Transport and uptake of retinol during chicken oocyte growth

Managing genomic diversity in conservation programs of Chinese domestic chickens

BACKGROUND

Effective conservation and utilization of farm animals are fundamental for realizing sustainable increases in food production. In situ and ex situ conservation are the two main strategies that are currently used to protect the genetic integrity of Chinese domestic chicken breeds. However, genomic diversity and population structure have not been compared in these conserved populations.

RESULTS

Three hundred and sixty-one individuals from three Chinese domestic chicken breeds were collected from populations conserved in situ and ex situ and genotyped using genotyping-by-sequencing (GBS). First, we used different parameters based on heterozygosity, genomic inbreeding, and linkage disequilibrium to estimate the genomic diversity of these populations, and applied principal component analysis (PCA), neighbor-joining tree, and ADMIXTURE to analyze population structure. We found that the small ex situ conserved populations, which have been maintained in controlled environments, retained less genetic diversity than the in situ conserved populations. In addition, genetic differentiation was detected between the in situ and ex situ conserved populations of the same breed. Next, we analyzed signatures of selection using three statistical methods (fixation index (FST), nucleotide diversity (Pi), and cross-population extended haplotype homozygosity (XP-EHH) to study the genetic footprints that underlie the differentiation between in situ and ex situ conserved populations. We concluded that, in these small populations, differentiation might be caused by genetic drift or by mutations from the original populations. The differentiation observed in the population of Beijing You chicken probably reflects adaptation to environmental changes in temperature and humidity that the animals faced when they were moved from their place of origin to the new site for ex situ conservation.

CONCLUSIONS

Conservation programs of three Chinese domestic chicken breeds have maintained their genomic diversity to a sustainable degree. The small ex situ conserved populations, which are maintained in controlled environments, retain less genetic diversity than populations conserved in situ. In addition, the transfer of populations from their place of origin to another site for conservation purposes results in genetic differentiation, which may be caused by genetic drift or adaptation. This study provides a basis for further optimization of in situ and ex situ conservation programs for domestic chicken breeds in China.

Transport of the Thyroid Hormone Carrier Protein Transthyretin into Human Epidermoid Cells

Purpose: Transthyretin (TTR) is a protein with a growing number of biological functions in addition to its well-established binding and circulatory transport of thyroxine, and indirect retinoid transport through interaction with retinol-binding protein. Misfolded and aggregated wild-type and mutant TTRs are involved in amyloid diseases. Several aspects of TTR pathology and physiology remain poorly understood. Receptor-mediated cellular transport of TTR has been described in a few cell types; and such studies suggest the possibility of different TTR receptors and endocytic pathways. Our main objective was to further understand the endocytic pathways for TTR.Methods: In the current study, analyses of TTR endocytic transport were performed in the human A431 cell line. The results of TTR uptake were compared with those of the iron-carrier protein transferrin (Tf, a common stardard for endocytosis studies) in the same cell types.Results: A comparison of TTR and Tf endocytosis suggested similar early, 5-10 min, accumulation kinetics. But at a later time, 30 min, TTR accumulation was 20-30% lower than that of Tf (p < .05), a result that suggests different post-endocytic fates for these two ligands. Through the use of multiple endocytosis inhibitors, biochemical evidence is provided for an internalization pathway that differs from the clathrin-mediated endocytosis of Tf.Conclusions: These results for A431 cells are compared with others reported for different cell types; and it is suggested that this same hormone carrier protein can transit into cells through multiple endocytic pathways.

Comparative proteome analysis of egg yolk plasma proteins during storage

BACKGROUND

Physical changes such as chicken egg white thinning and egg yolk flattening occur during storage, implying a decline in egg quality. To reveal the deteriorative process related to chicken egg internal quality, a comparative proteomic method was used in this study to analyze the alterations in egg yolk plasma proteins at different storage times (0, 20 and 40 days) under an ambient temperature of 22 ± 2 °C.

RESULTS

Using two-dimensional electrophoresis followed by matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry, 33 protein spots representing 12 proteins were identified with significant (P < 0.05) alterations in abundance at different storage times. The proteins that showed significant changes in abundance included serum albumin, vitellogenin fragments, IgY chains, ovalbumin, ovoinhibitor, α₂ -macroglobulin-like protein 1-like, hemopexin, transthyretin, apolipoprotein A-I and β₂ -glycoprotein I precursor. Accelerating degradation for most egg yolk plasma proteins was observed after prolonged storage (from day 20 to day 40).

