Retinoid-binding proteins in craniofacial development

Cephalic neural crest cells are known to form the frontonasal mesenchyme and contribute to the mesenchyme of the visceral arches. Retinoids affect neural crest cells and their derivatives during development, and thus cause craniofacial, thymus, and conotruncal heart malformations. In addition, retinoids induce malformations of the central nervous system (CNS). Retinoic acid (RA) and its congeners accumulate in a saturable manner in neural crest and neural crest-derived cells, in the hindbrain, and the spinal cord of mouse embryos. Cellular retinoic acid-binding protein (CRABP) was localized by immunohistochemistry in the same areas as were the labelled RA congeners. Thus, CRABP and RA congeners were found in the transitional zone between surface ectoderm and neuropeithelium, from where neural crest cells are known to emanate (day 8 1/2). Later, specific labelling was found in the frontonasal mesenchyme and in the visceral arches. Also in the trunk, neural crest cells were labelled. In CNS, strong staining was seen in the rhombomeres (especially numbers 4-6) of the hindbrain and in the spinal cord. Retinol and cellular retinol-binding protein (CRBP) were more evenly distributed, with exception of surface ectoderm, epithelium of gut, and myocardium, where CRBP was specifically expressed. These findings are discussed in relation to the differential expression of nuclear RA receptors and homeobox genes in the craniofacial region and in the hindbrain. It is possible that RA is important for the normal pattern formation in these regions and acts as a morphogen as previously proposed in limb development.

Identification and partial characterization of a retinal pigment epithelial membrane receptor for plasma retinol-binding protein

We have developed a membrane binding assay by which we have been able to characterize the interaction between 125I-labeled retinol-binding protein and its receptor in microsome fractions derived from retinal pigment epithelial cells. The binding of retinol-binding protein to the membranes was fast, with a dissociation constant in the range of 31-72 nM, and maximum binding occurred at neutral pH. Receptor binding sites were also found in microsome fractions of liver and kidney, whereas lung and muscle contained few, if any. Chemical cross-linking of retinol-binding protein to the microsomal membranes yielded a major molecular complex of Mr 86,000 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The protein responsible for binding of retinol-binding protein was identified as a Mr 63,000 protein using a label transfer cross-linking technique. Further characterization demonstrated that the receptor for retinol-binding protein is a terminally glycosylated membrane protein noncovalently associated with a high molecular weight complex.

Comparison of acid ethanol extraction and acid gel filtration prior to IGF-I and IGF-II radioimmunoassays: improvement of determinations in acid ethanol extracts by the use of truncated IGF-I as radioligand

Insulin-like growth factor binding proteins interfere in the IGF-I and -II radioimmunoassays. In an attempt to overcome this problem, we have compared the use of truncated IGF-I, with reduced IGFBP affinity, and IGF-I as radioligands for IGF-I RIA measurements in serum separated by acid gel filtration or acid ethanol extraction followed by cryo-precipitation. With truncated IGF-I as radioligand the IGF-I measurements in acid gel filtrates and acid ethanol extracts were significantly correlated in healthy subjects (N = 42, r = 0.91, p less than 0.001) and in patients with acromegaly (N = 10, r = 0.85, p less than 0.01), GH deficiency (N = 10, r = 0.88, p less than 0.001) or Type I diabetes mellitus (N = 10, r = 0.90, p less than 0.001). In contrast, the IGF-I concentrations in acid ethanol extracts determined with IGF-I as radioligand did not correlate with those in acid gel filtrates using truncated IGF-I radioligand in patients with acromegaly (r = 0.61, NS) or GH deficiency (r = 0.46, NS). In the latter group the mean IGF-I concentrations measured in acid ethanol extracts were erroneously elevated by 112%. Low-affinity antibodies used for IGF-II RIA determinations failed to give reliable results in acid ethanol extracts from patients with Type I diabetes mellitus or GH deficiency. In conclusion, erroneously high IGF-I concentrations owing to binding of the radioligand to IGFBPs not completely removed by acid ethanol extraction can be avoided by the use of truncated IGF-I as radioligand.

