Chapter 6 Interphotoreceptor retinol-binding protein: A possible role in retinoid transport to the retina

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.

A cleavable affinity biotinylating agent reveals a retinoid binding role for RPE65

Retinal pigment epithelial (RPE) membranes contain the full biochemical apparatus capable of processing all-trans-retinol (vitamin A) into 11-cis-retinal, the visual chromophore. As many of these proteins are integral membrane proteins and resistant to traditional methods of identification, alternate methods of identifying these proteins are sought. The approach described here involves affinity biotinylation with alkali cleavable linkers. A vitamin A containing affinity-labeling haloacetate is described which facilitates the identification of retinoid binding proteins (RBPs). Treatment of crude bovine RPE membranes with (3R)-3-[boc-lys(biotinyl)-O]-all-trans-retinol chloroacetate 1 in the low micromolar range led to the specific labeling of RPE65 and lecithin retinol acyltransferase (LRAT). Only RPE65 is labeled at 5 microM 1 at 4 degrees C. Labeled RPE65 was readily isolated by binding the labeled protein to avidin-containing beads, followed by cleavage of the protein from the beads at pH 11. Trypsin digestion of RPE65 modified by 1, followed by mass spectrometry, demonstrates that C231 and C448 are alkylated by 1. These studies validate the approach that was used, and furthermore demonstrate that RPE65, a major membrane-associated protein of the RPE, is a RBP.

Characterization of SPACR, a sialoprotein associated with cones and rods present in the interphotoreceptor matrix of the human retina: immunological and lectin binding analysis

Rod and cone photoreceptors project from the outer retinal surface into a carbohydrate-rich interphotoreceptor matrix (IPM). Unique IPM glycoconjugates are distributed around rods and cones. Wheat germ agglutinin (WGA) strongly decorates the rod matrix domains and weakly decorates the cone matrix domains. This study characterizes the major WGA-binding glycoprotein in the human IPM, which we refer to as SPACR (sialoprotein associated with cones and rods). SPACR, which has a molecular weight of 147 kDa, was isolated and purified from the IPM by lectin affinity chromatography. A polyclonal antibody to SPACR was prepared that colocalizes in tissue preparations with WGA-binding domains in the IPM. Sequential digestion of SPACR with N- and O-glycosidases results in a systematic increase in electrophorectic mobility, indicating the presence of both N- and O-linked glycoconjugates. Complete deglycosylation results in a reduction in the relative molecular mass of SPACR by about 30%. Analysis of lectin binding allowed us to identify some of the structural characteristics of SPACR glycoconjugates. Treatment with neuraminidase exposes Galbeta1-3GalNAc disaccharide as indicated by positive peanut agglutinin (PNA) staining, accompanied by the loss of WGA staining. Maackia amurensis agglutinins (MAA-1 and MAA-2), specific for sialic acid in alpha2-3 linkage to Gal, bind SPACR, while Sambucus nigra agglutinin (SNA), specific for alpha2-6 linked sialic acid, does not, indicating that the dominant glycoconjugate determinant on SPACR is the O-linked carbohydrate, NeuAcalpha2-3Galbeta1-3GalNAc. The abundance of sialic acid in SPACR suggests that this glycoprotein may contribute substantially to the polyanionic nature of the IPM. The carbohydrate chains present on SPACR could also provide sites for extensive crosslinking and participate in the formation of the ordered IPM lattice that surrounds the elongate photoreceptors projecting from the outer retinal surface.

The visual input stage of the mammalian circadian pacemaking system: I. Is there a clock in the mammalian eye?

Threads of evidence from recent experimentation in retinal morphology, neurochemistry, electrophysiology, and visual perception point toward rhythmic ocular processes that may be integral components of circadian entrainment in mammals. Components of retinal cell biology (rod outer-segment disk shedding, inner-segment degradation, melatonin and dopamine synthesis, electrophysiological responses) show self-sustaining circadian oscillations whose phase can be controlled by light-dark cycles. A complete phase response curve in visual sensitivity can be generated from light-pulse-induced phase shifting. Following lesions of the suprachiasmatic nuclei, circadian rhythms of visual detectability and rod outer-segment disk shedding persist, even though behavioral activity becomes arrhythmic. We discuss the converging evidence for an ocular circadian timing system in terms of interactions between rhythmic retinal processes and the central suprachiasmatic pacemaker, and propose that retinal phase shifts to light provide a critical input signal.

