11-cis-retinol.     

RPE65 operates in the vertebrate visual cycle by stereospecifically binding all-trans-retinyl esters

RPE65 is a major protein of unknown function found associated with the retinyl pigment epithelial (RPE) membranes [Hamel, C. P., Tsilou, E., Pfeffer, B. A., Hooks, J. J., Detrick, B., and Redmond, T. M. (1993) J. Biol. Chem. 268, 15751-15757; Bavik, C. O., Levy, F., Hellman, U., Wernstedt, C., and Eriksson, U. (1993) J. Biol. Chem. 268, 20540-20546]. RPE65 knockouts fail to synthesize 11-cis-retinal, the chromophore of rhodopsin, and accumulate all-trans-retinyl esters in the RPE. Previous studies have also shown that RPE65 is specifically labeled with all-trans-retinyl ester based affinity labeling agents, suggesting a retinyl ester binding role for the protein. In the present work, we show that purified RPE65 binds all-trans-retinyl palmitate (tRP) with a K(D) = 20 pM. These quantitative experiments are performed by measuring the quenching of RPE65 fluorescence by added tRP. The binding for tRP is highly specific because 11-cis-retinyl palmitate binds with a K(D) = 14 nM, 11-cis-retinol binds with a K(D) = 3.8 nM, and all-trans-retinol (vitamin A) binds with a K(D) = 10.8 nM. This stereospecificity for tRP is to be compared to the binding of retinoids to BSA, where virtually no discrimination is found in the binding of the same retinoids. This work provides further evidence that RPE65 functions by binding to and mobilizing the highly hydrophobic all-trans-retinyl esters, allowing them to enter the visual cycle.     

CoA- and non-CoA-dependent retinol esterification in retinal pigment epithelium

Washed, buffered microsomes from bovine retinal pigment epithelium catalyze retinyl ester synthesis from retinol in the absence of an exogenous acyl donor. A plot of retinyl ester synthesis versus time reaches a plateau at 123 +/- 26 nmol of retinyl ester mg-1 microsomal protein, providing a minimum value of the concentration of the endogenous acyl donor. Fatty acyl-CoA analysis by three different methods employing high performance liquid chromatography resulted in the detection of less than 1 nmol mg-1 protein of acyl-CoA, indicating that fatty acyl-CoA is not the endogenous acyl donor. Stimulation of the rate of retinyl ester synthesis by palmitoyl-CoA or ATP, CoA, and palmitate is observed following its addition at the beginning of the reaction or after the endogenous acyl source has been exhausted by 20 min of reaction with retinol. Palmitate from [14C]palmitoyl-CoA is incorporated into retinyl ester at a rate similar to that for the incorporation of [3H] retinol, demonstrating the presence of an apparent acyl-CoA:retinol acyl transferase activity. The acyl group from palmitoyl-CoA can be transferred initially to a component of the microsomes and subsequently to retinol. The product of retinyl ester synthesis from all-trans-retinol and palmitoyl-CoA is all-trans-retinyl palmitate, indicating that the stereochemical configuration is retained during esterification. The kinetic parameters for the esterification of 11-cis-retinol and all-trans-retinol are similar.     

Preferential release of 11-cis-retinol from retinal pigment epithelial cells in the presence of cellular retinaldehyde-binding protein

In photoreceptor cells of the retina, photoisomerization of 11-cis-retinal to all-trans-retinal triggers phototransduction. Regeneration of 11-cis-retinal proceeds via a complex set of reactions in photoreceptors and in adjacent retinal pigment epithelial cells where all-trans-retinol is isomerized to 11-cis-retinol. Our results show that isomerization in vitro only occurs in the presence of apo-cellular retinaldehyde-binding protein. This retinoid-binding protein may drive the reaction by mass action, overcoming the thermodynamically unfavorable isomerization. Furthermore, this 11-cis-retinol/11-cis-retinal-specific binding protein potently stimulates hydrolysis of endogenous 11-cis-retinyl esters but has no effect on hydrolysis of all-trans-retinyl esters. Apo-cellular retinaldehyde-binding protein probably exerts its effect by trapping the 11-cis-retinol product. When retinoid-depleted retinal pigment epithelial microsomes were preincubated with different amounts of all-trans-retinol to form all-trans-retinyl esters and then [3H]all-trans-retinol was added, as predicted, the specific radioactivity of [3H]all-trans-retinyl esters increased during subsequent reaction. However, the specific radioactivity of newly formed 11-cis-retinol stayed constant during the course of the reaction, and it was largely unaffected by expansion of the all-trans-retinyl ester pool during the preincubation. The absence of dilution establishes that most of the ester pool does not participate in isomerization, which in turn suggests that a retinoid intermediate other than all-trans-retinyl ester is on the isomerization reaction pathway.     

