The laying hen expresses two different low density lipoprotein receptor-related proteins

Identification and functional characterization of a novel member of low-density lipoprotein receptor-related protein (LRP)-like family in amphioxus

Low-density lipoprotein receptor-related protein (LRP) is a group of important endocytic receptors contributing to binding ligands and maintaining internal environment. In this study, we identified a soluble LRP-like molecule in the amphioxus B. japonicum, BjLRP, with an uncharacterized domain structure combination of LY-EGF-CRD-EGF-CRD. It was mainly expressed in the gill, muscle, notochord and testis, and was significantly up-regulated following the challenge with bacteria. Recombinant BjLRP was capable of interacting with both Gram-negative and positive bacteria as well as PAMPs including lipopolysaccharide (LPS), lipoteichoic acid (LTA) and peptidoglycan (PGN). Interestingly, recombinant LY peptide was also able to bind to the Gram-negative and positive bacteria as well as the PAMPs LPS, LTA and PGN. By contrast, none of recombinant EGF1, EGF2, CRD1 and CRD2 had affinity to the bacteria and the PAMPs. In addition, BjLRPΔLY had no affinity to the PAMPs, although BjLRPΔLY showed slight affinity to the bacteria. These suggest that the interaction of BjLRP with the bacteria and PAMPs was primarily attributable to the LY domain. It is clear that BjLRP is a novel pattern recognition protein capable of identifying and interacting with invading bacteria in amphioxus.

Lrp13 is a novel vertebrate lipoprotein receptor that binds vitellogenins in teleost fishes

Transcripts encoding a novel member of the lipoprotein receptor superfamily, termed LDL receptor-related protein (Lrp)13, were sequenced from striped bass (Morone saxatilis) and white perch (Morone americana) ovaries. Receptor proteins were purified from perch ovary membranes by protein-affinity chromatography employing an immobilized mixture of vitellogenins Aa and Ab. RT-PCR revealed lrp13 to be predominantly expressed in striped bass ovary, and in situ hybridization detected lrp13 transcripts in the ooplasm of early secondary growth oocytes. Quantitative RT-PCR confirmed peak lrp13 expression in the ovary during early secondary growth. Quantitative mass spectrometry revealed peak Lrp13 protein levels in striped bass ovary during late-vitellogenesis, and immunohistochemistry localized Lrp13 to the oolemma and zona radiata of vitellogenic oocytes. Previously unreported orthologs of lrp13 were identified in genome sequences of fishes, chicken (Gallus gallus), mouse (Mus musculus), and dog (Canis lupus familiaris). Zebrafish (Danio rerio) and Nile tilapia (Oreochromis niloticus) lrp13 loci are discrete and share genomic synteny. The Lrp13 appears to function as a vitellogenin receptor and may be an important mediator of yolk formation in fishes and other oviparous vertebrates. The presence of lrp13 orthologs in mammals suggests that this lipoprotein receptor is widely distributed among vertebrates, where it may generally play a role in lipoprotein metabolism.

A novel estrogen-regulated avian apolipoprotein

In search for yet uncharacterized proteins involved in lipid metabolism of the chicken, we have isolated a hitherto unknown protein from the serum lipoprotein fraction with a buoyant density of ≤1.063 g/ml. Data obtained by protein microsequencing and molecular cloning of cDNA defined a 537 bp cDNA encoding a precursor molecule of 178 residues. As determined by SDS-PAGE, the major circulating form of the protein, which we designate apolipoprotein-VLDL-IV (Apo-IV), has an apparent M_r of approximately 17 kDa. Northern Blot analysis of different tissues of laying hens revealed Apo-IV expression mainly in the liver and small intestine, compatible with an involvement of the protein in lipoprotein metabolism. To further investigate the biology of Apo-IV, we raised an antibody against a GST-Apo-IV fusion protein, which allowed the detection of the 17-kDa protein in rooster plasma, whereas in laying hens it was detectable only in the isolated ≤1.063 g/ml density lipoprotein fraction. Interestingly, estrogen treatment of roosters caused a reduction of Apo-IV in the liver and in the circulation to levels similar to those in mature hens. Furthermore, the antibody crossreacted with a 17-kDa protein in quail plasma, indicating conservation of Apo-IV in avian species. In search for mammalian counterparts of Apo-IV, alignment of the sequence of the novel chicken protein with those of different mammalian apolipoproteins revealed stretches with limited similarity to regions of ApoC-IV and possibly with ApoE from various mammalian species. These data suggest that Apo-IV is a newly identified avian apolipoprotein.

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Apo-VLDL-IV (Apo-IV) is a newly identified avian apolipoprotein.

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Apo-IV expression is suppressed by estrogen.

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Apo-IV containing VLDL particles are excluded from uptake into yolk.

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Apo-IV has limited similarity to mammalian ApoC-IV.

The restricted ovulator chicken strain: an oviparous vertebrate model of reproductive dysfunction caused by a gene defect affecting an oocyte-specific receptor

A unique non-laying strain of chickens with heritable hyperlipidemia and aortic atherosclerosis was first described in 1974. Subsequent work established that the phenotype results from a naturally occurring point mutation in the gene specifying the very low density lipoprotein (VLDL) receptor, a 95-kDa membrane protein which normally mediates the massive uptake of the main circulating hepatically-synthesized yolk precursors, VLDL and vitellogenin. As a result, hens of the mutant strain termed "restricted ovulator" (R/O) have approximately 5-fold elevations in circulating cholesterol and triglyceride concentrations compared with normal layers, and hepatic lipogenesis and cholesterogenesis are markedly attenuated due to feedback inhibition. R/O hens also exhibit hyperestrogenemia, hypoprogesteronemia, elevated circulating gonadotropins, and up-regulated pituitary progesterone receptor mRNA and isoforms. The ovaries of R/O hens are abnormal in that they lack a follicular hierarchy and contain many small preovulatory follicles of various colors, shapes, and sizes. However, since R/O hens occasionally lay eggs, it is possible that endocytic receptors other than the VLDL receptor may be able to facilitate oocyte growth and/or that yolk precursor uptake can occur via a nonspecific bulk process. A mammalian model of impaired fecundity with abnormal lipoprotein metabolism also has been described, but different mechanisms are likely responsible for its reproductive dysfunction. Nevertheless, as our understanding of the molecular physiology and biochemistry of avian oocyte growth continues to expand, in part due to studies of the R/O model, new analogies may emerge between avian and mammalian systems, which ultimately could help to answer important questions in reproductive biology.