Processing and transport of retinoids by the retinal pigment epithelium

Recent developments regarding our understanding of retinoid processing and transport during the visual cycle and related events are reviewed. Retinoids are bound and protected by a cohort of retinoid binding proteins, each of which is unique. The production of retinol (Vitamin A) derivatives is accomplished by a group of membrane-bound enzymes, some of which appear to be coupled in their actions.

Photoreceptor survival-promoting activity in interphotoreceptor matrix preparations: characterization and partial purification

It is well established that many types of cells are dependent on trophic factors for their survival. We are investigating whether photoreceptor survival may also be dependent upon such a factor, as well as the possibility that the interphotoreceptor matrix (IPM), which lies between the outer retina and the RPE, may be a source of photoreceptor survival-promoting activities. Well-characterized cultures of embryonic chick retinal neurons and photoreceptors, in which the photoreceptors spontaneously degenerate between 7 and 10 days in culture, were used to test this hypothesis. Crude extracts of IPM were found to support three- to fourfold increases in photoreceptor survival in 10-day cultures. This response was dose-dependent, saturable, and specific for photoreceptors, since there was no difference in the number of non-photoreceptor neurons between treated and control cultures. This photoreceptor survival-promoting activity (PSPA) may function when present either as a substratum-bound molecule or as a medium supplement. PSPA is heat labile, sensitive to freeze-thawing, and stable only within a very narrow pH range. The activity binds to heparin affinity columns but, in contrast to the behavior of growth factors such as FGF, it can be eluted from the columns at low salt concentrations (0.5 M NaCl). PSPA is eluted from a Sephacryl S-200 gel filtration column in two regions with Mr = 400-450 kDa and 33 kDa, respectively. SDS-gel electrophoresis suggests that the high molecular weight activity is composed of aggregates of lower molecular weight molecules. While this factor has not been purified to homogeneity, the combination of heparin-agarose affinity chromatography and hydrophobic interaction chromatography on phenyl-Sepharose columns results in 720-fold purification of the activity, with one unit of PSPA activity corresponding to 40 ng protein. This partial purification is accompanied by substantial reduction in the complexity of protein patterns in silver-stained SDS-gels. These findings are potentially important since a macromolecular factor promoting photoreceptor survival could play a role in the normal development of the retina, and could also further our understanding of retinal degenerations such as retinitis pigmentosa.

Interphotoreceptor retinoid-binding protein: role in delivery of retinol to the pigment epithelium

The ability of interphotoreceptor retinoid-binding protein (IRBP) to facilitate the incorporation of retinol into retinyl esters by the retinal pigment epithelium (RPE) was examined in toad (Bufo marinus) eyecup preparations devoid of neural retina (RPE-eyecup). Solutions containing purified bovine IRBP and all-trans[3H]retinol were introduced into the vitreal cavity of the RPE-eyecup. After incubation at 22 degrees C, [3H]retinyl ester was extracted from the RPE cells and isolated by high performance liquid chromatography. All-trans[3H]retinyl ester formed in the RPE increased with time of incubation (up to 2 hr) and with concentration of IRBP (up to 10 microM). The increase with IRBP concentration accompanied, and presumably resulted from, an increased transfer of [3H]retinol to the RPE-eyecup. With higher concentration of IRBP (20-30 microM), both the amount of [3H]retinyl ester formed (relative to the peak value at 10 microM IRBP) and the overall molar content of endogenous retinyl ester were reduced. On the other hand, bovine serum albumin at relatively high concentration (90 microM) was less effective than 3 microM IRBP in supporting the formation of [3H]retinyl ester, and it did not reduce the level of native retinyl ester in the RPE. Using 3 microM IRBP, levels of [3H]retinyl ester formed were comparable to or exceeded those obtained with phosphatidyl choline (0.9 mg ml-1) or serum retinol-binding protein (3 microM). The data are consistent with the hypothesized role of IRBP as a carrier of retinol between the retina and RPE in the operation of the visual cycle.