Retinoic acid stimulates neurite outgrowth in the amphibian spinal cord

There is increasing evidence that retinoic acid (RA), a vitamin A metabolite, plays a role in the development of the nervous system. Here we specifically test this notion by examining the effect of RA on neurite outgrowth from explanted segments of the axolotl spinal cord. We show that there is a threshold concentration in the region of 0.1-1 nM above which neurite outgrowth is stimulated 4-5 fold. Retinol, by contrast, only stimulated the migration of glial cells from the explants. Using HPLC we demonstrate that RA and retinol are present endogenously in the axolotl spinal cord. In addition, we have identified by immunocytochemistry with antipeptide antibodies the cells of the spinal cord that contain the binding proteins for RA (cellular RA-binding protein; CRABP) and retinol (cellular retinol-binding protein; CRBP). CRABP is found in the axons and CRBP is found in the ependyma and glial cells. These results provide strong evidence for a role for RA in the developing nervous system, and we propose a specific hypothesis involving CRBP, CRABP, retinol, and RA in the control of axon outgrowth in the spinal cord.

Retinoic acid-binding protein, rhombomeres and the neural crest

We have investigated by immunocytochemistry the spatial and temporal distribution of cellular retinoic acid-binding protein (CRABP) in the developing nervous system of the chick embryo in order to answer two specific questions: do neural crest cells contain CRABP and where and when do CRABP-positive neuroblasts first arise in the neural tube? With regard to the neural crest, we have compared CRABP staining with HNK-1 staining (a marker of migrating neural crest) and found that they do indeed co-localise, but cephalic and trunk crest behave slightly differently. In the cephalic region in tissues such as the frontonasal mass and branchial arches, HNK-1 immunoreactivity is intense at early stages, but it disappears as CRABP immunoreactivity appears. Thus the two staining patterns do not overlap, but are complementary. In the trunk, HNK-1 and CRABP stain the same cell populations at the same time, such as those migrating through the anterior halves of the somites. In the neural tube, CRABP-positive neuroblasts first appear in the rhombencephalon just after the neural folds close and then a particular pattern of immunoreactivity appears within the rhombomeres of the hindbrain. Labelled cells are present in the future spinal cord, the posterior rhombencephalon up to rhombomere 6 and in rhombomere 4 thus producing a single stripe pattern. This pattern is dynamic and gradually changes as anterior rhombomeres begin to label. The similarity of this initial pattern to the arrangement of certain homeobox genes in the mouse stimulated us to examine the expression of the chicken Hox-2.9 gene. We show that at stage 15 the pattern of expression of this gene is closely related to that of CRABP. The relationship between retinoic acid, CRABP and homeobox genes is discussed.

Localization of specific retinoid-binding sites and expression of cellular retinoic-acid-binding protein (CRABP) in the early mouse embryo

Retinoids (vitamin A derivatives) are important for normal embryogenesis and retinoic acid, an acidic derivative of vitamin A, was recently proposed to be an endogenous morphogen. Several retinoids are also potent teratogens. Using an autoradiographic technique, we have identified tissues and cells in early mouse embryos that are able to specifically accumulate a radiolabelled synthetic derivative of retinoic acid. Strong accumulation of radioactivity was seen in several neural crest derivatives and in specific areas of the CNS. Gel filtration analyses of cytosols from embryos that received the radiolabelled retinoid in utero suggested that cellular retinoic acid-binding protein (CRABP) was involved in the accumulation mechanism. Immunohistochemical localization confirmed that cells accumulating retinoids also expressed CRABP. Strong CRABP immunoreactivity was found in neural crest-derived mesenchyme of the craniofacial area, in visceral arches, in dorsal root ganglia and in cells along the gut and the major vessels of the trunk region. In CNS, CRABP expression and retinoid binding was largely restricted to the hindbrain, to a single layer of cells in the roof of the midbrain and to cells in the mantle layer of the neural tube. Our data suggest that cells in the embryo expressing CRABP are target cells for exogenous retinoids as well as endogenous retinoic acid. Retinoic acid may thus play an essential role in normal development of the CNS and of tissues derived from the neural crest. We propose that the teratogenic effects of exogenous retinoids are due to an interference with mechanisms by which endogenous retinoic acid regulates differentiation and pattern formation in these tissues.