CONCLUSION

It is likely that the increased degradation of protease inhibitors such as ovoinhibitor and α₂ -macroglobulin-like protein 1-like during prolonged storage lead to an imbalance of protease and antiprotease in egg yolk, which may play a key role in the degradation of egg yolk proteins. These findings will provide an insight into the effects of storage on egg yolk protein changes and give a deeper understanding of the deteriorative process of chicken egg yolk. © 2016 Society of Chemical Industry.

Comparative analysis of granulosa cell gene expression in association with oocyte competence in FSH-stimulated Holstein cows

Ovarian stimulation with exogenous FSH followed by FSH withdrawal or ‘coasting’ is an effective means of increasing the number of oocytes obtainable for the in vitro production of cattle embryos. However, the quality of the oocytes thus obtained varies considerably from one cow to the next. The aim of the present study was to gain a better understanding of the follicular conditions associated with low oocyte developmental competence. Granulosa cells from 94 Holstein cows in a commercial embryo production facility were collected following ovarian stimulation and coasting. Microarray analysis showed 120 genes expressed with a differential of at least 1.5 when comparing donors of mostly competent with donors of mostly incompetent oocytes. Using ingenuity pathway analysis, we revealed the main biological functions and potential upstream regulators that distinguish donors of mostly incompetent oocytes. These are involved in cell proliferation, apoptosis, lipid metabolism, retinol availability and insulin signalling. In summary, we demonstrated that differences in follicle maturity at collection could explain differences in oocyte competence associated with individual animals. We also revealed deficiencies in lipid metabolism and retinol signalling in granulosa cells from donors of mostly incompetent oocytes.

The developing chicken yolk sac acquires nutrient transport competence by an orchestrated differentiation process of its endodermal epithelial cells

During chicken yolk sac (YS) growth, mesodermal cells in the area vasculosa follow the migrating endodermal epithelial cell (EEC) layer in the area vitellina. Ultimately, these cells form the vascularized YS that functions in nutrient transfer to the embryo. How and when EECs, with their apical aspect directly contacting the oocytic yolk, acquire the ability to take up yolk macromolecules during the vitellina-to-vasculosa transition has not been investigated. In addressing these questions, we found that with progressive vascularization, the expression level in EECs of the nutrient receptor triad, LRP2-cubilin-amnionless, changes significantly. The receptor complex, competent for uptake of yolk proteins, is produced by EECs in the area vasculosa but not in the area vitellina. Yolk components endocytosed by LRP2-cubilin-amnionless, preformed and newly formed lipid droplets, and yolk-derived very low density lipoprotein, shown to be efficiently endocytosed and lysosomally processed by EECs, probably provide substrates for resynthesis and secretion of nutrients, such as lipoproteins. In fact, as directly demonstrated by pulse-chase experiments, EECs in the vascularized, but not in the avascular, region efficiently produce and secrete lipoproteins containing apolipoprotein A-I (apoA-I), apoB, and/or apoA-V. In contrast, perilipin 2, a lipid droplet-stabilizing protein, is produced exclusively by the EECs of the area vitellina. These data suggest a differentiation process that orchestrates the vascularization of the developing YS with the induction of yolk uptake and lipoprotein secretion by EECs to ensure embryo nutrition.

Transthyretin binds to glucose-regulated proteins and is subjected to endocytosis by the pancreatic β-cell

Transthyretin (TTR) is a functional protein in the pancreatic β-cell. It promotes insulin release and protects against β-cell death. We now demonstrate by ligand blotting, adsorption to specific magnetic beads, and surface plasmon resonance that TTR binds to glucose-regulated proteins (Grps)78, 94, and 170, which are members of the endoplasmic reticulum chaperone family, but Grps78 and 94 have also been found at the plasma membrane. The effect of TTR on changes in cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) was abolished if the cells were treated with either dynasore, a specific inhibitor of dynamin GTPase that blocks clathrin-mediated endocytosis, or an antibody against Grp78, that prevents TTR from binding to Grp78. The conclusion is that TTR binds to Grp78 at the plasma membrane, is internalized into the β-cell via a clathrin-dependent pathway, and that this internalization is necessary for the effects of TTR on β-cell function.