Chicken retinas contain a retinoid isomerase activity that catalyzes the direct conversion of all-trans-retinol to 11-cis-retinol

Vertebrate retinas contain two types of light-detecting cells. Rods subserve vision in dim light, while cones provide color vision in bright light. Both contain light-sensitive proteins called opsins. The light-absorbing chromophore in most opsins is 11-cis-retinaldehyde, which is isomerized to all-trans-retinaldehyde by absorption of a photon. Restoration of light sensitivity requires chemical re-isomerization of retinaldehyde by an enzymatic pathway called the visual cycle in the retinal pigment epithelium. The isomerase in this pathway uses all-trans-retinyl esters synthesized by lecithin retinol acyl transferase (LRAT) as the substrate. Several lines of evidence suggest that cone opsins regenerate by a different mechanism. Here we demonstrate the existence of two catalytic activities in chicken retinas. The first is an isomerase activity that effects interconversion of all-trans-retinol and 11-cis-retinol. The second is an ester synthase that effects palmitoyl coenzyme A-dependent synthesis of all-trans- and 11-cis-retinyl esters. Kinetic analysis of these two activities suggests that they act in concert to drive the formation of 11-cis-retinoids in chicken retinas. These activities may be part of a new visual cycle for the regeneration of chromophores in cones.     

Hydrolysis of 11-cis- and all-trans-retinyl palmitate by homogenates of human retinal epithelial cells

The retinal epithelium plays an important role in the storage and metabolism of retinoids in the eye. Studies were conducted to examine the enzymatic hydrolysis of retinyl esters by human retinal epithelial cells. Homogenates prepared from these cells were found to hydrolyze both the 11-cis- and all-trans-isomers of retinyl palmitate. Retinyl ester hydrolysis was time-, protein-, and pH-dependent. The 11-cis isomer was hydrolyzed at a rate which was approximately 20 times greater than that of the all-trans isomer. The 11-cis-retinyl palmitate hydrolase activity did not require detergents, unlike the all-trans-retinyl palmitate hydrolase activity, which required detergents for activity. The 11-cis-retinyl palmitate hydrolase activity was maximally active with the addition of 1.0% sodium taurocholate at about pH 8.5, was abolished by incubation at 50 degrees C for 10 min, and was quantitatively recovered in the pellet after centrifugation at 100,000 X g for 1 h. The rate of hydrolysis of 11-cis-retinyl palmitate became saturated with increasing concentrations of 11-cis-retinyl palmitate; under the assay conditions employed, the hydrolase activity had an apparent Km of 19 microM toward 11-cis-retinyl palmitate. All-trans-retinol and 11-cis-retinyl did not affect the rate of hydrolysis of 11-cis-retinyl palmitate, and addition of all-trans-retinyl palmitate only weakly inhibited the 11-cis-retinyl palmitate hydrolytic activities. These data indicate that the human retinal epithelium possesses distinct activities for the hydrolysis of 11-cis- and all-trans-retinyl esters and raise the possibility that these activities may provide a means of distinguishing the stereoisomers of retinol in this tissue.     