Impaired release of vitamin A from liver in primary biliary cirrhosis

In 44 patients with primary biliary cirrhosis serum levels of vitamin A, retinol-binding protein and transthyretin (prealbumin) were found to be significantly lower than in 25 sex- and age-matched controls. Liver biopsies were available for chemical analyses in 28 of the patients. Their mean liver vitamin A concentration (2.8 +/- 2.0 mumoles per gm wet weight) did not differ significantly from that in 22 cases of sudden death which served as controls (2.0 +/- 1.5 mumoles per gm wet weight). Immunohistochemical investigation showed a normal distribution of serum retinol-binding protein in the patients' livers, whereas the staining pattern of cellular retinol-binding protein, believed to be involved in the intrahepatic transport of vitamin A, was abnormal. Thus, the number size and cellular retinol-binding protein staining intensity of fat-storing (Ito) cells were clearly higher in the patients as compared with controls. The results suggest that the low serum vitamin A levels in primary biliary cirrhosis are not a consequence of vitamin A deficiency but instead reflect a defective mobilization of vitamin A from the liver.

Quantitation and tissue localization of the cellular retinoic acid-binding protein

The distribution of the cellular retinoic acid-binding protein (CRABP) in some rat tissues has been determined, and the protein has been localized by immunocytochemical techniques in sections from rat testis. In the testis CRABP was found in the seminiferous tubuli with Sertoli cells and the spermatogonia most intensely stained. All other cells of the germinal epithelium appeared largely devoid of CRABP. By use of an enzyme-linked immunosorbent assay CRABP was quantitatively estimated in several tissues and the highest levels were found in testis and eye. Comparisons of the tissue levels of CRABP and of the cellular retinol-binding protein (CRBP) did not reveal any apparent correlation.

Antibodies against mammary derived growth inhibitor (MDGI) react with a fibroblast growth inhibitor and with heart fatty acid binding protein

A polypeptide growth inhibitor, designated as mammary derived growth inhibitor, has previously been purified from lactating bovine mammary glands. Polyclonal rabbit antiserum raised against mammary derived growth inhibitor cross-reacts with bovine heart fatty acid binding protein and bovine peripheral myelin P2 protein. These results are consistent with the observation that the amino acid sequence of mammary derived growth inhibitor showed homology to the sequences of these proteins. In a parallel series of immunoblotting experiments, rabbit anti-mammary derived growth inhibitor also reacted specifically with a fibroblast growth inhibitor purified from the conditioned medium of cultured mouse 3T3 fibroblasts. These data suggest that bovine mammary derived growth inhibitor and mouse fibroblast growth inhibitor may share common structural features and raise the possibility that these growth inhibitors may together define a new family of growth regulatory molecules.

Expression of mouse beta 2-microglobulin in frozen and formaldehyde-fixed central nervous tissues: comparison of tissue behind the blood-brain barrier and tissue in a barrier-free region

Previous work indicates that the weak expression in neural tissues of beta 2-microglobulin (beta 2-m) and major histocompatibility complex (MHC) class I gene products can be increased experimentally. Physiologic conditions in which greater neural MHC expression occurs are not well defined. Here we have asked whether protection from blood-borne antigens afforded by the blood-brain barrier is related to the lack of MHC expression. A rabbit antiserum raised against purified mouse beta 2-m was used in an immunocytochemical assay. The serum reacted strongly with lymphoid tissues and was inhibited by purified beta 2-m. No beta 2-m was detected in neurons or glia in any brain area examined. A barrier-free region, the area postrema, showed the same lack of neural cell staining. Blood vessel walls in the same sections were beta 2-m+. It is unlikely that these staining patterns are due to cell type-specific beta 2-m degradation, since frozen and formaldehyde-perfused, paraffin-embedded preparations gave similar results. Failure to detect beta 2-m in the area postrema suggests that passive exposure to environmental antigens, immunomodulators, or immunocompetent cells is not sufficient to induce neural class I expression. Rather, if increased expression of beta 2-m and class I occurs in vivo, additional stimulus is required.

Expression of mouse beta 2-microglobulin in frozen and formaldehyde-fixed central nervous tissues: comparison of tissue behind the blood-brain barrier and tissue in a barrier-free region

Previous work indicates that the weak expression in neural tissues of beta 2-microglobulin (beta 2-m) and major histocompatibility complex (MHC) class I gene products can be increased experimentally. Physiologic conditions in which greater neural MHC expression occurs are not well defined. Here we have asked whether protection from blood-borne antigens afforded by the blood-brain barrier is related to the lack of MHC expression. A rabbit antiserum raised against purified mouse beta 2-m was used in an immunocytochemical assay. The serum reacted strongly with lymphoid tissues and was inhibited by purified beta 2-m. No beta 2-m was detected in neurons or glia in any brain area examined. A barrier-free region, the area postrema, showed the same lack of neural cell staining. Blood vessel walls in the same sections were beta 2-m+. It is unlikely that these staining patterns are due to cell type-specific beta 2-m degradation, since frozen and formaldehyde-perfused, paraffin-embedded preparations gave similar results. Failure to detect beta 2-m in the area postrema suggests that passive exposure to environmental antigens, immunomodulators, or immunocompetent cells is not sufficient to induce neural class I expression. Rather, if increased expression of beta 2-m and class I occurs in vivo, additional stimulus is required.