Thyroid system-disrupting chemicals: interference with thyroid hormone binding to plasma proteins and the cellular thyroid hormone signaling pathway

In vertebrates, thyroid hormones are essential for post-embryonic development, such as establishing the central nervous system in mammals and metamorphosis in amphibians. The present paper summarizes the possible extra-thyroidal processes that environmental chemicals are known to or suspected to target in the thyroid hormone-signaling pathway. We describe how such chemicals interfere with thyroid-hormone-binding protein functions in plasma, thyroid-hormone-uptake system, thyroid-hormone-metabolizing enzymes, and activation or suppression of thyroid-hormone-responsive genes through thyroid-hormone receptors in mammals and amphibian tadpoles. Several organohalogens affect different aspects of the extra-thyroidal thyroid-hormone-signaling pathway but hardly affect thyroid hormone binding to receptors. Rodents and amphibian tadpoles are most sensitive to the effects of environmental chemicals during specific thyroid-hormone-related developmental windows. Possible mechanisms by which environmental chemicals exert multipotent activities beyond one hormone-signaling pathway are discussed.

Studies on retinol-binding protein during vitellogenesis in the Rainbow Trout (Oncorhynchus mykiss)

Retinoids are important regulatory signaling molecules during embryonic development and therefore, should be present in the eggs of oviparous animals that develop independently of the maternal organism. Studies were initiated in Rainbow Trout to elucidate the role of retinol-binding protein (RBP), the specific retinol carrier protein in vertebrate plasma, during vitellogenesis. Plasma levels of RBP in pre-vitellogenic, vitellogenic, and post-vitellogenic females were compared to plasma of male trout, using a rabbit polyclonal antiserum that was generated to His-tagged RBP recombinant protein. Western-blot analyses showed that there were no differences in the relative plasma levels of RBP between pre-vitellogenic, vitellogenic or post-vitellogenic females and similar levels were also found in males. In contrast, strong elevation of vitellogenin (VTG) was observed in the plasma from vitellogenic females. Northern-blot analysis of hepatic mRNA revealed that there were no dramatic changes in the abundance of RBP transcripts in the liver of females during vitellogenesis, but showed a significant increase in the expression of VTG in the livers of vitellogenic females. These results indicate differences in the regulation of RBP and VTG during vitellogenesis, suggesting that RBP may not be the main transporter protein for retinoids to fish egg. Recent publications on the association of retinal with VTG in fish and the occurrence of RBP transcripts in ovarian tissues raise the need for reevaluation of the role of RBP during vitellogenesis in oviparous non-mammalian vertebrate species.

Carotenoid and retinoid transport to fish oocytes and eggs: what is the role of retinol binding protein?

Fish eggs contain carotenoids, retinals (retinal and dehydroretinal) and retinols (retinol, dehydroretinol and retinyl-esters) that are utilized during embryonic development, after fertilization. The carotenoids (mainly astaxanthins) are transported in the plasma by the low density lipoproteins, high density lipoproteins, and very high density lipoproteins (VHDL) and were found to be associated also with serum albumin. Retinals were found to be associated vitellogenin (VTG), a component of the plasma VHDL fraction that is internalized by oocytes during vitellogenesis. However, the transport of retinols and retinyl-esters that were located in the oil droplet fraction of homogenized eggs, has yet to be elucidated. Retinols are more abundant in freshwater fish eggs than in eggs of marine fish species. Since retinol is transported in the plasma of vertebrates in association with retinol binding protein (RBP), recent studies on the molecular characterization and expression sites of RBP, could contribute to determining the involvement of RBP in transporting retinol to developing oocytes in vertebrates.Recently, results from our laboratory show that RBP mRNA levels in the liver and RBP plasma levels did not significantly change with the onset and during vitellogenesis in the Rainbow trout. These results were in contrast with a dramatic elevation in the mRNA levels of VTG in the liver and an increase in VTG plasma levels that was observed in the same females. Moreover, 17beta-estradiol treatment of immature fish, resulted in relatively lower mRNA levels of RBP in the liver, concomitantly with an increase in the level of VTG transcripts and the appearance of VTG in the plasma of treated fish. In addition, RBP was localized in the cytosol of ovulated oocytes. These results for Rainbow trout are similar to those reported for the chicken but differ from those of Xenopus, where an increase in RBP mRNA was reported in the liver and higher levels of retinal and retinol were found in the plasma of 17beta-estradiol treated animals. The results, reported here for the first time in Rainbow trout, showing RBP transcripts in the ovary, oviduct (the ovarian tissue adjacent to the gonopore) and oocytes, suggest a modulating role for RBP in follicular development, as has been suggested for the bovine ovary.