Role of LRAT on the retinoid isomerase activity and membrane association of Rpe65

Absorption of a photon by a vertebrate opsin pigment induces 11-cis to all-trans isomerization of its retinaldehyde chromophore. Restoration of light sensitivity to the bleached opsin requires chemical re-isomerization of the chromophore via an enzyme pathway called the visual cycle. The retinoid isomerase in this pathway is Rpe65, a membrane-associated protein in the retinal pigment epithelium (RPE) with no predicted membrane-spanning segments. It has been suggested that Rpe65 is S-palmitoylated by lecithin:retinol acyl transferase (LRAT) on Cys(231), Cys(329), and Cys(330), and that this palmitoylation is required for isomerase activity and the association of Rpe65 with membranes. Here we show that the affinity of Rpe65 for membranes is similar in wild-type and lrat(-/-) mice. The isomerase activity of Rpe65 is also similar in both strains when all-trans-retinyl palmitate is used as substrate. With all-trans-retinol substrate, isomerase activity is present in wild-type but undetectable in RPE homogenates from lrat(-/-) mice. Substitution of Cys(231), Cys(329), and Cys(330) with Ser or Ala did not affect the affinity of Rpe65 for membranes. Further, these Cys residues are not palmitoylated in Rpe65 by mass spectrometric analysis. Global inhibition of protein palmitoylation by 2-bromopalmitate did not affect the solubility or isomerase activity of Rpe65. Finally, we show that soluble and membrane-associated Rpe65 possesses similar isomerase specific activities. These results indicate that LRAT is not required for isomerase activity beyond synthesis of retinyl-ester substrate, and that the association of Rpe65 with membranes is neither dependent upon LRAT nor the result of S-palmitoylation. The affinity of Rpe65 for membranes is probably an intrinsic feature of this protein.     

Purification and characterization of a transmembrane domain-deleted form of lecithin retinol acyltransferase

Lecithin retinol acyltransferase (LRAT) catalyzes the esterification of all-trans-retinol into all-trans-retinyl ester, an essential reaction in the vertebrate visual cycle. Since all-trans-retinyl esters are the substrates for the isomerization reaction that generates 11-cis-retinoids, this esterification reaction is essential in the operation of the visual cycle. In addition, LRAT is the founder member of a series of proteins, which are of novel sequence and have unknown functions. Native LRAT is an integral membrane protein and has never been purified. To obtain a pure LRAT, the N- and C-transmembrane termini were deleted and replaced with a poly His tag for the purpose of purification. This truncated form of LRAT, referred to as tLRAT, has been expressed in bacteria and fully purified. tLRAT is catalytically active and processes all-trans-retinol at least 10-fold more efficiently than 11-cis-retinol, the precursor to the visual chromophore. While tLRAT can be robustly expressed in bacteria, it requires detergent for extraction, as the enzyme still contains hydrophobic domains, which may interact. Indeed, tLRAT can oligomerize and forms dimers. Native LRAT also forms functional homodimers. These studies pave the way for the preparation of large-scale amounts of pure tLRAT for further mechanistic and structural studies.     

Membrane phospholipids as an energy source in the operation of the visual cycle

Biology depends on the coupling of the free energy of hydrolysis of phosphate esters, such as ATP, to drive processes which would otherwise be thermodynamically unfavorable. Carboxyl esters are like phosphate esters in their ability to hydrolyze with substantial negative free energies, enabling them to participate in group transfer processes as well. In particular, membrane phospholipids constitute an enormous store of potential energy that could be used to fuel energetically unfavorable processes. One such process involves the biosynthesis of 11-cis-retinal, the chromophore of rhodopsin, from all-trans-retinol (vitamin A). The difference in free energy between an all-trans retinoid and its corresponding 11-cis retinoid is approximately 4 kcal/mol. This energy is provided for in a minimally two-step process involving membrane phospholipids as the energy source. First, all-trans-retinol is esterified in the retinal pigment epithelium by lecithin retinol acyl transferase (LRAT) to produce an all-trans-retinyl ester. Second, this ester is transformed into 11-cis-retinol by an isomerohydrolase in a process that couples the negative free energy of hydrolysis of the acyl ester to the formation of the strained 11-cis-retinol.     

Molecular logic of 11-cis-retinoid biosynthesis in a cone-dominated species

The biochemical pathway to visual chromophore biosynthesis in rod-dominated animals involves minimally a two component system in which all-trans-retinyl esters, generated by the action of lecithin retinol acyltransferase (LRAT) on vitamin A, are processed into 11-cis-retinol by isomerohydrolase. Possible differences in retinoid metabolism in cone-dominated animals have been noted in the literature, so it was of interest to explore whether these differences are tangential or fundamental. Central to this issue is whether cone-dominated animals use an isomerohydrolase (IMH)-based mechanism in the predominant pathway to 11-cis-retinoids. Here, it is shown that all-trans-retinyl esters (tREs) are the direct precursors of 11-cis-retinol formation in chicken retinyl pigment epithelium/retina preparations. This conclusion is based on at least three avenues of evidence. First, reagents that block tRE synthesis from vitamin A also block 11-cis-retinol synthesis. Second, pulse-chase experiments also establish that tREs are the precursors to 11-cis-retinol. Finally, 11-cis-retinyl-bromoacetate, a known affinity-labeling agent of isomerohydrolase, also blocks chromophore biosynthesis in the cone system.     

