Characterization of a plasma retinol-binding protein membrane receptor expressed in the retinal pigment epithelium

Retinoids in chemoprevention and differentiation therapy

Retinoids are essential for the maintenance of epithelial differentiation. As such, they play a fundamental role in chemoprevention of epithelial carcinogenesis and in differentiation therapy. Physiological retinoic acid is obtained through two oxidation steps from dietary retinol, i.e. retinol-->retinal-->retinoic acid. The latter retinal-->retinoic acid step is irreversible and eventually marks disposal of this essential nutrient, through cytochrome P450-dependent oxidative steps. Mutant mice deficient in aryl hydrocarbon receptor (AHR) accumulate retinyl palmitate, retinol and retinoic acid. This suggests a direct connection between the AHR and retinoid homeostasis. Retinoids control gene expression through the nuclear retinoic acid receptors (RARs) alpha, beta and gamma and 9-cis-retinoic acid receptors alpha, beta and gamma, which bind with high affinity the natural ligands all-trans-retinoic acid and 9-cis-retinoic acid, respectively. Retinoids are effective chemopreventive agents against skin, head and neck, breast, liver and other forms of cancer. Differentiation therapy of acute promyelocytic leukemia (APL) is based on the ability of retinoic acid to induce differentiation of leukemic promyelocytes. Patients with relapsed, retinoid-resistant APL are now being treated with arsenic oxide, which results in apoptosis of the leukemic cells. Interestingly, induction of differentiation in promyelocytes and consequent remission of APL following retinoid therapy depends on expression of a chimeric PML-RAR alpha fusion protein resulting from a t(15;17) chromosomal translocation. This protein functions as a dominant negative against the function of both PML and RARs and its overexpression is able to recreate the phenotypes of the disease in transgenic mice. The development of new, more effective and less toxic retinoids, alone or in combination with other drugs, may provide additional avenues for cancer chemoprevention and differentiation therapy.

Rapid restoration of visual pigment and function with oral retinoid in a mouse model of childhood blindness

Mutations in the retinal pigment epithelium gene encoding RPE65 are a cause of the incurable early-onset recessive human retinal degenerations known as Leber congenital amaurosis. Rpe65-deficient mice, a model of Leber congenital amaurosis, have no rod photopigment and severely impaired rod physiology. We analyzed retinoid flow in this model and then intervened by using oral 9-cis-retinal, attempting to bypass the biochemical block caused by the genetic abnormality. Within 48 h, there was formation of rod photopigment and dramatic improvement in rod physiology, thus demonstrating that mechanism-based pharmacological intervention has the potential to restore vision in otherwise incurable genetic retinal degenerations.

Vitamin A in pregnancy: requirements and safety limits

Most of the functions of vitamin A are mediated through the binding of retinoic acid to specific nuclear receptors that regulate genomic expression. Recent experimental work in transgenic mice showed clearly that normal embryonic development depends on the correct spatial and temporal expression of the receptors in the differentiating cells and on the binding of specific forms of retinoic acid. This implies that the parent compound, vitamin A, is available in adequate forms and quantities. Excessive dietary intake of vitamin A has been associated with teratogenicity in humans in <20 reported cases over 30 y. However, caution must be exercised to avoid unnecessary supplementation of women of childbearing age. Hypovitaminosis A affects millions of women and children worldwide. The main consequence of a poor vitamin A supply during pregnancy is a low vitamin A status at birth and in the next few months. Vitamin A deficiency is strongly associated with depressed immune function and higher morbidity and mortality due to infectious diseases such as diarrhea, measles, and respiratory infections. Vitamin A deficiency is often associated with an increased mother-to-child transmission of HIV-1. The initiation of vitamin A supplementation should be carefully examined in each case according to the risk-to-benefit ratio. The final decision should take into account the estimated vitamin A status of the woman, the availability of vitamin A-rich foods in her diet, and whether supplementation can be supervised.