Expression of retinol-binding protein messenger RNA and retinoic acid receptors in preattachment bovine embryos

In cattle, retinoic acid (RA) has been indirectly associated with developmental potential of the embryo. RA is transported by retinol-binding protein (RBP) and actions of RA are mediated by several subtypes of nuclear retinoic acid receptors (RAR). Bovine embryos, produced in vitro from oocytes harvested from ovaries collected at a local abattoir, were frozen in liquid nitrogen at the oocyte, 2-, 4-, 8-, 16 to 20-cell, morula, blastocyst, and hatched blastocyst stages. Employing reverse transcription polymerase chain reaction (RT-PCR) we investigated mRNA expression for RBP, RARalpha, RARbeta, RARgamma, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Total RNA was extracted from 25 pooled embryos at each stage and RT-PCR analysis was repeated thrice. GAPDH transcript was detected in all stages. Transcripts for RBP, RARalpha, and RARgamma were also detected in all stages from the oocyte through to the hatched blastocyst. Expression of RARbeta was not detected at any stage. Whole-mount immunohistochemistry was performed with intact and hatched blastocysts using polyclonal antibodies against RARalpha and RARgamma2 to investigate if these embryonic mRNAs were translated to the mature protein. Strong immunostaining was observed for both RARalpha and RARgamma2 in the inner cell mass and trophectoderm of intact and hatched blastocysts. Expression of mRNA for RBP, RARalpha, RARgamma, and of the RARalpha and RARgamma2 receptor proteins in the bovine embryo suggests that RA is likely to directly regulate gene expression during preimplantation development in that species.

Retinol binding protein in rainbow trout: molecular properties and mRNA expression in tissues

Retinoids are important regulatory signaling molecules during embryonic development. The molecular properties of rainbow trout (Oncorhynchus mykiss) retinol-binding protein (rtRBP), the specific retinol carrier in vertebrate plasma, were studied to elucidate its role in transporting retinols to developing fish oocytes. A 954-nucleotide rtRBP cDNA was cloned from the liver coding for a 176-amino-acid (aa) mature protein, with an estimated molecular mass of 20,267 Da. The nucleotide sequence suggests a putative 16-aa signal peptide and shows all the aa residues that were previously identified as critical for the retinol binding pocket. Five of the eight amino acid residues that are associated with the interaction of RBP and transthyretin in mammalian and non-mammalian species are conserved. The deduced aa sequence of rtRBP shows 60-66% identity with zebrafish, chicken, mouse, rat, horse, bovine, and human RBPs and 56% identity with Xenopus RBP. Northern blot analysis revealed a approximately 1.1-kb hepatic mRNA transcript. RBP is highly expressed in the liver, but low levels were also detected in the spleen, kidney, ovary, and brain. In the rainbow trout, 17beta-estradiol treatment led to a decrease in the RBP mRNA signal relative to that of the controls. The efficacy of the 17beta-estradiol treatment was verified by an induction of vitellogenin (VTG) mRNA expression in the liver and occurrence of VTG in the plasma.