Stereochemical inversion at C-15 accompanies the enzymatic isomerization of all-trans- to 11-cis-retinoids

all-trans-Retinol (vitamin A) is processed by membranes from the pigment epithelium of the amphibian or bovine eye to form 11-cis-retinoids. When the isomerization reaction is performed with either [15(S)-3H,14C]-all-trans-retinol or [15(R)-3H,14C]-all-trans-retinol as substrate, the resultant 11-cis-retinals, formed by the in vitro enzymatic oxidation of the retinols, retain their 3H in the former case and lose it in the latter. The ocular all-trans- (pro-R specific) and 11-cis-retinol (pro-S specific) dehydrogenases operate with different stereochemistries with respect to the prochiral methylene hydroxyl centers of their substrates. Inversion of stereochemistry at the prochiral retinol centers was shown to accompany the isomerization process in both the amphibian and bovine systems. The 11-cis-retinol formed from [15(S)-3H,14C]-all-trans-retinol was chemically isomerized with I2 to produce [15(R)-3H,14C]-all-trans-retinol. The 11-cis-retinol formed from [15(R)-3H,14C]-all-trans-retinol was chemically isomerized with I2 to produce [15(S)-3H,14C]-all-trans-retinol. The stereochemistry at the prochiral center of retinol is not affected by the I2-catalyzed double-bond isomerization process and, hence, inversion of stereochemistry at C-15 must accompany isomerization. The same inverted stereochemistry was found with the associated retinyl palmitates. Possible mechanistic reasons for the observed inversion of stereochemistry during isomerization are discussed.     

Synthesis of the all-trans-retinal chromophore of retinal G protein-coupled receptor opsin in cultured pigment epithelial cells.

Light-dependent production of 11-cis-retinal by the retinal pigment epithelium (RPE) and normal regeneration of rhodopsin under photic conditions involve the RPE retinal G protein-coupled receptor (RGR) opsin. This microsomal opsin is bound to all-trans-retinal which, upon illumination, isomerizes stereospecifically to the 11-cis isomer. In this paper, we investigate the synthesis of the all-trans-retinal chromophore of RGR in cultured ARPE-hRGR and freshly isolated bovine RPE cells. Exogenous all-trans-[(3)H]retinol is incorporated into intact RPE cells and converted mainly into retinyl esters and all-trans-retinal. The intracellular processing of all-trans-[(3)H]retinol results in physiological binding to RGR of a radiolabeled retinoid, identified as all-trans-[(3)H]retinal. The ARPE-hRGR cells contain a membrane-bound NADPH-dependent retinol dehydrogenase that reacts efficiently with all-trans-retinol but not the 11-cis isomer. The NADPH-dependent all-trans-retinol dehydrogenase activity in isolated RPE microsomal membranes can be linked in vitro to specific binding of the chromophore to RGR. These findings provide confirmation that RGR opsin binds the chromophore, all-trans-retinal, in the dark. A novel all-trans-retinol dehydrogenase exists in the RPE and performs a critical function in chromophore biosynthesis.     