Retinoic acid: its biosynthesis and metabolism

This article presents a model that integrates the functions of retinoid-binding proteins with retinoid metabolism. One of these proteins, the widely expressed (throughout retinoid target tissues and in all vertebrates) and highly conserved cellular retinol-binding protein (CRBP), sequesters retinol in an internal binding pocket that segregates it from the intracellular milieu. The CRBP-retinol complex appears to be the quantitatively major form of retinol in vivo, and may protect the promiscuous substrate from nonenzymatic degradation and/or non-specific enzymes. For example, at least seven types of dehydrogenases catalyze retinal synthesis from unbound retinol in vitro (NAD+ vs. NADP+ dependent, cytosolic vs. microsomal, short-chain dehydrogenases/reductases vs. medium-chain alcohol dehydrogenases). But only a fraction of these (some of the short-chain de-hydrogenases/reductases) have the fascinating additional ability of catalyzing retinal synthesis from CRBP-bound retinol as well. Similarly, CRBP and/or other retinoid-binding proteins function in the synthesis of retinal esters, the reduction of retinal generated from intestinal beta-carotene metabolism, and retinoic acid metabolism. The discussion details the evidence supporting an integrated model of retinoid-binding protein/metabolism. Also addressed are retinoid-androgen interactions and evidence incompatible with ethanol causing fetal alcohol syndrome by competing directly with retinol dehydrogenation to impair retinoic acid biosynthesis.

Studies on the cellular uptake of retinol binding protein and retinol

The uptake and release of (125)I-RBP and of holoRBP labeled with [(3)H]retinol ((3)H-ROH) were studied in two cell lines which synthesize and secrete RBP, the HepG2 hepatocarcinoma cell line and the Caki-1 kidney adenocarcinoma cell line, and in HeLa cells that do not express the endogenous RBP gene. In all three cell lines a part of endocytosed (125)I-RBP is recycled to the extracellular medium and part is degraded. Nonspecific endocytosis of (125)I-RBP was estimated to be approximately 10% of total endocytosed (125)I-RBP. In HepG2 cells the (3)H-ROH from the [(3)H]retinol-RBP complex ((3)H-ROH-RBP) is recycled bound to RBP into serum-free chase medium. This (3)H-ROH recycling is blocked in HepG2 cells by cyclohexymide and by brefeldin A, an inhibitor of protein export from the main secretory route, and is absent in HeLa cells, which do not synthesize RBP. These data suggest that at least part of retinol taken up from exogenous holoRBP is delivered to newly synthesized RBP. (3)H-ROH recycled by HeLa cells is bound to serum albumin, as is a portion of that recycled by HepG2 cells. Transfer of (3)H-ROH from RBP to serum albumin does not occur in the absence of cells. We conclude that RBP is endocytosed through a specific pathway and that the RBP-associated retinol is transferred to newly synthesized RBP or to serum albumin.

Identification of RPE65 in transformed kidney cells

The protein RPE65 has an important role in retinoid processing and/or retinoid transport in the eye. Retinoids are involved in cell differentiation, embryogenesis and carcinogenesis. Since the kidney is known as an important site for retinoid metabolism, the expression of RPE65 in normal kidney and transformed kidney cells has been examined. The RPE65 mRNA was detected in transformed kidney cell lines including the human embryonic kidney cell line HEK293 and the African green monkey kidney cell lines COS-1 and COS-7 by reverse transcription PCR. In contrast, it was not detected in human primary kidney cells or monkey kidney tissues under the same PCR conditions. The RPE65 protein was also identified in COS-7 and HEK293 cells by Western blot analysis using a monoclonal antibody to RPE65, but not in the primary kidney cells or kidney tissues. The RPE65 cDNA containing the full-length encoding region was amplified from HEK293 and COS-7 cells. DNA sequencing showed that the RPE65 cDNA from HEK293 cells is identical to the RPE65 cDNA from the human retinal pigment epithelium. The RPE65 from COS-7 cells shares 98 and 99% sequence identity with human RPE65 at the nucleotide and amino acid levels, respectively. Moreover, the RPE65 mRNA was detected in three out of four renal tumor cultures analyzed including congenital mesoblastic nephroma and clear cell sarcoma of the kidney. These results demonstrated that transformed kidney cells express this retinoid processing protein, suggesting that these transformed cells may have an alternative retinoid metabolism not present in normal kidney cells.