Receptor-mediated mechanisms in ovarian follicle and oocyte development

The normal development of the chicken oocyte within the ovarian follicle depends on the coordinated expression and function of several members of the low density lipoprotein receptor gene family. The human low density lipoprotein receptor (LDLR) is the prototype of the gene family; since its discovery and the elucidation of the medical significance of mutations in the ldlr gene, many additional family members have been discovered and characterized, and some important advances have resulted from studies in the chicken. I describe the analogies as well as the differences that exist between the molecular genetics of the mammalian and avian members of this important gene family, with emphasis on receptor-mediated oocyte growth. Recent progress in the molecular characterization of the chicken genes whose products mediate oocyte growth, follicle development, and accessory pathways is described in detail, and emerging information of preliminary nature is included. As the availability of chicken genome sequence data has enhanced the rate of progress in the field, our understanding of the physiological roles of members of this receptor family in general has already gained from studies in the avian model system.

Variation in yolk precursor receptor mRNA expression is a key determinant of reproductive phenotype in the zebra finch (Taeniopygia guttata)

The vitellogenin/very low density lipoprotein receptor (VTG/VLDL-R), a 95 kDa protein that belongs to the low density lipoprotein receptor gene family,mediates the uptake of yolk precursors by developing follicles during oocyte growth. However, the extent to which variation in VTG/VLDL-R expression plays a role in determining inter-individual variation in reproductive phenotype(e.g. follicle or egg size) is not known. Here we show that the mRNA sequence of the zebra finch (Taeniopygia guttata) VTG/VLDL-R shows a high degree of sequence identity (92%) with chicken VTG/VLDL-R mRNA. Using quantitative real-time PCR we measured transcriptional expression of VTG/VLDL-R mRNA in various tissues, and for different stages of oocyte growth,in individual female zebra finches. VTG/VLDL-R mRNA was expressed at high levels in vitellogenic oocytes and in skeletal muscle, and was also detectable in liver, but these tissues expressed different splice variants: the short-form LR8–in oocytes and liver, and the LR8+ form in skeletal muscle. There was significant temporal variation in VTG/VLDL-R expression during follicle growth, with highest levels in ovary and a gradual decrease from pre-F3 to F1 vitellogenic follicles. Variation in ovary mRNA expression was correlated with inter-individual variation in clutch size and laying interval. Furthermore, variation in F3 follicle VTG/VLDL-R mRNA expression was correlated with inter-individual variation in egg mass and F1 follicle mass,suggesting that VTG/VLDL receptor mRNA expression is a key determinant of inter-individual variation in reproductive phenotype.

Low density lipoprotein receptor relatives in chicken ovarian follicle and oocyte development

The normal development of the chicken oocyte within the ovarian follicle depends on the coordinated expression and function of several members of the low density lipoprotein receptor gene family. The human low density lipoprotein receptor is the prototype of the gene family; since its discovery and the elucidation of the medical significance of mutations in the LDLR gene, many additional family members have been discovered and characterized, and some important advances have resulted from studies in the chicken. I describe the analogies as well as the differences that exist between the molecular genetics of the mammalian and avian members of this important gene family, with emphasis on receptor-mediated oocyte growth. Recent progress in the molecular characterization of the chicken genes whose products mediate oocyte growth, follicle development, and accessory pathways is described in detail, and emerging information of preliminary nature is included. As the availability of chicken genome sequence data has enhanced the rate of progress in the field, our understanding of the physiological roles of members of this receptor family in general has already gained from studies in the avian model system.

Pituitary progesterone receptor expression and plasma gonadotrophin concentrations in the reproductively dysfunctional mutant restricted ovulator chicken

Female mutant restricted ovulator (RO) chickens of the White Leghorn strain carry a naturally occurring single nucleotide mutation in the very low density lipoprotein receptor (VLDLR) gene. Due to this mutation, RO hens fail to express a functional VLDLR protein on the oocyte membrane, which results in an impaired uptake of circulating yolk precursor macromolecules. Mutant RO hens subsequently develop hyperlipidemia and generally fail to lay eggs due to follicular atresia. Since RO hens also reportedly have three-fold higher basal plasma estrogen concentrations, combined with four-fold lower levels of circulating progesterone as compared to wild-type (WT) hens, we hypothesized that RO hens would have an increased abundance of pituitary progesterone receptor (PR) mRNA and PR isoforms A and B as well as alterations in circulating gonadotrophin levels. Quantitative PCR assays revealed significantly greater (P<or=0.05) pituitary PR mRNA abundance in RO hens as compared to WT hens. Similarly, pituitary PR isoforms A and B quantities were significantly greater (P<or=0.05) in the RO hens compared to WT hens. In addition, mutant RO hens had significantly greater plasma concentrations of luteinizing hormone, follicle stimulating hormone, estrone, and estradiol, but lower circulating progesterone levels. Collectively, elevated circulating estrogen and/or decreased progesterone levels may have contributed to the upregulation of PR mRNA and PR isoforms A and B in the RO hen pituitary gland. Lastly, in order to gain a more complete understanding of why RO hens are reproductively dysfunctional, a model is proposed that links humoral and ovarian factors to observed and putative changes in the hypothalamic-pituitary axis.