Cloning of cDNAs encoding human interphotoreceptor retinoid-binding protein (IRBP) and comparison with bovine IRBP sequences

We have determined the sequence of the human interphotoreceptor retinoid-binding protein mRNA from three separately isolated cDNAs. The sequence is 4.28 kb long and encodes a protein of 1247 amino acids (aa) including a putative signal peptide and propeptide. The sequence is shorter (by about 1.67 kb) than the bovine mRNA with the major difference in the lengths located in the 3'-untranslated region. We suggest that this resulted from an insertion in the bovine gene or a large deletion from the human gene. The insertion/deletion is flanked on either side by sequences that are similar in the bovine and human sequences. Like the bovine polypeptide, the deduced protein sequence from the human cDNA contains a fourfold repeat, with each repeat containing about 300 aa. Among the four repeats, the identity is about 30-40%. The identity between the complete bovine and human polypeptide sequences is 84%. The identity between the nucleotide sequences is 83% (excluding the major insertion/deletion). Comparison with the bovine gene indicates that the human sequence may lack about 5-10 bp at the 5' end of the cDNA; it, however, includes a poly(A) tail at the 3' end. Thus, the human sequence is virtually full length, is similar to the bovine sequence, and contains a striking fourfold repeat.

Synthesis of an immunopathogenic fusion protein derived from a bovine interphotoreceptor retinoid-binding protein cDNA clone

We have extended the cDNA sequence of bovine interphotoreceptor retinoid-binding protein (IRBP) and subcloned one of the sequenced cDNA fragments into an expression vector. The nucleotide (nt) sequences of four bovine IRBP cDNA clones have been determined. These sequences when assembled cover the 3' proximal 3629 nt of the IRBP mRNA and encode the C-terminal 551 amino acids (aa) of IRBP. This cDNA sequence validates the intron: exon boundaries predicted from the gene. A 2-kb EcoRI insert from lambda IRBP2, one of the clones sequenced, encoding the C-terminal 136 aa of IRBP was subcloned into the expression vector pWR590-1. Escherichia coli carrying this plasmid construction, pXS590-IRBP, produced a fusion protein containing 583 N-terminal aa of beta-galactosidase, three linker aa residues, 136 C-terminal aa of IRBP and possibly a number of additional C-terminal residues due to suppressed termination. This 86-kDa fusion protein, purified by detergent/chaotrope extraction followed by reverse-phase high-performance liquid chromatography, cross-reacted with anti-bovine IRBP on Western blots. This protein induced an experimental autoimmune uveo-retinitis and experimental autoimmune pinealitis in Lewis rats indistinguishable from that induced by authentic bovine IRBP. Thus, it is evident that biological activity of this region of IRBP, as manifested by immuno-pathogenicity, is retained by the fusion protein.

The isolation of a gene encoding interphotoreceptor retinoid-binding protein

The gene locus for bovine IRBP, as well as several kilobases of sequence flanking the gene on either end, has been cloned. Two of the several clones seem to contain full-length copies of the protein encoding portion of the gene. Using these clones and cDNA clones, we have determined that there are one or perhaps two copies of the IRBP gene per haploid genome in several species. The gene is compact considering the large size of the protein (145,000 daltons) and its large mRNA (about 6.5 kb). Surprisingly, the gene is no more than 14 kb, being fully contained on lambda clones of maximum packaging size 20 kb. Small parts of the gene were sequenced for the purpose of proving the identity of the genomic clones. DNA sequencing of one of the IRBP gene clones demonstrates the existence of an intron in the gene. The sequence analysis of another fragment identified the N-terminus which has been sequenced at the protein level. The DNA sequence analysis showed the existence of a putative signal sequence and the potential existence of a short five amino acid sequence between the signal sequence and the authentic N-terminus of the secreted extracellular IRBP. This confirms and validates the finding of the extra five amino acid sequence that is present on 40-50% of the polypeptides in monkey and human IRBP which have been isolated from the subretinal space. The presence of the appropriate gene sequence for the pentapeptide but its absence in bovine IRBP indicates differences in processing among the vertebrate IRBPs.