Retinoids and mammalian development

All vertebrate embryos require retinoic acid (RA) for fulfilment of the developmental program encoded in the genome. In mammals, maternal homeostatic mechanisms minimize variation of retinoid levels reaching the embryo. Retinol is transported as a complex with retinol-binding protein (RBP): transplacental transfer of retinol and its uptake by the embryonic tissues involves binding to an RBP receptor at the cell surface. Embryonic tissues in which this receptor is present also contain the retinol-binding protein CRBP I and the enzymes involved in RA synthesis; the same tissues are particularly vulnerable to vitamin A deficiency. In the nucleus, the RA signal is transduced by binding to a heterodimeric pair of retinoid receptors (RAR/RXR). In general, the receptors show functional plasticity, disruption of one RAR or RXR gene having minor or no effects on embryogenesis. However, genetic studies indicate that RXR alpha is essential for normal development of the heart and eye. Excess RA causes abnormalities of many systems; altered susceptibility to RA excess in mice lacking RAR gamma or RXR alpha suggests that the teratogenic signal is transduced through different receptors compared with physiological RA function in the same tissue.

Retinol-binding protein is produced by rabbit chondrocytes and responds to parathyroid hormone (PTH)/PTH-related peptide-cyclic adenosine monophosphate pathway

PTH and dibutyryl cAMP [(Bu)2cAMP] induced the expression of a 19-kDa protein in the conditioned media of rabbit growth plate chondrocyte cultures. The 19-kDa protein was identified as plasma retinol-binding protein (RBP) by aminoterminal sequence analysis and immunoblot analysis with an anti-RBP monoclonal antibody. Northern blot analysis showed that PTH, PTH-related peptide (PTHrP), and (Bu)2cAMP increased the RBP messenger RNA (mRNA) level in chondrocyte cultures. Further, both PTH and (Bu)2cAMP markedly induced the expression of RBP mRNA by about 10-fold at 3 h and by about 40-fold at 24 h, indicating a pretranslational regulation. The level of the mRNA expression induced by PTH, PTHrP, and (Bu)2cAMP was as high as that by retinoic acid (RA), known as a potent inducer of RBP in hepatoma cells. RBP mRNA was also detected in cartilage tissues at higher levels than in the other tissues examined except liver. Both RBP and PTH/PTHrP inhibited the dedifferentiative activity of RA on growth plate chondrocytes when added to the culture medium. These results demonstrate that chondrocytes synthesize and secrete RBP in vivo and in vitro and suggest that PTH/PTHrP modulates the effect of RA by means of RBP production in chondrocytes.

Intracellular localization and membrane topology of 11-cis retinol dehydrogenase in the retinal pigment epithelium suggest a compartmentalized synthesis of 11-cis retinaldehyde

11-cis retinol dehydrogenase (EC 1.1.1.105) catalyses the last step in the biosynthetic pathway generating 11-cis retinaldehyde, the common chromophore of all visual pigments in higher animals. The enzyme is abundantly expressed in retinal pigment epithelium of the eye and is a member of the short chain dehydrogenase/reductase superfamily. In this work we demonstrate that a majority of 11-cis retinol dehydrogenase is associated with the smooth ER in retinal pigment epithelial cells and that the enzyme is an integral membrane protein, anchored to membranes by two hydrophobic peptide segments. The catalytic domain of the enzyme is confined to a lumenal compartment and is not present on the cytosolic aspect of membranes. Thus, the subcellular localization and the membrane topology of 11-cis retinol dehydrogenase suggest that generation of 11-cis retinaldehyde is a compartmentalized process.

Retinitis pigmentosa: defined from a molecular point of view

Retinitis pigmentosa (RP) denotes a group of hereditary retinal dystrophies, characterized by the early onset of night blindness followed by a progressive loss of the visual field. The primary defect underlying RP affects the function of the rod photoreceptor cell, and, subsequently, mostly unknown molecular and cellular mechanisms trigger the apoptotic degeneration of these photoreceptor cells. Retinitis pigmentosa is very heterogeneous, both phenotypically and genetically. In this review we propose a tentative classification of RP based on the functional systems affected by the mutated proteins. This classification connects the variety of phenotypes to the mutations and segregation patterns observed in RP. Current progress in the identification of the molecular defects underlying RP reveals that at least three distinct functional mechanisms may be affected: 1) the daily renewal and shedding of the photoreceptor outer segments, 2) the visual transduction cascade, and 3) the retinol (vitamin A) metabolism. The first group includes the rhodopsin and peripherin/RDS genes, and mutations in these genes often result in a dominant phenotype. The second group is predominantly associated with a recessive phenotype that results, as we argue, from continuous inactivation of the transduction pathway. Disturbances in the retinal metabolism seem to be associated with equal rod and cone involvement and the presence of deposits in the retinal pigment epithelium.