Identification of a novel chondroitin-sulfated collagen in the membrane separating theca and granulosa cells in chicken ovarian follicles: the granulosa-theca cell interface is not a bona fide basement membrane

The membranous structure separating the granulosa from theca cells in the developing ovarian follicles of birds is generally perceived as a genuine basement membrane (BM). Previously, we suggested that this membrane is unusual in that it lacks several typical BM components, e.g. collagen IV, laminin B, perlecan, and fibronectin (Hummel, S., Osanger, A., Bajari, T. M., Balasubramani, M., Halfter, W., Nimpf, J., and Schneider, W. J. (2004) J. Biol. Chem. 279, 23486-23494). We have now identified a novel chondroitin sulfate-modified collagen, tentatively termed ggBM1 (Gallus gallus basement membrane protein1) as a major component of the border between the vascularized theca and the epitheloid granulosa cells. In biosynthetic experiments using [3H]proline and [35S]sulfate, ggBM1 was shown to be synthesized by and secreted from the granulosa cells that support the developing oocyte. The acidic heterogeneous 135-kDa proteoglycan was converted to a protein with an apparent Mr of 95,000 by treatment with chondroitinase ABC and was completely degraded by collagenase. Sequencing of tryptic fragments revealed peptides typical of collagens. The follicular BM accumulated apolipoprotein B and apo-VLDLII, the major resident proteins of the yolk precursor very low density lipoprotein. Interestingly, and likely indicating an analogous situation to the follicle, ggBM1 is also a component of Bruch's membrane of the eye, which separates the vascularized choroid from retinal pigmented epithelial cells. Based on our data we propose that in addition to thecal perlecan, ggBM1 is involved in the transfer of yolk precursors from the thecal capillary bed to oocyte surface lipoprotein receptors mediating their uptake into oocytes.

Effects of atorvastatin on lipid metabolism in normolipidemic and hereditary hyperlipidemic, non-laying hens

As a result of a hereditable point mutation in the oocyte very low density lipoprotein (VLDL) receptor, sexually mature restricted ovulator (RO) female chickens (Gallus gallus), first described as a non-laying strain, exhibit endogenous hyperlipidemia and develop atherosclerotic lesions. In a 20-day study, RO hens and their normolipidemic (NL) siblings were fed either a control diet, or the control diet supplemented with 0.06% atorvastatin (AT), a potent 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) inhibitor. Compared to NL hens, RO birds exhibited greatly elevated baseline plasma total cholesterol (CHOL) and triglyceride (TG) concentrations (1.56 vs. 4.55 g/l and 30.7 vs. 138.4 g/l, respectively). AT attenuated plasma CHOL and TG concentrations by 60.3% and 70.1%, respectively, in NL hens and by 45.1% and 34.3%, respectively, in RO hens. Messenger RNA levels of several key genes involved in hepatic VLDL assembly were suppressed in RO vs. NL hens, but were unaffected by AT. In contrast, AT elevated liver HMGR mRNA levels in NL and RO birds, but only NL hens exhibited an AT-associated increase in hepatic HMGR immunoreactive protein levels. Down-regulation of HMGR gene expression due to higher baseline levels of circulating CHOL may explain why RO birds responded less robustly than NL hens to AT administration.

Extracellular matrices of the avian ovarian follicle. Molecular characterization of chicken perlecan

In egg-laying species, such as the chicken, the mode of transport of lipoprotein particles from the capillary plasma to endocytic receptors on the oocyte surface is largely unknown. Here we show by molecular characterization that the large prominent heparan sulfate proteoglycan of extracellular matrices, termed perlecan or HSPG2 (the product of the hspg2 gene), is a component of ovarian follicles that may participate in this process. However, although normally a major HSPG of basement membranes or basal laminae, in chicken follicles, perlecan is absent from the membranous structure between the theca interna and granulosa cell layers, which to date has been considered a bona fide basement membrane. Rather, the protein is localized in the extracellular matrix of theca externa cells, which produce this HSPG. Furthermore, in chicken testes, perlecan is localized in the peritubular spaces but in less organized fashion than the classical basement membrane components, agrin and laminin. All five domains and structural hallmarks of chicken perlecan (4071 residues) have been conserved in its mammalian counterparts. We have produced the recombinant domain II (containing low density lipoprotein (LDL) receptor-like binding repeats) of chicken perlecan and demonstrate its capacity to bind LDL and very low density lipoprotein (VLDL), apolipoprotein B-containing lipoproteins ultimately destined for uptake into oocytes via members of the low density lipoprotein receptor family. Binding to perlecan heparan sulfate side chains may facilitate the interaction of lipoproteins with domain II. Based on the current results and on domain-domain interactions revealed by recent ultrastructural investigations of the LDL receptor, nidogen, and laminin (Rudenko, G., Henry, L., Henderson, K., Ichtchenko, K., Brown, M. S., Goldstein, J. L., and Deisenhofer, J. (2002) Science 298, 2353-2358 and Takagi, J., Yang, Y., Liu, J. H., Wang, J. H., and Springer, T. A. (2003) Nature 424, 969-974), we propose a novel role of perlecan in mediating plasma-to-oocyte surface transport of VLDL particles.