Deleted ring chromosome 22 in a mentally retarded boy

A mentally retarded boy with a ring chromosome 22, where most of band q13 was deleted, is reported. The fact that the leucocyte beta-galactosidase and alpha-galactosidase B activities were normal, but the arylsulphatase A activity only half of the normal is consistent with a gene dosage effect and that the arylsulphatase A locus is located more distally, than the gene loci for the other two enzymes, in the deleted part of 22q13.

Increased levels of several retinoid binding proteins resulting from retinoic acid-induced differentiation of F9 cells

The embryonal carcinoma cell line F9 is known to differentiate when exposed to retinoic acid. We have examined the quantities of two intracellular retinoid-binding proteins in undifferentiated and differentiated F9 cells. The existence of a cell surface receptor that recognizes the plasma retinol-binding protein was also explored. It was shown that undifferentiated F9 cells contain low concentrations of the two retinoid-binding proteins. The cellular retinoic acid-binding protein was present in approximately 3-fold molar excess over the cellular retinol-binding protein. Upon culture in the presence of retinoic acid, F9 cells display elevated concentrations of both cellular retinol-binding protein and cellular retinoic acid-binding protein. Since the levels of beta 2-microglobulin, a marker of the differentiated state with no known involvement in the metabolism of vitamin A, increased in parallel with the retinoid-binding proteins, it seems unlikely that retinoic acid selectively increased the levels of the two retinoid-binding proteins. The differentiated, in contrast to the undifferentiated cells, can accumulate retinol from plasma retinol-binding protein and display a cell surface receptor for this protein. Despite the fact that retinoic acid-induced differentiation of F9 cells promotes increased levels of several proteins involved in the normal metabolism of vitamin A, no evidence was obtained to suggest that the cells were dependent on retinoids to maintain their differentiated state.

Hepatic retinol metabolism. Distribution of retinoids, enzymes, and binding proteins in isolated rat liver cells

The main retinoids and some binding proteins and enzymes involved in retinol metabolism have been quantified in different types of rat liver cells. Hepatic perisinusoidal stellate cells contained 28-34 nmol of retinoids/10(6) cells, and parenchymal liver cells contained 0.5-0.8 nmol of retinoids/10(6) cells, suggesting that as much as 80% of more of total liver retinoids might be stored in stellate cells with the rest stored in parenchymal cells. Isolated endothelial cells and Kupffer cells contained very low levels of retinoids. More than 98% of the retinoids recovered in stellate cells were retinyl esters. Isolated parenchymal and stellate cell preparations both contained considerable retinyl palmitate hydrolase and acyl-CoA:retinol acyltransferase activities. Parenchymal cells accounted for about 75-80% of the total hepatic content of these two enzyme activities, with the rest located in stellate cells. On a cell protein basis, the concentrations of both of these activities were much greater in stellate cells than in parenchymal cells. In contrast, cholesteryl oleate and triolein hydrolase activities were fairly evenly distributed in all types of liver cells. Large amounts of cellular retinol binding proteins were also found in parenchymal and stellate cells. Although parenchymal cells accounted for more than 90% of hepatic cellular retinol binding protein, the concentration of the protein in stellate cells (per unit protein) was 22 X greater than that in parenchymal cells. Stellate cells were also enriched in cellular retinoic acid binding protein. Thus, both parenchymal and stellate cells contain substantial amounts of retinoids and of the enzymes and intracellular binding proteins involved in retinol metabolism. Stellate cells are particularly enriched in these several components.