Cloning and localization of RPE65 mRNA in salamander cone photoreceptor cells1

RPE65 is a potential retinoid-processing protein expressed in the retinal pigment epithelium. Mutations in the RPE65 gene have been shown to cause certain inherited retinal dystrophies. Previous studies have shown that salamander cone photoreceptor cells have a unique retinoid processing mechanism which is distinct from that of rods. To determine whether RPE65 is expressed in photoreceptors, the RPE65 cDNA was cloned from a salamander retinal cDNA library. The deduced protein consists of 533 amino acids and is 85% identical to human and bovine RPE65. The RPE65 mRNA was detected in all of the single cone cells isolated from the salamander retina, as well as in the retinal pigment epithelium by RT-PCR, but not in the isolated rods. The RT-PCR products have been confirmed to be RPE65 by DNA sequencing. The results indicate that this potential retinoid processing protein is expressed in the cone photoreceptor cells but not in rods. Therefore, this protein may contribute to the unique retinoid processing capabilities in salamander cones.

A receptor for the lipocalin placental protein 14 on human monocytes

Human placental protein 14 (PP14), a member of the lipocalin structural superfamily, is an abundant amniotic fluid glycoprotein with documented immunoinhibitory activities. While receptors have been characterized for several other lipocalins, none have been reported to date for PP14. In the present study, two-color immunofluorescence and flow cytometry was used to screen peripheral blood mononuclear cell subpopulations for their capacity to engage fluoresceinated recombinant PP14. The tagged PP14 bound strongly in a specific and saturable fashion to CD14+ (monocyte lineage) cells, but not to CD20+ (B cell lineage) or CD3+ (T cell lineage) cells. This binding was both pH- and temperature-sensitive, and was reduced by proteolytic pre-digestion of the cells with trypsin or proteinase K. Scatchard analysis demonstrated a single class of receptors on CD14+ cells, with a K(D) of approximately 1 x 10(-8) and approximately 10-35,000 receptors per cell. These findings constitute the first report of a cell surface-associated binding protein for PP14 and set the stage for exploring the molecular mechanisms of PP14-mediated signaling and immunomodulation.

Molecular cloning and hormonal regulation of a murine epididymal retinoic acid-binding protein messenger ribonucleic acid

A complementary DNA encoding the mouse epididymal secretory protein MEP 10 (mouse epididymal protein 10) was cloned and is now renamed murine epididymal retinoic acid binding protein (mE-RABP). The analysis of the predicted primary amino acid sequence showed that mE-RABP has a 75% identity with rat ESP I (epididymal secretory protein I), another epididymal retinoic acid-binding protein. The homology strongly suggests that mE-RABP is the mouse orthologue of rat ESP I. A computer analysis of the predicted three-dimensional structure confirmed that mE-RABP can accommodate retinoic acid as ligand. In the rat, ESP I messenger RNA (mRNA) is expressed in the efferent ducts and in the entire caput epididymidis. However, in the mouse, the expression of a 950-bp mE-RABP mRNA was detected only in principal cells of the mid/distal caput epididymidis, suggesting that the regulation of region-specific expression is different in rat and mouse. Northern blot analyses showed that mE-RABP gene expression is no longer detected 10 days after castration but progressively rebounds between days 15 and 60. However, mE-RABP protein could not be detected by Western blot 30 days after castration. Androgen replacement, begun 5 days after castration and continued for 4 days restored significant expression of mE-RABP mRNA. Efferent duct ligation for 10 days did not affect gene expression. Taken together, these results indicate that mE-RABP mRNA expression is regulated by androgens but not by testicular factors. The overall similarity in the primary amino acid sequence of mE-RABP with ESP I and other members of the lipocalin superfamily suggests that they are evolutionarily related.