Cyanogen bromide fragments of bovine interphotoreceptor retinoid-binding protein induce experimental autoimmune uveoretinitis in Lewis rats

Interphotoreceptor retinoid-binding protein (IRBP), a retinal specific antigen, induces experimental autoimmune uveoretinitis (EAU) when injected into Lewis rats. Here we report that certain cyanogen bromide fragments of IRBP are capable of inducing EAU. Bovine IRBP, reduced and S-carboxymethylated, was subjected to cyanogen bromide cleavage. This CNBr digest was subjected to reversed-phase high performance liquid chromatography. Three fragments were purified to apparent homogeneity. These three fragments were subjected to gas-phase amino-terminal sequencing analysis. All three yielded single sequences, confirming their purity. On the basis of this amino-terminal sequencing and sequencing of cDNAs encoding bovine IRBP, two of these sequences, named CB-58 and CB-71, were localized to the C-terminal one-third of the IRBP molecule, whereas the third, a subfragment thought to result from cleavage at a tryptophan residue and named CB-47, was localized to the N-terminal one third of the protein. CB-71 and CB-47 shared a strong homology, suggesting a putative internal gene duplication event in the evolution of IRBP. All three of these fragments when injected into Lewis rats caused moderately severe EAU with early onset at relatively low doses. The histopathologic changes induced were indistinguishable from those caused by the intact protein. It would seem, therefore, that bovine IRBP contains multiple uveitogenic sites.

Immunocytochemical markers revealing retinal and pineal but not hypothalamic photoreceptor systems in the Japanese quail

The retinal proteins opsin, alpha-transducin, S-antigen and interstitial retinol-binding protein (IRBP) are essential for the processes of vision. By use of immunocytochemistry we have employed antibodies directed against these "photoreceptor proteins" in an attempt to identify the photoreceptor systems (retina, pineal and deep brain) of the Japanese quail. Opsin immunostaining was identified within many outer (basal portion) and inner segments of retinal photoreceptor cells and limited numbers of photoreceptor perikarya. Opsin immunostaining was also demonstrated in limited numbers of pinealocytes with all parts of these cells being immunoreactive. These results differ from previous observations. In contrast to the results obtained with the antibody against opsin, S-antigen and alpha-transducin immunostaining was seen throughout the entire outer segments and many photoreceptor perikarya of the retina, in the pineal organ immunostaining was seen in numerous pinealocytes in all follicles. These results conform to previous findings in birds. In addition, IRBP has been demonstrated for the first time in the avian retina and pineal organ. These findings underline the structural and functional similarities between the retina and pineal organ and provide additional support for a photoreceptive role of the avian pineal. No specific staining was detected in any other region of the brain in the Japanese quail; the hypothalamic photoreceptors of birds remain unidentified.

Dominantly inherited retinitis pigmentosa. Ultrastructure and biochemical analysis

A 66-year-old white man had dominant retinitis pigmentosa. He developed progressive restriction of his visual field, night blindness, pallor of the optic discs, pigmentary retinopathy and posterior subcapsular cataracts. Postmortem examination of the eyes included electron microscopy and biochemical analysis of cyclic nucleotides and interphotoreceptor retinoid-binding protein (IRBP). Except for the fovea and periphery, the retina showed extensive gliosis and neuronal loss with loss of photoreceptor cells. The choriocapillaris was variably occluded in the regions of absent retinal pigment epithelium (RPE). In places, the pigment epithelium invaded the retina to the level of the internal limiting membrane. Biochemical analysis revealed that the interphotoreceptor retinoid-binding protein (IRBP), an important glycoprotein of the interphotoreceptor space, was virtually absent even in retinal areas where photoreceptor cells were still present. Cyclic nucleotide determinations indicated a decrease in the cyclic GMP concentration that reflected the general loss of photoreceptor elements. On the other hand the cyclic AMP levels in all retinal areas tested were abnormally elevated, indicating the possible involvement of this nucleotide in the pathogenesis of the disease.

Distribution and axial diffusion of retinol in bleached rod outer segments of frogs (Rana pipiens)

Isolated retinas and rod outer segments from frogs (Rana pipiens) were exposed to light that produced axially uniform total bleaches of rhodopsin. Using fluorescence video microscopy, it was shown that the formation and equilibrium distribution of all-trans-retinol, the final chromophore product of rhodopsin bleaching is axially uniform. This result shows that the rate and amount of oxidoreductase-mediated reduction of all-trans-retinal to all-trans-retinol is not affected by the relative age of the disk membranes to which the enzymes are bound. Therefore previously reported axial differences in regeneration of rhodopsin and recovery of photocurrent after exposure to bright light probably are not due to axial differences in the formation of rhodopsin photoproducts. In addition, measurements on individual rod cells show that there is no significant redistribution of retinol for up to 2 hr following localized partial bleaches of rhodopsin. This raises the perplexing question of how retinol is shuttled between disk membranes and the pigment epithelium during visual pigment regeneration following substantial bleaches.