Molecular characterization of the first avian LDL receptor: role in sterol metabolism of ovarian follicular cells

Low levels of expression and sluggish sterol-mediated regulation have been likely reasons for the failure to molecularly characterize a bona fide LDL receptor (LDLR) in egg-laying species to date. The overall structure of the chicken LDLR, delineated here by cDNA cloning, has been conserved in evolution, since hallmark properties of mammalian LDLRs are already present in the avian protein. The chicken receptor appears to prefer LDL over VLDL as ligand, in compliance with its main role in providing lipoprotein-derived cholesterol for steroid production in ovarian follicular cells. This is also compatible with the fact that estrogen administration increased hepatic LDLR expression in roosters despite dramatically stimulated VLDL production. In cultured chicken embryo fibroblasts, expression of the receptor was induced by incubation with cholesterol synthesis inhibitors such as a statin. Furthermore, preincubation of induced cells with a specific anti-receptor antibody blocks LDL endocytosis, demonstrating that the receptor is ligand-endocytosis competent. Finally, the distribution of LDLRs among the extraoocytic cell populations lends support to a three-cell model for estrogen production within the ovarian follicle. In summary, the molecular characterization of the first avian LDLR reveals novel information about evolutionary, structural, and functional aspects of members of the supergene family of LDLR-related proteins.

Receptor-mediated chicken oocyte growth: differential expression of endophilin isoforms in developing follicles

Receptor-mediated endocytosis of yolk precursors via clathrin-coated structures is the key mechanism underlying rapid chicken oocyte growth. In defining oocyte-specific components of clathrin-mediated events, we have to date identified oocyte-specific yolk transport receptors, but little is known about the oocytes' supporting endocytic machinery. Important proteins implicated in clathrin-mediated endocytosis and recycling are the endophilins, which thus far have been studied primarily in synaptic vesicle formation; in the present study, as a different highly active endocytic system, we exploit rapidly growing chicken oocytes. Molecular characterization of the chicken endophilins I, II, and III revealed that their mammalian counterparts have been highly conserved. All chicken endophilins interact via their SH3 domain with the avian dynamin and synaptojanin homologues and, thus, share key functional properties of mammalian endophilins. The genes show different expression patterns: As in mammals, expression is low to undetectable in the liver and high in the brain; in ovarian follicles harboring oocytes that are rapidly growing via receptor-mediated endocytosis, levels of endophilins II and III, but not of endophilin I, are high. Immunohistochemical analysis of follicles demonstrated that endophilin II is mainly present in the theca interna but that endophilin III predominates within the oocyte proper. Moreover, in a chicken strain with impaired oocyte growth and absence of egg-laying because of a genetic defect in the receptor for yolk endocytosis, endophilin III is diminished in oocytes, whereas endophilin III levels in the brain and endophilin II localization to theca cells are unaltered. Thus, the present study reveals that the endophilins differentially contribute to oocyte endocytosis and development.

Validation of a modified PCR-based method for identifying mutant restricted ovulator chickens: substantiation of genotypic classification by phenotypic traits

Upon photostimulation, restricted ovulator (RO) female chickens exhibit endogenous hyperlipidemia, develop atherosclerotic lesions, and generally fail to lay eggs. This phenotype results from a point mutation in the gene specifying the very low density lipoprotein receptor (VLDLR), whose protein product normally mediates the massive oocytic uptake of egg yolk precursors from the circulation. Taking advantage of the single base change in the mutant VLDLR allele, a PCR-based method for the rapid identification of RO chickens was developed at the Biocenter and University of Vienna, Austria. However, this procedure was incompletely validated because phenotypic data were not obtained and conventional progeny testing of sons and grandsons was not performed. Here, the assay validation was completed by providing plasma lipid concentrations, plasma very low density lipoprotein particle sizes, or egg production records of PCR-genotyped females and their brothers and sires to demonstrate that each bird's phenotypic traits substantiated their genotypic classification. Moreover, several methodological modifications resulted in improved chemical safety, speed, and cost of preparing and analyzing genomic DNA from chicken erythrocytes. Because the ovaries of mutant RO females generally contain numerous vitellogenic follicles in the absence of a functional oocyte plasma membrane VLDLR, the existence of an alternate system for the oocytic uptake of plasma very low density lipoprotein and vitellogenin is suggested, whereas a physiological explanation as to why some, but not all, mutant RO hens are able to ovulate and lay eggs is lacking.

The low-density lipoprotein receptor family: genetics, function, and evolution

With ever increasing sophistication in molecular biological approaches, the low-density lipoprotein receptor supergene family continues to grow rapidly. From the well-defined key role of these receptors in lipoprotein metabolism, the new members move the field into many different and diverse physiologic and developmental areas. We observe an expansion of the functional spectrum of the family members, which is due to 1) the binding to their extracellular domains of more and more components lacking homology to apolipoproteins, and 2) the recently uncovered interaction of the receptors' cytoplasmic tails with adaptor proteins that are part of signaling pathways. As this review attempts to describe, the task of delineation of the evolutionary history of the gene family may be aided by concepts that consider events, both divergent and convergent, within and between the intra- and extracellular domains.

Multiple involvement of clusterin in chicken ovarian follicle development. Binding to two oocyte-specific members of the low density lipoprotein receptor gene family