The transfer of retinol from serum retinol-binding protein to cellular retinol-binding protein is mediated by a membrane receptor

The hypothesis that the cellular uptake of retinol involves the specific interaction of a plasma membrane receptor with serum retinol-binding protein (RBP) at the extracellular surface followed by ligand transfer to cytoplasmic cellular retinol-binding protein (CRBP) has been investigated. The experimental system consisted of the [3H]retinol-RBP complex, Escherichia coli-expressed recombinant apo-CRBP containing the 10 amino acid long streptavidin-binding peptide sequence at its C terminus (designated as CRBP-Strep) and permeabilized human placental membranes. [3H]Retinol transfer from RBP to CRBP-Strep was monitored by measuring the radioactivity associated with CRBP-Strep retained by an immobilized streptavidin resin. Using this assay system, we have demonstrated that optimal retinol uptake is achieved with holo-RBP, the membrane receptor and apo-CRBP. The effects are specific: other binding proteins, including beta-lactoglobulin and serum albumin, despite their ability to bind retinol, failed to substitute for either RBP or apo-CRBP. The process is facilitated by membranes containing the native receptor suggesting that this protein is an important component in the transfer mechanism. Taken together, the data suggest that the RBP receptor, through specific interactions with the binding proteins, participates (either directly or via associated proteins) in the mechanism which mediates the transfer of retinol from extracellular RBP to intracellular CRBP.

Identification of a mechanism to localize generation of retinoic acid in rat embryos

Vitamin A (retinol) is essential for normal mammalian development. However, its biological activity depends upon its conversion to retinoic acid (RA), a local mediator of cellular proliferation and differentiation. Previous studies have shown that embryonic RA is found specifically in tissues known to depend upon vitamin A for normal development and that its production follows uptake of maternal retinol. The aim of this study was to identify the mechanism for tissue-specific generation of RA in developing rat embryos. Here we show immunohistochemical localization of the retinol binding protein receptor, cellular retinol binding protein, retinol dehydrogenase and retinal dehydrogenase in rat embryos (presomitic to the 25-30 somite pair stage). These proteins are proposed to be responsible for cellular uptake of retinol, its intracellular transport and its conversion to RA. Thus, they potentially constitute the entire metabolic pathway from vitamin A to RA. All four proteins were detected specifically in tissues that are known to depend upon vitamin A for normal development including the yolk sac, heart, gut, notochord, somites, sensory placodes and the limb. Furthermore, our previous studies have demonstrated that uptake of retinol into the yolk sac depends upon a retinol binding protein receptor. Here we provide evidence that this mechanism functions also in the heart. Colocalization of cellular retinol binding protein, retinol and retinal dehydrogenase with the retinol binding protein receptor in tissues dependent upon vitamin A for normal development suggests that coordinate functioning of these proteins is responsible for cellular uptake of circulating retinol and its metabolism to RA. This is the first evidence of a tissue-specific mechanism for generation of RA from its precursor retinol in the developing embryo.

Mutations in RPE65 cause autosomal recessive childhood-onset severe retinal dystrophy

Autosomal recessive childhood-onset severe retinal dystrophy (arCSRD) designates a heterogeneous group of disorders affecting rod and cone photoreceptors simultaneously. The most severe cases are termed Leber congenital amaurosis (LCA), while the less aggressive forms are usually considered juvenile retinitis pigmentosa. Recently, mutations in the retinal-specific guanylate cyclase gene were found in patients with LCA. Disease genes implicated in other forms of arCSRD are expected to encode proteins present in the neuroretina or in the retinal pigment epithelium (RPE). The RPE, a monolayer of cells separating the vascular-rich choroid and the neuroretina, is in intimate contact with the outer segments of rods and cones via the microvilli surrounding the photoreceptors. The RPE expresses a tissue-specific and evolutionarily highly conserved 61 kD protein (RPE65) present at high levels in vivo. Although the function of RPE65 is not yet known, an important role in the RPE/photoreceptor vitamin-A cycle is suggested by the fact that RPE65 associates both with serum retinol-binding protein and with the RPE-specific 11-cis retinol dehydrogenase, an enzyme active in the synthesis of the visual pigment chromophore 11-cis retinal. Here we report that the analysis of RPE65 in a collection of about 100 unselected retinal-dystrophy patients of different ethnic origin revealed five that are likely to be pathogenic mutations, including a missense mutation (Pro363Thr), two point mutations affecting splicing (912 + 1G-->T and 65 + 5G-->A) and two small re-arrangements (ins144T and 831del8) on a total of nine alleles of five patients with arCSRD. In contrast to other genes whose defects have been implicated in degenerative retinopathies, RPE65 is the first disease gene in this group of inherited disorders that is expressed exclusively in the RPE, and may play a role in vitamin-A metabolism of the retina.

