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

Expression of microsomal triglyceride transfer protein in lipoprotein-synthesizing tissues of the developing chicken embryo

In contrast to mammals, in the chicken major sites of lipoprotein synthesis and secretion are not only the liver and intestine, but also the kidney and the embryonic yolk sac. Two key components in the assembly of triglyceride-rich lipoproteins are the microsomal triglyceride transfer protein (MTP) and apolipoprotein B (apoB). We have analyzed the expression of MTP in the embryonic liver, small intestine, and kidney, and have studied the expression of MTP in, and the secretion of apoB from, the developing yolk sac (YS). Transcript and protein levels of MTP increase during embryogenesis in YS, liver, kidney, and small intestine, and decrease in YS, embryonic liver, and kidney after hatching. In small intestine, the MTP mRNA level rises sharply during the last trimester of embryo development (after day 15), while MTP protein is detectable only after hatching (day 21). In the YS of 15- and 20-day old embryos, apoB secretion was detected by pulse-chase metabolic radiolabeling experiments and subsequent immunoprecipitation. Taken together, our data reveal the importance of coordinated production of MTP and apoB in chicken tissues capable of secreting triglyceride-rich lipoproteins even before hatching.

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MTP is expressed in liver, small intestine, and kidney of chicken embryos.

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MTP is expressed in the chicken yolk sac.

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ApoB is secreted from the chicken yolk sac.

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Embryonic tissues contribute to the lipoprotein pool of the developing chick.

Increased expression of bradykinin type-1 receptors in endothelium of intramyocardial coronary vessels in human failing hearts

In experimental animals, bradykinin type-1 receptors (BK-1Rs) are induced during inflammation and ischemia, and, by exerting either cardioprotective or cardiotoxic effects, they may contribute to the pathogenesis of heart failure. Nothing is known about the expression of BK-1Rs in human heart failure. Human heart tissue was obtained from excised hearts of patients undergoing cardiac transplantation (n = 13), due to idiopathic dilated cardiomyopathy (IDC; n = 7) or to coronary heart disease (CHD; n = 6), and from normal hearts (n = 6). The expression of BK-1Rs was analyzed by means of competitive RT-PCR, Western blot analysis, and immunohistochemistry. Expression of BK-1R mRNA was increased in both IDC (2.8-fold) and CHD (2.1-fold) hearts compared with normal hearts. The observed changes were verified at the protein level. Expression of BK-1Rs in failing hearts localized to the endothelium of intramyocardial coronary vessels and correlated with an increased expression of TNF-alpha in the vessel wall. Treatment of human coronary artery endothelial cells with TNF-alpha increases their BK-1R expression. These novel results show that BK-1Rs are induced in the endothelium of intramyocardial coronary vessels in failing human hearts and so may participate in the pathogenesis of heart failure.

A role for cardiac mast cells in the pathogenesis of hypertensive heart disease

OBJECTIVE

Cardiac mast cells participate in myocardial dysfunction, but the mechanisms are presently unknown.

DESIGN

By examining spontaneously hypertensive rats (SHRs) during their entire lifespan, we attempted to define the role of mast cells in the induction of cardiac hypertrophy and transition to heart failure.

METHODS AND RESULTS

By contrast to normotensive littermates, hearts of newborn SHRs already contained mast cells. In the prehypertensive (2-week-old) SHRs, the increased expression of c-kit and soluble stem cell factor correlated with an increased number of cardiac mast cells. The mast cells contained tumour necrosis factor-alpha which, together with nuclear factor kappa-B (NF-kappaB) and interleukin (IL)-6, was significantly induced in the prehypertensive SHRs. Stimulation of cardiac mast cells with compound 48/80 in an ex-vivo Langendorff heart perfusion system resulted in increased expression of nuclear factor Kappa-B (NF-kappaB) (four-fold) and IL-6 (nine-fold) mRNA in the left ventricles of adult rat hearts. In the presence of an inhibitor of mast cell degranulation, disodium cromoglycate, the induced expression of NF-kappaB and IL-6 was inhibited. In the late hypertensive stage, the hearts of SHRs with advanced cardiac hypertrophy (12-month-old) and heart failure (20-month-old) had significantly increased levels of transforming growth factor (TGF)-beta1 and basic fibroblast growth factor (bFGF), and displayed increased myocardial fibrosis. Activated mast cells were a major source of TGF-beta1 and bFGF, and localized to areas of myocardial fibrosis.

CONCLUSIONS

By synthesizing and secreting prohypertrophic cytokines and profibrotic growth factors, cardiac mast cells participate in the induction of cardiac hypertrophy and cardiac fibrosis, which are the key steps in the transition to heart failure.