The interaction of the female germ cell with somatic cells during the development of the ovarian follicle in the chicken provides a prime system to study gene expression. Here, we have uncovered the involvement of clusterin, the function(s) of which is still poorly understood, in this complex process. As revealed by molecular cloning, chicken clusterin is a 428-residue protein that migrates at 70 kDa on SDS-polyacrylamide gel electrophoresis and possesses most of the structural features of its mammalian successors. However, in contrast to mammalian clusterin, the chicken protein appears not to be cleaved intracellularly into a disulfide-linked heterodimer; possibly as a consequence thereof, it is not secreted constitutively and is absent from the circulation, where most of clusterin is found in mammals. In the ovary, clusterin is a major product of the somatic granulosa cells, in a pattern correlating with the developmental phases of individual follicles. In that, transcript levels are high not only at onset of vitellogenesis, but also in atretic follicles and in the postovulatory follicle sac, i.e. in situations characterized by apoptotic events. Yolk of growing oocytes contains a 43-kDa truncated form of clusterin that does not appear to be synthesized within the oocyte. Rather, we here show for the first time that 70-kDa clusterin interacts not only with megalin, but also with two chicken oocyte-specific members of the low density lipoprotein receptor (LDLR) gene family. These receptors, termed LDLR-related protein with eight ligand binding repeats (LR8) and LDLR-related protein (380 kDa), likely internalize granulosa cell-derived 70-kDa clusterin, which may subsequently be processed to the 43-kDa product. Thus, chicken clusterin could serve as a marker for follicular atresia and resorption, and, based on its ability to bind several other proteins, it may serve as carrier for the receptor-mediated endocytosis into oocytes of components important for embryonic development, two hitherto unknown functions of this intriguing protein.

Very-low-density lipoprotein receptor fragment shed from HeLa cells inhibits human rhinovirus infection

The large family of human rhinoviruses, the main causative agents of the common cold, is divided into the major and the minor group based on receptor specificity. Major group viruses attach to intercellular adhesion molecule 1 (ICAM-1), a member of the immunoglobulin superfamily, whereas minor group viruses use low-density lipoprotein receptors (LDLR) for cell entry. During early attempts aimed at isolating the minor group receptor, we discovered that a protein with virus binding activity was released from HeLa cells upon incubation with buffer at 37 degreesC (F. Hofer, B. Berger, M. Gruenberger, H. Machat, R. Dernick, U. Tessmer, E. Kuechler, and D. Blaas, J. Gen. Virol. 73:627-632, 1992). In light of the recent discovery of several new members of the LDLR family, we reinvestigated the nature of this protein and present evidence for its being derived from the human very-low density lipoprotein receptor (VLDLR). A soluble VLDLR fragment encompassing the eight complement type repeats and representing the N-terminal part of the receptor was then expressed in the baculovirus system; both the shed protein and the recombinant soluble VLDLR bind minor group viruses and inhibit viral infection of HeLa cells in a concentration-dependent manner.

Identification and characterization of receptors for riboflavin carrier protein in the chicken oocyte. Role of the phosphopeptide in mediating receptor interaction

Riboflavin carrier protein (RCP) is a phosphoglycoprotein found in the egg and the serum of laying birds and other animals. We have investigated the binding of chicken RCP (cRCP) to membranes prepared from the whole chicken oocytes. RCP binding had an absolute requirement for calcium, with an affinity (Kd 10(-8) M) high enough to be physiologically relevant. Ligand blotting experiments using labeled RCP and vitellogenin, with proteins solubilized from oocyte membranes, indicated that RCP and vitellogenin bound specifically to three proteins of Mr 380, 260 and 110 kDa. Vitellogenin also bound to proteins of Mr 515 kDa and 97 kDa, similar in size to those identified by receptor associated protein of RAP. Reduced and carboxyamidated RCP inhibited the binding of 125I-labeled RCP to chicken oocyte membranes, but recombinant RCP expressed in E. coli, and dephosphorylated RCP, failed to interact with the receptors, indicating that post-translational modifications were necessary for ligand-receptor interaction. The purified phosphopeptide, prepared from tryptic digests of egg white RCP, was able to inhibit the binding of RCP to the receptor proteins, with an affinity comparable to native RCP indicating that the phosphopeptide sequence present in RCP serves as the focal point for RCP-receptor interactions.

Receptor-associated protein in an oviparous species is correlated with the expression of a receptor variant

The biosynthesis of proteins containing cysteine-rich domains requires chaperones for their correct folding. For instance, the 39-kDa receptor-associated protein (RAP) aides in the cell-surface targeting of newly synthesized members of the mammalian low density lipoprotein receptor (LDLR) gene family, which contains tandemly arranged clusters of hexacysteine repeats. In the chicken, an LDLR relative with eight such repeats is expressed as two different splice variant forms in cell type-specific fashion (Bujo, H., Lindstedt, K. A., Hermann, M., Mola Dalmau, L., Nimpf, J., and Schneider, W. J. (1995) J. Biol. Chem. 270, 23546-23551). To learn more about evolutionary aspects of RAP, its role in escorting of these different receptor splice variants, and other potential functions, we have extended our studies on the avian LDLR family to RAP. cDNA cloning, determination of tissue expression at both the transcript and the protein level, stable expression in COS cells, and binding studies with chicken RAP revealed that mammalian RAPs have retained many features of the non-amniotic proteins. However, structural details, e.g. the well defined internal triplicate repeats in the chicken protein, have been somewhat diluted during evolution. Interestingly, chicken RAP was found to correlate positively with the expression levels in somatic cells of the larger splice variant of the eight-cysteine repeat receptor, but not with those of the smaller variant, expressed only in germ cells. This is compatible with the possibility that RAP may play a role in receptor biology that could be complementing its function in assisting folding. Chicken RAP in crude extracts of the stable expressor COS cells is able to bind to LDLR relatives in ligand blots without requirement for prior purification of the ligand. Thus, in conjunction with the avian model of massive lipid transport to germ cells, these cells provide a novel comparative system amenable to investigation of the biological functions of RAP.

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.