RPE65, the major retinal pigment epithelium microsomal membrane protein, associates with phospholipid liposomes

The retinal pigment epithelium (RPE)-specific protein RPE65 is the major protein of the RPE microsomal membrane fraction. Though RPE65 lacks transmembrane domains or signal peptide, detergents are required for its maximally effective solubilization in isotonic buffers. However, in 0.75-1.0 M KCl, RPE65 is as soluble without detergent, indicating a peripheral membrane association. We wished to understand why this non-membrane-inserted protein was so closely associated with RPE microsomal membranes. To explore the possible involvement of interactions with phospholipids, an isotonic salt-soluble extract of RPE was incubated with phosphatidylcholine (PC)/phosphatidylserine (PS)/phosphatidylinositol liposomes and centrifuged to sediment the liposomes. RPE65 cosedimented with the liposome pellet. RPE65 also cosedimented with synthetic dipalmitoyl-, 1-palmitoyl, 2-docosahexaenoyl-PC or dipalmitoyl-PS liposomes. Incubation with 1 mM Ca2+ or 1 mM EGTA had no effect, indicating a Ca2+-independent association. A spectrophotometric assay showed that this interaction of RPE65 with phospholipid vesicles resulted in increased light scattering, consistent with phospholipid vesicle aggregation. Resonance energy transfer experiments showed that any putative aggregation occurred without subsequent vesicle fusion. This PC affinity was further confirmed by incubation of RPE extract with dimyristoyl-PC-immobilized artificial membrane (IAM.PC) matrix. The RPE65 selectively bound and was elutable with 2% detergent. This RPE65-phospholipid liposome association may explain the solubilization characteristics of RPE65 and may be related to the function of RPE65 and to its physical association with the RPE smooth endoplasmic reticulum.

Uptake and metabolism of [3H]retinoic acid delivered to human foreskin keratinocytes either bound to serum albumin or added directly to the culture medium

Retinoic acid (RA), a potent modulator of cell proliferation and differentiation is present in plasma bound to serum albumin. The biologic significance or source of plasma RA is not clear. Although most cellular RA is believed to be made in situ via the oxidation of retinol, plasma RA could potentially provide target cells with a source of preformed RA. To investigate RA uptake, we have used a model system of human foreskin keratinocytes (HKc) cultured in serum-free media to compare the uptake and metabolism of [3H]RA added directly to the culture medium in ethanol to that delivered bound to bovine serum albumin (BSA). [3H]RA added directly to the culture medium was rapidly taken up by HKc during the first 10 min of incubation (25-35% of the applied RA), no further accumulation occurred between 10 min and 90 min, and then cell-associated radioactivity rapidly decreased to about 3-5% of the applied dose by 12 h. In contrast, when [3H]RA was delivered to HKc bound to BSA, total cell-associated radioactivity reached about 2.5% of the applied dose by 5 min, increased to 3-5% of the applied radioactivity by 1 h, and no further accumulation or loss occurred over the next 23 h. The uptake by HKc of [3H]RA delivered bound to BSA or added directly to the culture medium was not influenced by pre-treatment of the cells for 72 h with unlabeled RA or by excess unlabeled RA added at the time of uptake. Analysis of the cells and media by high-performance liquid chromatography for RA metabolites found that [3H]RA added directly to the medium is rapidly converted by HKc to polar compounds that are subsequently excreted back into the medium. Also, RA added directly to the medium was susceptible to degradation in the absence of cells. In marked contrast, [3H]RA added to the media bound to BSA was much less susceptible to degradation in the absence of cells, and few [3H]RA metabolites were found in the media even after exposure to HKc for 24 h. The binding of RA to albumin clearly protects RA from conversion to polar metabolites, and also provides for a controlled delivery of RA from the aqueous extracellular environment to the cell surface.