Receptor clustering is involved in Reelin signaling

The Reelin signaling cascade plays a crucial role in the correct positioning of neurons during embryonic brain development. Reelin binding to apolipoprotein E receptor 2 (ApoER2) and very-low-density-lipoprotein receptor (VLDLR) leads to phosphorylation of disabled 1 (Dab1), an adaptor protein which associates with the intracellular domains of both receptors. Coreceptors for Reelin have been postulated to be necessary for Dab1 phosphorylation. We show that bivalent agents specifically binding to ApoER2 or VLDLR are sufficient to mimic the Reelin signal. These agents induce Dab1 phosphorylation, activate members of the Src family of nonreceptor tyrosine kinases, modulate protein kinase B/Akt phosphorylation, and increase long-term potentiation in hippocampal slices. Induced dimerization of Dab1 in HEK293 cells leads to its phosphorylation even in the absence of Reelin receptors. The mechanism for and the sites of these phosphorylations are identical to those effected by Reelin in primary neurons. These results suggest that binding of Reelin, which exists as a homodimer in vivo, to ApoER2 and VLDLR induces clustering of ApoER2 and VLDLR. As a consequence, Dab1 becomes dimerized or oligomerized on the cytosolic side of the plasma membrane, constituting the active substrate for the kinase; this process seems to be sufficient to transmit the signal and does not appear to require any coreceptor.

Expression of bradykinin receptors in the left ventricles of rats with pressure overload hypertrophy and heart failure

OBJECTIVES

Bradykinin exerts cardioprotective effects through bradykinin type-2 receptors (BK-2Rs). After acute myocardial infarction in rat, the heart adapts by increasing its number of BK-2Rs. However, in human chronic end-stage heart failure, the number of BK-2Rs is significantly decreased. Thus, the presence of a cardioprotective BK-2R signaling system may be critical in the prevention of pressure overload-induced heart failure.

DESIGN

To explain differences in myocardial BK-2R expression during cardiac overload, we studied: (1). spontaneously hypertensive rats (SHRs) of different ages, and (2). normotensive Sprague-Dawley rats subjected to aortic banding or angiotensin II infusion.

METHODS AND RESULTS

The mRNA levels of BK-2Rs were found to be significantly (P < 0.05) increased in the aging (12 and 20-month-old) SHRs (2.9- and 3-fold, respectively). Similarly, in the Sprague-Dawley rats, the expression of BK-2Rs was increased at 12 h (1.8-fold, P < 0.05) and at 3 days (3.1-fold, P < 0.05) after aortic banding, and at 2 weeks (2.2-fold) after angiotensin II infusion. In the 12-month-old SHRs, with compensated left ventricular hypertrophy (no fibrosis or left ventricular dysfunction), the amount of BK-2Rs was also significantly increased (1.8-fold, P < 0.05). However, in the 20-month-old SHRs, with a dramatic increase in fibrosis and development of diastolic dysfunction and heart failure, the amount of BK-2Rs were significantly decreased (63%, P < 0.05) specifically in the cardiac endothelial cells.

CONCLUSIONS

The present results show that, during pressure overload and compensated left ventricular hypertrophy, the expression of BK-2Rs is increased. However, ongoing pressure overload leads to a loss of BK-2Rs with a dramatic increase in left ventricular fibrosis followed by diastolic dysfunction and heart failure.

Down-regulation of cardioprotective bradykinin type-2 receptors in the left ventricle of patients with end-stage heart failure

OBJECTIVES

We sought to study the expression of bradykinin type-2 receptors (BK-2Rs) in patients with heart failure (HF).

BACKGROUND

Recent work in experimental animals has suggested that bradykinin (BK) exerts cardioprotective effects through specific BK-2Rs. However, nothing is known about the regulation of BK-2R expression in the pathogenesis of human HF.

METHODS

Human heart tissue was obtained from excised hearts of patients undergoing cardiac transplantation (n = 13) and from normal hearts (n = 6) unsuitable for donation. The patients had HF due to idiopathic dilated cardiomyopathy (IDC) (n = 7) or coronary heart disease (CHD) (n = 6). Tissue samples from the left ventricles were analyzed by competitive reverse-transcriptase-polymerase chain reaction and Western blotting for the expression of BK-2R messenger ribonucleic acid (mRNA) and protein.

RESULTS

In both the IDC and CHD hearts, the level of BK-2R mRNA expression was found to be significantly lower (30% and 38% of control values, respectively) than that in normal hearts. Correspondingly, the BK-2R protein level was significantly reduced in both the IDC and CHD hearts (45% and 62% of control values, respectively) and apparently involved all myocardial cell types. The down-regulation of BK-2R expression in failing hearts did not correlate with decreased cellularity or with the expression pattern of other members of the G-protein-coupled receptor superfamily. However, BK-2R down-regulation in the failing hearts was associated with a decrease in endothelial nitric oxide synthase in both IDC (53% of control value) and CHD (43% of control value) hearts.