Germ cell-somatic cell dichotomy of a low-density lipoprotein receptor gene family member in testis

Members of the low-density lipoprotein receptor (LDLR) supergene family interact with a large number of diverse ligands. One of the relevant receptors is the recently characterized LDLR relative with eight ligand-binding repeats, termed LR8, which exists in two splice variant forms. The gonads, relying on receptor-mediated lipoprotein supply for steroidogenesis, and on interplay of germ cells with somatic cells, provide a particularly attractive setting to study details of the expression of LR8. Here we show by polymerase chain reactions and Northern analysis, as well as by in situ hybridization, that the longer of the two splice variants (LR8+), containing an additional region defining an O-linked sugar domain, is produced in the somatic cells of chicken testis, whereas the shorter form lacking this domain (LR8-) is expressed in the male germ cells. Interestingly, as shown by transcript analysis and at the functional level by ligand blotting, LR8- expression in the spermatoids increases with germ cell maturation, but is absent from ejaculated sperm. This constitutes a scenario reminiscent of the situation in growing vitellogenic oocytes, which express very high levels of LR8-, but lack the receptor following ovulation. Thus, the cell-specific expression of different LR8 splice variants may relate to the requirements of extensive communication and cooperation between germ cells and somatic cells in the gonads.

Blockade of the alpha 2-macroglobulin receptor/low-density-lipoprotein-receptor-related protein on rat liver parenchymal cells by the 39-kDa receptor-associated protein leaves the interaction of beta-migrating very-low-density lipoprotein with the lipoprotein remnant receptor unaffected

The nature of the liver binding site which is responsible for the initial recognition and clearance of chylomicron-remnants and beta-migrating very-low-density lipoprotein (beta-VLDL) is under active dispute. We have investigated the effect of the 39-kDa receptor-associated protein (RAP) on the recognition site for activated alpha 2-macroglobulin and beta-VLDL on rat liver parenchymal cells in vivo and in vitro in order to analyze whether both substrates are recognized and internalized by the same receptor system. Radiolabelled trypsin-activated alpha 2-macroglobulin (alpha 2M-T) was cleared rapidly by the liver (maximal uptake of 80.8 +/- 1.0% of the injected dose). Prior injection of 5, 15, or 50 mg gluthathione-S-transferase-linked RAP (GST-RAP)/kg rat reduced the liver uptake to 62.2 +/- 2.3%, 59.3 +/- 1.1%, or 2.9 +/- 0.1% of the injected dose, respectively. Concurrently the serum decay was strongly delayed after injection of 50 mg GST-RAP/kg rat but this did not affect the serum decay and liver uptake of 125I-beta-VLDL. Binding studies with isolated liver parenchymal cells in vitro demonstrated that the binding of 125I-alpha 2M-T was 98% inhibited by GST-RAP with an IC50 of 0.3 microgram/ml (4.2 nM), whereas the binding of 125I-beta-VLDL and 125I-beta-VLDL + recombinant apolipoprotein E (rec-apoE) was unaffected by GST-RAP up to 50 micrograms/ml (700 nM). Also, the cell association and degradation of alpha 2M-T was blocked by RAP, while the association and degradation of beta-VLDL and beta-VLDL + rec-apoE were not influenced. The inhibitory effect of RAP on the cell association and degradation of alpha 2M-T lasted for 1-2 h of incubation at 37 degrees C. The binding of the radioiodinated RAP to isolated liver parenchymal cells was highly efficiently coupled to lysosomal degradation. Upon in vivo injection into rats, 125I-labeled RAP is rapidly cleared from the serum and taken up by the liver, which is also coupled to efficient degradation. Since RAP blocks binding of all known ligands to the alpha 2-macroglobulin receptor/low-density lipoprotein receptor-related protein (the alpha 2Mr/LRP) and at high concentrations the binding to the LDL receptor, we conclude that the initial binding and internalization of beta-VLDL by rat liver parenchymal cells is not mediated by the alpha 2Mr/LRP. The properties of binding of beta-VLDL to rat liver parenchymal cells points to an apoE-specific recognition site for lipoprotein remnants which differs from the alpha 2Mr/LRP, proteoglycans and the LDL receptor and is tentatively called the lipoprotein remnant receptor.

Preferential recognition of the very low-density lipoprotein receptor ligand binding site by antibodies from phage display libraries

Screening of a phage library displaying single chain fragments of the variable regions of human immunoglobulins (scFv) for binding to the ovarian chicken very low-density lipoprotein/vitellogenin receptor (OVR) led to the isolation of several antibody fragments with high affinity. As for the natural ligands of OVR, receptor binding of all antibody fragments is strictly Ca(2+)-dependent and is prevented by receptor-associated protein (RAP). Moreover, attachment of human rhinovirus serotype 2 (HRV2) to this receptor is inhibited by all scFvs. In contrast to conventional immunization, the in vitro selection method thus exclusively led to antibodies that attach to or close to the ligand binding site and thereby block the receptor-ligand interaction.

Cloning and sequencing of human gp330, a Ca(2+)-binding receptor with potential intracellular signaling properties

We present here the complete primary structure of human gp330, the human variant of the principal kidney autoantigen causing Heymann membranous glomerulonephritis in rats. The deduced 4655 amino acid residues give a calculated molecular mass of 519636 Da for the mature protein and consists of a probable 25-amino-acid N-terminal signal peptide sequence, an extracellular region of 4398 amino acids, a single transmembrane-spanning domain of 23 amino acids, and an intracellular C-terminal region of 209 amino acid residues. Three types of cysteine-rich repeats characteristic of the low density lipoprotein receptor (LDLR) superfamily are present in human gp330. In the extracellular region, there are a total of 36 LDLR ligand-binding repeats, comprising four distinct domains, 16 growth factor repeats separated by eight YWTD spacer regions, and one epidermal growth factor-like repeat. No consensus cleavage sequence for the processing endoprotease furin is detected in human gp330. The intracellular tail contains not only two copies of the F(X)NPXY coated-pit mediated internalization signal characteristic of LDLR superfamily members, but also intriguing and potentially functional motifs including several Src-homology 3 recognition motifs, one Src-homology 2 recognition motif for the p85 regulatory subunit of phosphatidylinositol 3-kinase, and additional sites for protein kinase C, casein kinase II and cAMP-/cGMP-dependent protein kinase. There is approximately 77% amino acid identity between human and rat gp330 with minor differences between the extracellular and intracellular regions. Recently gp330 has been implicated in Ca2+ regulation in the parathyroid, the placenta, and the renal tubule, but its overall physiological and pathological role still remains uncertain.