Retinol-binding protein and asialo-orosomucoid are taken up by different pathways in liver cells

The intracellular transport and degradation of in vivo endocytosed retinol-binding protein was compared with that of asialo-orosomucoid, a marker for receptor-mediated endocytosis through coated pits. The transport pathways were studied in rat liver cells by means of subcellular fractionation in Nycodenz and sucrose density gradients and by immunoelectron microscopy. Retinol-binding protein and asialo-orosomucoid were labeled by covalent attachment of radioiodinated tyramine cellobiose, an adduct which is incapable of crossing cellular membranes and thus provides a marker for the organelles where the protein has been taken up and degraded. The data obtained from subcellular fractionation studies, as well as from immunoelectron microscopy, showed that retinol-binding protein and asialo-orosomucoid were initially localized in different endocytic vesicles. Retinol-binding protein co-localized in density gradients with markers for potocytosis, an alternative endocytic pathway which uses internalization through caveolae instead of clathrin-coated pits. Later, retinol-binding protein and asialo-orosomucoid comigrated in the gradients and they were also observed in the same larger vesicles by immunoelectron microscopy. These data suggest that retinol-binding protein is taken up by liver cells by potocytosis and that a fraction of the retinol-binding protein is later transferred to larger vesicles located deeper in the cytoplasm where degradation takes place.

Multiple molecular recognition properties of the lipocalin protein family

The lipocalins, a diverse family of small extracellular ligand binding proteins, display a remarkable range of different molecular recognition properties. While their binding of small hydrophobic molecules, and to a lesser extent their binding to cell surface receptors, is well known, it is shown here that formation of macromolecular complexes is also a common feature of this family. Analysis of known crystallographic structures reveals that the lipocalins possess a conserved common structure: an antiparallel beta-barrel with a repeated +1 topology. Comparisons show that within this overall similarity the structure of individual proteins is specifically adapted to bind their particular ligands, forming a binding site from an internal cavity (within the barrel) and/or an external loop scaffold, which gives rise to different binding modes that reflects the need to accommodate ligands of different shape, size, and chemical structure. The architecture of the lipocalin fold suggests that both the ends and sides of this barrel are topologically distinct, differences also apparent in analyses of structural and sequence variation within the family. These differences can be linked to experimental evidence suggesting a possible functional dichotomy between the two ends of the lipocalin fold. The structurally invariant end of the molecule may be implicated in general binding to common cell surface receptors, while the more variable end is adapted to the specialised tasks of binding small ligands and forming macromolecular complexes via an exposed binding surface.

Retinol in avian oogenesis: molecular properties of the carrier protein

Normal embryo development in oviparous (egg-laying) species requires the coordinated targeting to growing oocytes of nutrients and regulatory molecules such as retinol, the precursor of active retinoids. Serum retinol-binding protein (RBP) is the major carrier protein for retinol in the circulatory system of vertebrates. In oviparous animals, RBP is thought to function in the delivery of retinol to yolk, in analogy to other important nutrients and vitamins known to accumulate within the oocyte. Here, immunoelectron microscopy revealed that RBP indeed accumulates in yolk, in particular in the electron-lucent phase of yolk organelles known to harbor other serum-derived yolk proteins and their receptors. To gain understanding of the RBP-mediated serum-to-yolk transport of retinol, we have characterized the chicken carrier protein at the molecular level. The essential function of RBPs is emphasized by the first known avian RBP structure, which confirms that these vitamin carriers are among the most highly conserved serum proteins known. Interestingly, by analysis of RBP hepatic RNA and serum protein levels, we identified a unique property of chicken RBP relative to other known RBPs and yolk precursors, i.e., the absence of estrogen induction. One cause of the observed reduction in RBP RNA is an estrogen-dependent decrease of RBP gene transcription. Furthermore, Northern blot analysis of tissues of the hen demonstrated a lack of RBP synthesis by the oocyte or other ovarian cells, confirming the exogenous (hepatic) origin of yolk RBP. These results provide strong evidence that chicken RBP is an essential serum-to-yolk vitamin carrier with certain properties different from those of other such transporters.