Cellular retinoid binding proteins

The cellular retinol-binding protein (CRBP) and the cellular retinoic acid binding protein (CRABP) have similar physicochemical characteristics. The amino acid sequences of rat CRBP and bovine CRABP have been elucidated and they display 40% sequence identity. Both protein sequences appear to be evolutionarily highly conserved. The amino acid sequence of human CRBP, deduced from a cDNA-clone, is 96% identical to the rat CRBP sequence. CRBP and CRABP are members of a protein family, all members of which may bind hydrophobic ligands and interact with membrane components. All members of the protein family are probably related in tertiary structure and might interact with membrane components through two regions with a high probability for alpha-helix. The tissue distribution of CRBP and CRABP, together with their relation to lipid transporting proteins suggests that CRBP and CRABP are cellular transporting proteins for retinol and retinoic acid, respectively.

The primary structure of rat liver cellular retinol-binding protein

The complete amino acid sequence of a cellular retinol-binding protein (CRBP) has been determined for the first time. The primary structure of rat liver CRBP was elucidated by analyses of cyanogen bromide fragments and peptides obtained by tryptic and thermolytic digestions. The single polypeptide chain of rat CRBP consists of 134 amino acid residues. Under reducing conditions, CRBP exists as a monomer, but, in the absence of reducing agents, dimers and multimers of the protein emerge. This is explained by the observation that CRBP contains 3 cysteines, one of which seems to be highly reactive. Whether CRBP contains a disulfide bond is not yet established. The present data extend the previously described homology between CRBP and a family of low molecular weight proteins, all members of which may bind hydrophobic ligands. Since some of these proteins apparently display intracellular transport functions, a similar role for CRBP is envisaged.

The primary structure of bovine cellular retinoic acid-binding protein

The complete amino acid sequence of bovine adrenal gland cellular retinoic acid-binding protein (CRABP) has been determined. The primary structure was established by analyses of cyanogen bromide fragments and peptides obtained by trypsin and Staphylococcus aureus protease digestions. The polypeptide chain of bovine CRABP comprises 136 amino acid residues. From partial sequence information, CRABP has been shown to be homologous to cellular retinol-binding protein, myelin protein P2, and the fatty acid-binding Z-protein. A comparison of the complete amino acid sequences of the members of this protein family, which also includes the rat intestinal fatty acid-binding protein, shows that CRABP is more similar to cellular retinol-binding protein and protein P2 than to the fatty acid-binding proteins. All five proteins are very similar in their NH2-terminal regions, suggesting that this part is important for a property common to the members of this protein family. This is the first report of a complete amino acid sequence of a CRABP.

The primary structure of rabbit and rat prealbumin and a comparison with the tertiary structure of human prealbumin

The primary structures of rabbit and rat prealbumin have been determined. The amino acid sequence of rabbit prealbumin was determined by analyses of peptides obtained by trypsin and Staphylococcus aureus protease digestions. The rat prealbumin sequence was deduced by analyses of tryptic peptides as well as by nucleotide sequencing of cDNA clones. Both amino acid sequences contain 127 amino acid residues, the same as human prealbumin. Pairwise comparisons show that the three sequences are more than 80% identical. All three prealbumins were found to display significant sequence homology with human thyroxine-binding globulin. A comparison of the primary structures of the prealbumins with the tertiary structure of human prealbumin shows that amino acid replacements are preferentially located at the surface of the molecule and in the loops connecting the beta-strands. The locations of the replacements are discussed as regards the different molecular interactions in which prealbumin is involved.

Amino acid sequence homologies between rabbit, rat, and human serum retinol-binding proteins

The main transporting protein for vitamin A in rabbit serum, the retinol-binding protein (RBP), was isolated and its amino acid sequence determined. Rabbit RBP was found to be highly homologous to human RBP, whose amino acid sequence was elucidated earlier, and to rat RBP. The rat RBP sequence was obtained by combining information deduced from the nucleotide sequences of two overlapping cDNA clones with the NH2-terminal sequence of the isolated protein determined by automated Edman degradation. The identity between the three proteins is approximately 90%. The high degree of homology between RBP molecules from different species is probably explained by the fact that RBP participates in at least three types of molecular interactions: in the binding of prealbumin, in the interaction with retinol, and in the recognition of a specific cell surface receptor. All these interactions should lead to a conservation of RBP structure. The amino acid differences between rabbit, rat, and human RBP are discussed in light of the recent elucidation of the three-dimensional structure of human RBP. Hybridization of a probe isolated from a rat RBP cDNA clone to restriction enzyme-digested genomic DNA from rat and mouse suggests that RBP is encoded by a single gene.