CONCLUSIONS

These results are the first to suggest that a loss of BK-2Rs is involved in the pathogenesis of human HF.

Lipoprotein receptors in extraembryonic tissues of the chicken

Yolk is the major source of nutrients for the developing chicken embryo, but molecular details of the delivery mechanisms are largely unknown. During oogenesis in the chicken, the main yolk components vitellogenin and very low density lipoprotein (VLDL) are taken up into the oocytes via a member of the low density lipoprotein receptor gene family termed LR8 (Bujo, H., Hermann, M., Kaderli, M. O., Jacobsen, L., Sugawara, S., Nimpf, J., Yamamoto, T., and Schneider, W. J. (1994) EMBO J. 13, 5165-5175). This endocytosis is accompanied by partial degradation of the yolk precursor protein moieties; however, fragmentation does not abolish binding of VLDL to LR8. The receptor exists in two isoforms that differ by a so-called O-linked sugar domain; the shorter form (LR8-) is the major form in oocytes, and the longer protein (LR8+) predominates in somatic cells. Here we show that both LR8 isoforms are expressed at ratios that vary with embryonic age in the extraembryonic yolk sac, which mobilizes yolk for utilization by the embryo, and in the allantois, the embryo's catabolic sink. Stored yolk VLDL interacts with LR8 localized on the surface of the yolk sac endodermal endothelial cells (EEC), is internalized, and degraded, as demonstrated by the catabolism of fluorescently labeled VLDL in cultured EEC. Addition to the incubation medium of the 39-kDa receptor-associated protein, which inhibits all known LR8/ligand interactions, blocks the uptake of VLDL by EEC. The levels of endogenous receptor-associated protein correspond to those of LR8+ but not LR8-, suggesting that it may play a role in the modulation of surface presentation of LR8+. Importantly, EEC express significant levels of microsomal triglyceride transfer protein and protein disulfide isomerase, key components required for lipoprotein synthesis. Because the apolipoprotein pattern of VLDL isolated from the yolk sac-efferent omphalomesenteric vein is very different from that of yolk VLDL, these data strongly suggest that embryo plasma VLDL is resynthesized in the EEC. LR8 is a key mediator of a two-step pathway, which affects the uptake of VLDL from the yolk sac and the subsequent delivery of its components to the growing embryo.

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.

Domain organization of the 39-kDa receptor-associated protein

The 39-kDa receptor-associated protein (RAP) is an endoplasmic reticulum resident protein that binds to the low density lipoprotein receptor-related protein (LRP) as well as certain members of the low density lipoprotein receptor superfamily and antagonizes ligand binding. In order to identify important functional regions of RAP, studies were performed to define the domain organization and domain boundaries of this molecule. Differential scanning calorimetry (DSC) experiments revealed that the process of thermal denaturation of RAP is highly reversible and occurs in a broad temperature range with two well resolved heat absorption peaks. A good fit of the endotherm was obtained with four two-state transitions suggesting these many cooperative domains in the molecule. A number of recombinant fragments of RAP were expressed in bacteria, and their domain composition and stability were characterized by DSC, circular dichroism, and fluorescence spectroscopy. The results confirmed that RAP is composed of four independently folded domains, D1, D2, D3, and D4, that encompass residues 1-92, 93-163, 164-216, and 217-323, respectively. The first and the fourth domains preserved their structure and stability when isolated, whereas the compact structure of the fragment corresponding to D2 seems to be altered when isolated from the parent molecule. Isolated D3 was partially degraded during isolation from bacterial lysates. The isolated D4 was capable of binding with high affinity to LRP whereas neither D1 nor D2 bound. At the same time a fragment containing both D1 and D2 exhibited high affinity binding to LRP. These facts combined with the thermodynamic analysis of the melting process of the fragments containing D1 and D2 indicate that these two domains interact with each other and that the proper folding of the second domain into a native-like active conformation requires presence of the first domain.

Localization of a single transglutaminase-reactive glutamine in the third domain of RAP, the alpha2-macroglobulin receptor-associated protein

The 39-kDa receptor-associated protein (RAP) is an intracellular glycoprotein that interacts with hitherto unknown sites in several members of the low-density-lipoprotein receptor gene family. Upon binding to these receptors, RAP inhibits all ligand interactions with the receptors. In the present study, the transglutaminase-catalyzed incorporation of radioactively labeled putrescine and a dansylated glutamine-containing peptide into human RAP has been studied. The results indicate the presence of both glutamine and lysine residues in RAP, accessible for transglutaminase cross-linking. Moreover, enzymatic digestion followed by sequence analysis of radiolabeled fractions demonstrated that Gln261 acts as the amine acceptor site. This residue is located in the third domain of RAP and is conserved among the RAP interspecies homologues. Insertion of a reporter group into the protein could prove useful to assess ligand/receptor interactions.