Cellular retinol-binding protein type I is prominently and differentially expressed in the sensory epithelium of the rat cochlea and vestibular organs

To understand the possible role of retinoic acid during inner ear development and cellular regeneration, we have examined the expression pattern of two intracellular retinoid-binding proteins, the cellular retinol- and retinoic acid-binding proteins of type I in the developing and mature rat inner ear. Expression of cellular retinol-binding protein type I was seen in the supporting cells of the organ of Corti and vestibular organs as soon as the first signs of differentiation of the adjacent hair cells were seen. In the developing organ of Corti, the expression pattern followed the basal-to-apical coil differentiation gradient. After the 1st postnatal week, detectable expression of cellular retinol-binding protein type I disappeared from the organ of Corti, but persisted in the supporting cells of vestibular organs throughout life. Expression of cellular retinoic acid-binding protein type I was not found in the inner ear sensory epithelia. Cellular retinol-binding protein type I has previously been shown to act as a substrate carrier in the synthesis of retinoic acid from its precursor, retinol. Our data suggest that retinoic acid is synthesized in the developing sensory epithelium of the cochlear and vestibular organs and that a concentration gradient formed by retinoic acid may have a role in differentiation of the cochlear sensory epithelium. Furthermore, retinoic acid may have a role in damage-induced hair cell regeneration in the developing and mature vestibular organs as well as in the developing auditory organ. The absence of cellular retinol-binding protein type I from the supporting cells of the mature organ of Corti may be associated with the inability of this organ to regenerate hair cells after damage.

The lipocalin protein family: a role in cell regulation

The lipocalins, a large, diverse, but relatively poorly understood family of small extracellular proteins, are characterized by the ability to bind small hydrophobic molecules, such as retinol, and by their binding to specific cell surface receptors. These general properties suggest such proteins as appropriate transporters transferring biologically hazardous molecules in a safe and controlled manner between cells. Moreover, many lipocalins have been implicated in the regulation of cell homeostasis: apolipoprotein D, quiescience specific protein, purpurin, alpha-1-microglobulin, and NGAL. This combination of direct and indirect evidence suggests that the lipocalin protein family may be involved, in a quite general way, in the mediation of cell regulation and that many presently functionless family members might act in this way.

Localization of cellular retinoid-binding proteins suggests specific roles for retinoids in the adult central nervous system

Retinoic acid, the active metabolite of retinoids (vitamin A compounds), is thought to act as a gene regulator via ligand-activated transcription factors. In order to investigate possible roles of retinoids and retinoid-controlled gene expression in brain function, we have used immunohistochemistry to localize the possible presence of two intracellular retinoid-binding proteins, cellular retinol-binding protein type I and cellular retinoic acid-binding protein type I, in the adult rat central nervous system. We find a widespread, yet distinct, presence of these two binding proteins in the brain and spinal cord. Most of the immunoreactivity is neuronal, including cell somata, as well as dendritic and axonal processes and axon terminals. Cellular retinol-binding protein type I-immunoreactivity is also found in the walls of cerebral blood vessels, the meninges, the choroid plexus, certain ependymal cells, tanocytes and certain other glial elements. The cellular retinol-binding protein type I- and cellular retinoic acid-binding protein type I-immunoreactivity patterns appear to be almost exclusively non-overlapping. Very strong cellular retinol-binding protein type I-immunoreactivity is found in the dendritic layers of the hippocampal formation and dentate gyrus. Cellular retinol-binding protein type I-immunoreactivity is also present in layer 5 cortical pyramidal neurons and neurons in the glomerular layer of the olfactory bulb. Many other areas, e.g. hypothalamic nuclei and amygdala areas, contain networks of varicose cellular retinol-binding protein type I-immunoreactive nerve fibers. The medial amygdaloid nucleus contains strongly cellular retinol-binding protein type I-positive neurons. Cellular retinoic acid-binding protein type I-immunoreactivity is more restricted in the adult brain. Strong cellular retinoic acid-binding protein type I-immunoreactivity is, however, found in a population of medium-sized neurons scattered throughout the striatum, in neurons in the glomerular layer of the olfactory bulb, the olfactory nerve and in a group of nerve cells close to the third ventricle in hypothalamus. The remarkably selective patterns of cellular retinol-binding protein type I- and cellular retinoic acid-binding protein type I-immunoreactivity discovered in the adult rat brain suggest that retinoids have important roles as regulators of gene expression in normal brain function. The high levels of cellular retinol-binding protein type I-immunoreactivity found in hippocampus suggest that one such role might relate to brain plasticity.