Developmental expression, tissue distribution and hormonal regulation of fish (Sparus aurata) serum retinol-binding protein

Retinol-binding protein (RBP) is the specific carrier of retinol in vertebrates and forms a 1:1 complex with transthyretin (TTR). A cDNA encoding serum RBP was cloned from liver and 7-day larvae of the marine fish Sparus aurata. The mature protein is 176 amino acids long and shows sequence identity of 77-78%, 56%, 63% and 62% with rainbow trout, Xenopus, chicken and human RBP, respectively. Northern blot analysis of hepatic RBP revealed two transcripts: a major one of approximately 1.4-1.5 kb and a minor of approximately 0.7 kb. Distribution of RBP mRNA in various tissues was studied by RT-PCR and showed high expression in liver and skin, and low expression in brain, kidney and gill filament (20-35% of the level in liver). RBP expression in intestine, pyloric caeca, muscle and pituitary was estimated to be approximately 7-14% of the level in liver. The ontogeny of RBP expression in S. aurata was examined in unfertilized eggs, embryos and larvae by using RT-PCR followed by hybridization with a specific probe. RBP transcript was found in all larval stages studied. Very low levels of RBP mRNA were detected in unfertilized eggs and in embryos 8 h after fertilization with a gradual increase at 12 h and 15-16 h post-fertilization. A single injection of estradiol-17beta to S. aurata immature, bisexual fish or to adult males reduced steady-state levels of hepatic RBP by 37 and 25%, respectively. The same treatment induced vitellogenin expression. The present data suggest that in fish, liver is the main site of RBP synthesis, but that RBP may have an important function in fish skin. RBP is expressed early in embryonic development and in fish its expression can be down regulated by estrogen.

Contrasting effects of estrogen on transthyretin and vitellogenin expression in males of the marine fish, Sparus aurata

A partial cDNA encoding for the C-terminus of vitellogenin (VTG) was cloned from liver of Sparus aurata male treated with 17beta-estradiol (E(2)). E(2) treatment of S. aurata males resulted in increased synthesis and secretion of VTG protein into the plasma, determined by a specific enzyme-linked immunosorbent assay (ELISA) in a time-dependent manner. While VTG mRNA was induced by E(2) treatment, transthyretin (TTR) mRNA levels were reduced. These data provide the first demonstration that estrogen exhibits contrasting effect on VTG and on TTR gene expression in teleosts.

Receptor-mediated endocytosis of transthyretin by ependymoma cells

Transthyretin (TTR) is involved in the transport of thyroxine (T4) and retinol-binding protein (RBP) in cerebrospinal fluid (CSF) and serum. TTR is secreted in the CSF by the epithelial cells of choroid plexus. The binding of [(125)I]TTR to cultured ependymoma cells which form the brain cerebrospinal barrier, was studied to determine whether these cells carry receptor(s) for TTR. TTR was bound by ependymoma cells in a time-dependent manner reaching equilibrium within 2 h. Scatchard analysis was consistent with a single class of high-affinity binding sites with a K(d) of approximately 18 nM. Saturable high-affinity binding of human TTR has previously been described in rat primary hepatocytes and human renal adenocarcinoma, neuroblastoma, hepatoma and astrocytoma cells, and also transformed lung cells. Endocytosis of fluorescent or biotinylated TTR was observed in ependymoma cells in cytoplasmic vesicles but TTR did not colocalize with clathrin in endocytic coated vesicles. Endocytosis of TTR was inhibited by high sucrose concentration (0.45 M). Finally, ligand blotting and chemical-linking experiments revealed the presence of a approximately 100 kDa putative TTR receptor on the ependymoma cell membrane. Receptor binding of TTR provides a potential mechanism for the delivery of T4 within the central nervous system.

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.

Expression of retinol binding protein and transthyretin during early embryogenesis