Recovery of visual functions in a mouse model of Leber congenital amaurosis

The visual process is initiated by the photoisomerization of 11-cis-retinal to all-trans-retinal. For sustained vision the 11-cis-chromophore must be regenerated from all-trans-retinal. This requires RPE65, a dominant retinal pigment epithelium protein. Disruption of the RPE65 gene results in massive accumulation of all-trans-retinyl esters in the retinal pigment epithelium, lack of 11-cis-retinal and therefore rhodopsin, and ultimately blindness. We reported previously (Van Hooser, J. P., Aleman, T. S., He, Y. G., Cideciyan, A. V., Kuksa, V., Pittler, S. J., Stone, E. M., Jacobson, S. G., and Palczewski, K. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 8623-8628) that in Rpe65-/- mice, oral administration of 9-cis-retinal generated isorhodopsin, a rod photopigment, and restored light sensitivity to the electroretinogram. Here, we provide evidence that early intervention by 9-cis-retinal administration significantly attenuated retinal ester accumulation and supported rod retinal function for more than 6 months post-treatment. In single cell recordings rod light sensitivity was shown to be a function of the amount of regenerated isorhodopsin; high doses restored rod responses with normal sensitivity and kinetics. Highly attenuated residual rod function was observed in untreated Rpe65-/- mice. This rod function is likely a consequence of low efficiency production of 11-cis-retinal by photo-conversion of all-trans-retinal in the retina as demonstrated by retinoid analysis. These studies show that pharmacological intervention produces long lasting preservation of visual function in dark-reared Rpe65-/- mice and may be a useful therapeutic strategy in recovering vision in humans diagnosed with Leber congenital amaurosis caused by mutations in the RPE65 gene, an inherited group of early onset blinding and retinal degenerations.     

Lecithin-retinol acyltransferase is essential for accumulation of all-trans-retinyl esters in the eye and in the liver

Lecithin-retinol acyltransferase (LRAT), an enzyme present mainly in the retinal pigmented epithelial cells and liver, converts all-trans-retinol into all-trans-retinyl esters. In the retinal pigmented epithelium, LRAT plays a key role in the retinoid cycle, a two-cell recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. We disrupted mouse Lrat gene expression by targeted recombination and generated a homozygous Lrat knock-out (Lrat-/-) mouse. Despite the expression of LRAT in multiple tissues, the Lrat-/- mouse develops normally. The histological analysis and electron microscopy of the retina for 6-8-week-old Lrat-/- mice revealed that the rod outer segments are approximately 35% shorter than those of Lrat+/+ mice, whereas other neuronal layers appear normal. Lrat-/- mice have trace levels of all-trans-retinyl esters in the liver, lung, eye, and blood, whereas the circulating all-trans-retinol is reduced only slightly. Scotopic and photopic electroretinograms as well as pupillary constriction analyses revealed that rod and cone visual functions are severely attenuated at an early age. We conclude that Lrat-/- mice may serve as an animal model with early onset severe retinal dystrophy and severe retinyl ester deprivation.     

11-cis-Acyl-CoA:retinol O-acyltransferase activity in the primary culture of chicken Muller cells

A novel retinoid cycle has recently been identified in the cone-dominated chicken retina, and this cone cycle accumulates 11-cis-retinyl esters upon light adaptation. The purpose of this study is to investigate how 11-cis-retinyl esters are formed in the retina. Primary cultures of chicken Muller cells and cell membrane were incubated with all-trans- or 11-cis-retinol to study retinyl ester synthesis. In Muller cells, esterification of 11-cis-retinol was four times greater than esterification of all-trans-retinol. In the presence of palmitoyl-CoA and CRALBP, Muller cell membranes synthesized 11-cis-retinyl ester from 11-cis-retinol at a rate which was 20-fold higher than that of all-trans-retinyl ester. In the absence of CRALBP, 11-cis-retinyl ester synthesis was greatly reduced (by 7-fold). In the absence of palmitoyl-CoA, retinyl ester synthesis was not observed. Muller cell membranes incubated with radiolabeled palmitoyl-CoA resulted in the transfer of the labeled acyl group to retinol. This acyl transfer was greatly reduced in the presence of progesterone, a known ARAT inhibitor. 11-cis-ARAT activity remained unchanged when assayed in the presence of all-trans-retinol, suggesting a distinct catalytic activity from that of all-trans-ARAT. Apparent kinetic rates for 11-cis-ARAT were 0.135 nmol min(-)(1) mg(-)(1) (V(max)) and 11.25 microM (K(M)) and for all-trans-ARAT were 0.0065 nmol min(-)(1) mg(-)(1) (V(max)) and 28.88 microM (K(M)). Our data indicate that Muller cells in the chicken retina possess 11-cis-ARAT activity, thus providing an explanation for the accumulation of 11-cis-retinyl esters in the cone cycle.