A new low density lipoprotein receptor homologue with 8 ligand binding repeats in brain of chicken and mouse

The blood-brain barrier necessitates disparate macromolecular transport systems in the brain and central nervous system. We now report the discovery of a new member of the low density lipoprotein receptor (LDLR) family whose expression is highly restricted to the brain. The full-length cDNA specifying the chicken receptor (open reading frame, 2754 base pairs) as well as a cDNA for the major portion of its murine homologue have been obtained. The novel receptor shows the greatest similarity to the group of LDLR relatives with 8 ligand binding repeats, in chicken termed LR8 and in mammals, very low density lipoprotein receptors. Thus, in addition to 8 tandemly arranged ligand binding repeats, the five-domain receptor contains an O-linked sugar region and the internalization signal, Phe-Asp-Asn-Pro-Val-Tyr, typical for all LDLR gene family members. In chicken, the 6.5-kb receptor transcript is present at high levels in brain and at much lower levels in extraoocytic cells of the ovary; in mouse, the same transcript of 6.5 kb was detected in brain, but not in heart (the major site of very low density lipoprotein receptor expression), lung, liver, kidney, and ovary. An antibody directed against the predicted carboxyl terminus of the avian receptor detected a 130-kDa protein in brain extracts. The apparent size of the immunoreactive protein is compatible with extensive glycosylation of the 894-residue mature form of the receptor. The presence of this novel receptor in brain of a bird and a rodent suggests an important and evolutionary conserved function.

Vitellogenin receptors: oocyte-specific members of the low-density lipoprotein receptor supergene family

Receptors that transport vitellogenin (VTG) into oocytes are of vital importance to egg-laying species, because they mediate a key step of oocyte maturation, a prerequisite to reproduction. Vitellogenins are lipophosphoglycoproteins that are produced under female hormonal control in large central organs (fat body in insects; liver in higher animals) and are transported in the circulation to the female gonads. VTG receptors localized in coated pits on the surface of growth-competent oocytes are able to accumulate in the yolk high concentrations of VTG and other ligands they recognize. The study of VTG receptors and their ligands has identified genes that specify related ligands, and a family of receptors. To date, all molecularly characterized VTG receptors belong to the low-density lipoprotein receptor supergene family, which ranges from a 600-kDa receptor in Caenorhabditis elegans to the 100-kDa so-called very-low-density lipoprotein receptors in mammals. These receptors, by and large, recognize ligands with similarities in structural elements first defined in the human apoplipoproteins B-100 and E. Recent studies on the receptor family have added VTG and lipoprotein lipase to the list of co-evolved ligands and have revealed that VTG receptors are able to interact with ligands other than VTG and also with some unrelated to lipoprotein metabolism. For example, the chicken VTG receptor also imports very-low-density lipoprotein, riboflavin-binding protein, and alpha-2-macroglobulin into growing oocytes. Such multifunctionality of receptors is likely the result of evolutionary pressure to provide the female germ cell with a highly economical machinery for vitellogenesis.

An antibody fragment from a phage display library competes for ligand binding to the low density lipoprotein receptor family and inhibits rhinovirus infection

Recently antibodies with a wide range of binding specificities have been isolated from large repertoires of antibody fragments displayed on filamentous phage, including those that are difficult to raise by immunization. We have used this approach to isolate an antibody fragment against chicken very low density lipoprotein (VLDL) receptor. It binds to the receptor with good affinity (Kaff = 2 x 10(8) M-1) as measured by plasmon surface resonance, and competes for binding of natural ligands (vitellogenin, VLDL, and receptor-associated protein). The antibody also binds to other members of the low density lipoprotein (LDL) receptor family including rat LDL receptor and human and rat low density lipoprotein receptor-related protein (LRP/alpha 2MR), and it competes for binding of receptor-associated protein to LRP/alpha 2MR. Moreover, the antibody fragment inhibits infection of human fibroblasts deficient in LDL-R but expressing LRP/alpha 2MR by human rhinovirus. Binding of the antibody is abolished upon reduction of the receptors and is strictly Ca2+ dependent. The phage antibody thus recognizes the ligand binding site(s) of several members of the LDL receptor family, in contrast to antibodies produced by hybridoma technology.

The chicken oocyte receptor for yolk precursors as a model for studying the action of receptor-associated protein and lactoferrin

Receptor-associated protein (RAP) was originally described as a 39-kDa intracellular protein copurifying with mammalian low density lipoprotein (LDL) receptor-related protein/alpha 2-macroglobulin receptor (LRP/alpha 2MR). RAP has a high affinity for LRP/alpha 2MR and interferes with the receptor's ability to bind a variety of ligands. The laying hen expresses, in a tissue-specific manner, at least four different proteins which belong to the same family of receptors as LRP/alpha 2MR. Here we show that the chicken also produces RAP, so far thought to be expressed only in mammals. Studies on the interaction of recombinant human RAP with the LDL receptor family in the chicken revealed that RAP binds with high affinity to the abundant oocyte receptor for yolk precursors (OVR) as well as to the somatic cell-specific LRP/alpha 2MR. Significantly, RAP interacts with a lower affinity with the LDL receptor, but does not bind to the oocyte-specific form of LRP. Binding of RAP to OVR inhibits the interaction of the receptor with all known physiological ligands, i.e. the yolk precursors very low density lipoprotein, vitellogenin, and alpha 2-macroglobulin. In COS cells transfected with OVR, RAP is internalized and degraded in a concentration-dependent and saturable manner. Lactoferrin, another protein with a high affinity for mammalian LRP/alpha 2MR, also binds to OVR and abolishes its interaction with yolk precursors. Cross-competition experiments show that RAP and lactoferrin recognize sites different from those involved in yolk precursor binding. The availability of pure OVR and LDLR enable us to determine kinetic parameters for the binding of RAP and lactoferrin to these receptors by surface plasmon resonance. Taken together, our results strongly suggest that chicken OVR, which is easily accessible and highly abundant in growing oocytes, represents a superior system for studying mechanistic and structural aspects of the interaction of ligands and modulating proteins with members of the LDL receptor gene family.