The VLDL receptor: an LDL receptor relative with eight ligand binding repeats, LR8

The discovery in 1992 of a member of the low density lipoprotein receptor (LDLR) family with eight ligand binding repeats (LR8) has raised more questions than have been answered to date. Here, we summarize the current status of knowledge about this intriguing molecule, generally termed VLDL receptor, at the molecular biological, cell biological, and physiological levels. On one hand, the wealth of reports concerning the role(s) of this receptor in lipoprotein metabolism in mammalian systems has revealed partially conflicting details, particularly in regards to its natural ligand(s) and site of action. On the other hand, molecular genetic and biochemical studies in the chicken have clearly demonstrated the multiple roles of LR8 in the physiology and reproduction of egg-laying species, and have generated insights into the evolutionary aspects of the LDLR gene family.

The low density lipoprotein receptor gene family. Differential expression of two alpha2-macroglobulin receptors in the brain

LR7/8B is a member of the low density lipoprotein receptor gene family that is specifically synthesized in the brain. Here we have functionally expressed in 293 cells the splice variant harboring eight ligand binding repeats (LR8B). As assessed by confocal microscopy, the expressed receptor is localized to the plasma membrane. Importantly, in cell binding experiments, we demonstrate that this protein is a receptor for activated alpha2-macroglobulin. Because to date low density lipoprotein receptor-related protein (LRP) has been shown to be the only alpha2-macroglobulin receptor in brain, we became interested in the expression pattern of both proteins at the cellular level in the brain. LR7/8B is expressed in large neurons and Purkinje cells of the cerebellum and in cells constituting brain barrier systems such as the epithelial cells of the choroid plexus, the arachnoidea, and the endothelium of penetrating blood vessels. Anti-LR7/8B antibody stains the plasma membrane, dendrites, and vesicular structures close to the cell membrane of neurons, especially of Purkinje cells. In contrast, LRP is present in patchy regions around large neurons and most prominently in the glomeruli of the stratum granulare of the cerebellum. This suggests that, contrary to LR7/8B, LRP is expressed in synaptic regions of the neurons; furthermore, there is a striking difference in the expression patterns of LR7/8B and LRP in the choroid plexus. Whereas LRP shows baso-lateral and apical localization in the epithelial cells, LR7/8B is restricted to the apical cell aspect facing the cerebrospinal fluid. Finally, these studies were extended to cultured primary rat neurons, where double immunofluorescence labeling with anti-LR7/8B and anti-microtubuli-associated protein 2 (MAP2) confirmed the somatodendritic expression of the receptor. Based upon these data, we propose that LR7/8B is involved in the clearance of alpha2-macroglobulin.proteinase complexes and/or of other substrates bound to alpha2-macroglobulin from the cerebrospinal fluid and from the surface of neurons.

S-phase DNA damage checkpoint in budding yeast

Eukaryotic cells must be able to coordinate DNA repair, replication and cell cycle progression in response to DNA damage. A failure to activate the checkpoints which delay the cell cycle in response to internal and external cues and to repair the DNA lesions results in an increase in genetic instability and cancer predisposition. The use of the yeast Saccharomyces cerevisiae has been invaluable in isolating many of the genes required for the DNA damage response, although the molecular mechanisms which couple this regulatory pathway to different DNA transactions are still largely unknown. In analogy with prokaryotes, we propose that DNA strand breaks, caused by genotoxic agents or by replication-related lesions, trigger a replication coupled repair mechanism, dependent upon recombination, which is induced by the checkpoint acting during S-phase.

Apolipoprotein A-I production by chicken granulosa cells

In avian species such as the chicken, development of the oocyte is associated with massive deposition of yolk in this cell. Oocytes grow within the follicle, a compartment consisting of a very specialized set of cells and acellular structures. The oocyte is surrounded by the perivitelline layer and granulosa cells, which are separated from the thecae by a pronounced basement membrane. In addition to the production of yolk precursors in the liver, we have long implied that cells within the follicle make a direct contribution to the growth of the oocyte. Here we show that chicken granulosa cells express and actively secrete apolipoprotein A-I (apoA-I) as a part of particles with very high density. The granulosa cell-derived, apoA-I-containing material is different from the small portion of yolk high density lipoprotein that arises via transfer from the peripheral circulation. We propose that the ApoA-I-containing particles secreted by granulosa cells 1) support the growth of the rapidly growing germ cell, possibly by direct lipid transfer to the plasma membrane of the oocyte, and/or 2) deliver cholesteryl esters to the steroid-producing cells of the theca layer. These findings are discussed with respect to the proposed functions of apoE (an apolipoprotein not found in chicken) within the mammalian follicle.