Previous studies have shown that anterior lateral plate endoderm from stage 6 chicken embryos is necessary and sufficient to enable precardiac mesoderm to complete its cardiogenic program in vitro, culminating in a rhythmically contractile multicellular vesicle (Sugi and Lough [1994] Dev. Dyn. 200:155-162). To identify cardiogenic factors, we have begun to characterize proteins that are secreted by endoderm cell explants. Fluorography of proteins from endoderm-conditioned medium revealed 1-2 dozen bands, the most prominent of which migrated at approximately 17 and 25 kD. The bulk of the 17-kD band, which migrates near FGFs and subunits of the transforming growth factor-beta family, was identified by N-terminal sequencing as transthyretin (TTR). A component of the 25-kD band was identified by Western blotting as retinol binding protein (RBP). RT/PCR analysis revealed that mRNAs for both proteins are in the embryo as early as stage 3. In situ hybridization localized these mRNAs to the extraembryonic endoderm at stage 6, after which they were detected in endoderm overlying the embryo proper, including the developing heart. Later, RBP and TTR mRNA and protein were detected in cells associated with the developing heart. Western blotting of whole embryo proteins revealed the presence of RBP by stage 7, followed by sequential increases to stage 25; by contrast, content of RBP in isolated hearts peaked at stage 14, then declined. Immunohistochemistry revealed the presence of RBP protein in the extracellular matrix subjacent to lateral plate endoderm beginning at stage 8; upon formation of the definitive heart, intense staining was observed in the cardiac "jelly." By contrast TTR was intracellular, first detected as subtle deposits in stage 6 embryonic endoderm, which by stage 8 were prominent in the dorsally invaginated endoderm subjacent to the precardiac splanchnic mesoderm. At stages 11-14, TTR was detected only in myocardial cells. Such localization of RBP and TTR may indicate a role in the transport and distribution of retinol and thyroid hormone, respectively, from yolk to embryo prior to establishment of the circulatory system, and is suggestive of a subsequent role in heart development.

Low density lipoprotein receptor gene family members mediate yolk deposition

Yolk represents the last growth stage of a single cell, the oocyte, which contains, besides bona fide cytoplasm, endocytosed serum-derived lipoproteins and minor components essential for normal embryo development. Transport of bulk lipoproteins, micronutrients, and morphogens to oocytes in parallel with maintenance of somatic homeostasis is achieved by ligand targeting via cell-specific expression of receptors and subtle differences in ligand structure. Lipoprotein metabolism is the prime example of these regulatory principles, in which receptors belonging to the low density lipoprotein receptor gene family play key roles. Here, we present the laying hen's features that make it an attractive model system to dissect macromolecular transport processes at the molecular level. In addition to the characterization of a family of yolk precursor receptors, studies on systemic vs. oocyte-directed transport have uncovered new aspects of the biological rationale for simultaneous expression of closely related genes in a single organism.

An oocytic membrane receptor for biotin-binding protein

The chicken oocyte accumulates a biotin-binding protein (BBP) in the yolk that is distinct from the avidin in the 'egg white'. An identical BBP to that of the yolk is also present in the circulation of the laying hen. We report the first evidence for the existence of a BBP receptor in the oocyte vitelline membrane. Reduction of the 100 kDa receptor results in loss of BBP-binding activity; this suggests that a disulfide bonded region of the receptor is necessary for maintaining BBP-binding activity. We show further that the levels of serum BBP are strictly dependent on the presence of estrogen. As expected, BBP is not detected in significant quantities in rooster serum. Thus, these results suggest that circulatory BBP, like other estrogen-dependent components of serum, has a cognate binding activity on the oocyte membrane that may mediate its endocytosis.

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.

Transport of serum transthyretin into chicken oocytes. A receptor-mediated mechanism

Transthyretin (TTR) is involved in the transport of thyroid hormones and, due to its interaction with serum retinol-binding protein, also of vitamin A. The importance of both ligands in vertebrate embryonic development has prompted us to investigate the molecular details of TTR transport function in a powerful germ cell system, the rapidly growing chicken oocytes. Yolk TTR is derived from the circulatory system, since biotinylated TTR was recovered by immunoaffinity chromatography of yolk obtained from a hen previously infused with in vitro biotinylated chicken serum proteins. In concordance with the intraoocytic localization in an endosomal compartment, ligand blotting and chemical cross-linking experiments revealed the presence of a approximately 115-kDa TTR-binding oocyte membrane protein. This putative TTR receptor was not detected in chicken ovarian granulosa cells or embryonic fibroblasts and was different from the previously described oocyte-specific receptor for two estrogen-induced chicken serum lipoproteins, vitellogenin and very low density lipoprotein (Barber, D. L., Sanders, E. J., Aebersold, R., and Schneider, W. J. (1991) J. Biol. Chem. 266, 18761-18770). Furthermore, in contrast to the serum levels of the yolk precursor lipoproteins, those of TTR were not significantly changed by estrogen; thus, TTR represents a newly defined, estrogen-independent class of yolk precursor proteins. These data strongly suggest that oocytic TTR is derived from the circulation, where it is a constitutive component, and deposited into yolk as a result of endocytosis mediated by a specific receptor.