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.

Visualization of the chicken oocyte lipoprotein receptor by ligand blotting with biotinylated plasma and yolk very low density lipoproteins

The laying hen 95-kDa oocyte membrane receptor that transports hepatically synthesized very low density lipoprotein (VLDL) and vitellogenin (VTG) from the plasma to growing follicles was visualized by ligand blotting with biotinylated VLDL followed by enhanced chemiluminescence (ECL) detection. Plasma and egg yolk VLDL were isolated by ultracentrifugation and free epsilon-amino groups of lysines of apolipoprotein B (apo B), the protein constituent of VLDL that mediates binding to the 95-kDa oocyte membrane receptor, were biotinylated using D-biotin-N-hydroxysuccinimide ester. An apo B concentration of approximately 223 pM was sufficient to give a signal on 2.5 micrograms of protein from a chicken oocyte membrane detergent extract. Western blotting (immunoblotting) of the laying hen 95-kDa receptor with polyclonal rabbit anti-chicken oocyte VLDL receptor IgG resulted in an ECL signal with the same position of migration as that observed in ligand blots using biotinylated plasma and yolk VLDL. Binding of biotinylated plasma or yolk VLDL to the 95-kDa receptor was abolished by excess unlabeled plasma or yolk VLDL, respectively, as well as by EDTA. Receptor binding activity of biotinylated plasma and yolk VLDL was also demonstrated by a reverse ligand blotting procedure. Compared with conventional techniques involving the use of 125I-labeled ligands or antibodies, the laying hen 95-kDa oocyte membrane lipoprotein receptor can be safely and rapidly visualized with excellent sensitivity using the present nonradioactive method. In addition, it is suggested that ECL detection can be employed to further study the ligand-binding properties and specificity of this protein, which is essential to vitellogenesis in the chicken.

Genetic deficiency in low density lipoprotein receptor-related protein confers cellular resistance to Pseudomonas exotoxin A. Evidence that this protein is required for uptake and degradation of multiple ligands

The low density lipoprotein receptor-related protein (LRP) is a large multifunctional receptor implicated in the cellular uptake of functionally diverse ligands. Biochemical evidence suggests that LRP is a clearance receptor for apoE-rich remnant lipoproteins, lipoprotein lipase, alpha 2-macroglobulin/protease complexes, plasminogen activator/inhibitor complexes, the active protease tissue-type plasminogen activator and exotoxin A from Pseudomonas aeruginosa. Mice genetically deficient in LRP die early during gestation, underscoring the essential physiological role of this gene in vivo. To study the effect of LRP deficiency at the cellular level, we have used Pseudomonas exotoxin A (PEA) to select murine embryonic fibroblasts that are genetically deficient in LRP. Our results demonstrate that this single gene defect is sufficient to confer resistance to PEA on cultured cells. In addition, embryonic fibroblasts lacking LRP are unable to bind, internalize and degrade methylamine-activated alpha 2-macroglobulin and complexes of urokinase with plasminogen activator inhibitor-1. Furthermore, cellular uptake and degradation of receptor-associated protein, a 39 kDa accessory protein of LRP, is reduced by 90% in the absence of LRP. These results provide genetic evidence for the multifunctional nature of LRP and its crucial role in protease/inhibitor complex metabolism.

Chicken oocyte growth: receptor-mediated yolk deposition

During the rapid final stage of growth, chicken oocytes take up massive amounts of plasma components and convert them to yolk. The oocyte expresses a receptor that binds both major yolk lipoprotein precursors, vitellogenin (VTG) and very low density lipoprotein (VLDL). In the present study, in vivo transport tracing methodology, isolation of coated vesicles, ligand- and immuno-blotting, and ultrastructural immunocytochemistry were used for the analysis of receptor-mediated yolk formation. The VTG/VLDL receptor was identified in coated profiles in the oocyte periphery, in isolated coated vesicles, and within vesicular compartments both outside and inside membrane-bounded yolk storage organelles (yolk spheres). VLDL particles colocalized with the receptor, as demonstrated by ultrastructural visualization of VLDL-gold following intravenous administration, as well as by immunocytochemical analysis with antibodies to VLDL. Lipoprotein particles were shown to reach the oocyte surface by passage across the basement membrane, which possibly plays an active and selective role in yolk precursor accessibility to the oocyte surface, and through gaps between the follicular granulosa cells. Following delivery of ligands from the plasma membrane into yolk spheres, proteolytic processing of VTG and VLDL by cathepsin D appears to correlate with segregation of receptors and ligands which enter disparate sub-compartments within the yolk spheres. In small, quiescent oocytes, the VTG/VLDL receptor was localized to the central portion of the cell. At onset of the rapid growth phase, it appears that this pre-existing pool of receptors redistributes to the peripheral region, thereby initiating yolk formation. Such a redistribution mechanism would obliterate the need for de novo synthesis of receptors when the oocyte's energy expenditure is to be utilized for plasma membrane synthesis, establishment and maintenance of intracellular topography and yolk formation, and preparation for ovulation.