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Wieczorek E, Kędracka–Krok S, Sołtys K, Jankowska U, Hołubowicz R, Seliga J, Ożyhar A. Is Transthyretin a Regulator of Ubc9 SUMOylation? PLoS One 2016; 11:e0160536. [PMID: 27501389 PMCID: PMC4976990 DOI: 10.1371/journal.pone.0160536] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2016] [Accepted: 07/20/2016] [Indexed: 12/21/2022] Open
Abstract
Ageing and mutations of transthyretin (TTR), the thyroid hormones and retinol transporting protein lead to amyloidosis by destabilizing the structure of TTR. Because protein structure is regulated through posttranslational modifications, we investigated the Small Ubiquitin-like Modifier (SUMO)ylation of TTR. We chose the widely used Ubc9 fusion-directed SUMOylation system, which is based on a fusion of the SUMOylation substrate of interest with Ubc9, a sole SUMO conjugating enzyme. Surprisingly, despite our presumptions, we found that Ubc9 fused to TTR was SUMOylated at a unique set of lysine residues. Three unknown SUMOylation sites of Ubc9-K154, K18 and K65-were revealed by mass spectrometry (MS). The previously reported SUMOylation at K49 of Ubc9 was also observed. SUMOylation of the lysine residues of TTR fused to Ubc9 was hardly detectable. However, non-fused TTR was SUMOylated via trans-SUMOylation by Ubc9 fused to TTR. Interestingly, mutating the catalytic residue of Ubc9 fused to TTR did not result in complete loss of the SUMOylation signal, suggesting that Ubc9 linked to TTR is directly cross-SUMOylated by the SUMO-activating enzyme E1. Ubc9, TTR or fusion proteins composed of TTR and Ubc9 specifically affected the global SUMOylation of cellular proteins. TTR or Ubc9 alone increased global SUMOylation, whereas concomitant presence of TTR and Ubc9 did not further increase the amount of high-molecular weight (HMW) SUMO conjugates. Our data suggest that TTR may influence the SUMOylation of Ubc9, thereby altering signalling pathways in the cell.
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Affiliation(s)
- Elżbieta Wieczorek
- Department of Biochemistry, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław, Poland
- * E-mail:
| | - Sylwia Kędracka–Krok
- Department of Physical Biochemistry, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Krakow, Poland
| | - Katarzyna Sołtys
- Department of Biochemistry, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław, Poland
| | - Urszula Jankowska
- Department of Structural Biology, Malopolska Centre of Biotechnology, Jagiellonian University, Krakow, Poland
| | - Rafał Hołubowicz
- Department of Biochemistry, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław, Poland
| | - Justyna Seliga
- Department of Biochemistry, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław, Poland
| | - Andrzej Ożyhar
- Department of Biochemistry, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław, Poland
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Hanak BW, Ross EF, Harris CA, Browd SR, Shain W. Toward a better understanding of the cellular basis for cerebrospinal fluid shunt obstruction: report on the construction of a bank of explanted hydrocephalus devices. J Neurosurg Pediatr 2016; 18:213-23. [PMID: 27035548 PMCID: PMC5915300 DOI: 10.3171/2016.2.peds15531] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
OBJECTIVE Shunt obstruction by cells and/or tissue is the most common cause of shunt failure. Ventricular catheter obstruction alone accounts for more than 50% of shunt failures in pediatric patients. The authors sought to systematically collect explanted ventricular catheters from the Seattle Children's Hospital with a focus on elucidating the cellular mechanisms underlying obstruction. METHODS In the operating room, explanted hardware was placed in 4% paraformaldehyde. Weekly, samples were transferred to buffer solution and stored at 4°C. After consent was obtained for their use, catheters were labeled using cell-specific markers for astrocytes (glial fibrillary acidic protein), microglia (ionized calcium-binding adapter molecule 1), and choroid plexus (transthyretin) in conjunction with a nuclear stain (Hoechst). Catheters were mounted in custom polycarbonate imaging chambers. Three-dimensional, multispectral, spinning-disk confocal microscopy was used to image catheter cerebrospinal fluid-intake holes (10× objective, 499.2-μm-thick z-stack, 2.4-μm step size, Olympus IX81 inverted microscope with motorized stage and charge-coupled device camera). Values are reported as the mean ± standard error of the mean and were compared using a 2-tailed Mann-Whitney U-test. Significance was defined at p < 0.05. RESULTS Thirty-six ventricular catheters have been imaged to date, resulting in the following observations: 1) Astrocytes and microglia are the dominant cell types bound directly to catheter surfaces; 2) cellular binding to catheters is ubiquitous even if no grossly visible tissue is apparent; and 3) immunohistochemical techniques are of limited utility when a catheter has been exposed to Bugbee wire electrocautery. Statistical analysis of 24 catheters was performed, after excluding 7 catheters exposed to Bugbee wire cautery, 3 that were poorly fixed, and 2 that demonstrated pronounced autofluorescence. This analysis revealed that catheters with a microglia-dominant cellular response tended to be implanted for shorter durations (24.7 ± 6.7 days) than those with an astrocyte-dominant response (1183 ± 642 days; p = 0.027). CONCLUSIONS Ventricular catheter occlusion remains a significant source of shunt morbidity in the pediatric population, and given their ability to intimately associate with catheter surfaces, astrocytes and microglia appear to be critical to this pathophysiology. Microglia tend to be the dominant cell type on catheters implanted for less than 2 months, while astrocytes tend to be the most prevalent cell type on catheters implanted for longer time courses and are noted to serve as an interface for the secondary attachment of ependymal cells and choroid plexus.
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Affiliation(s)
- Brian W. Hanak
- Center for Integrative Brain Research, Seattle Children’s Research Institute,Department of Neurological Surgery, University of Washington, Seattle, Washington
| | - Emily F. Ross
- Center for Integrative Brain Research, Seattle Children’s Research Institute
| | - Carolyn A. Harris
- Department of Neurosurgery, Wayne State University, Detroit, Michigan
| | - Samuel R. Browd
- Center for Integrative Brain Research, Seattle Children’s Research Institute,Department of Neurological Surgery, University of Washington, Seattle, Washington
| | - William Shain
- Center for Integrative Brain Research, Seattle Children’s Research Institute,Department of Neurological Surgery, University of Washington, Seattle, Washington
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Johanson CE, Stopa EG, McMillan PN. The blood-cerebrospinal fluid barrier: structure and functional significance. Methods Mol Biol 2011; 686:101-131. [PMID: 21082368 DOI: 10.1007/978-1-60761-938-3_4] [Citation(s) in RCA: 136] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
The choroid plexus (CP) of the blood-CSF barrier (BCSFB) displays fundamentally different properties than blood-brain barrier (BBB). With brisk blood flow (10 × brain) and highly permeable capillaries, the human CP provides the CNS with a high turnover rate of fluid (∼400,000 μL/day) containing micronutrients, peptides, and hormones for neuronal networks. Renal-like basement membranes in microvessel walls and underneath the epithelium filter large proteins such as ferritin and immunoglobulins. Type IV collagen (α3, α4, and α5) in the subepithelial basement membrane confers kidney-like permselectivity. As in the glomerulus, so also in CP, the basolateral membrane utrophin A and colocalized dystrophin impart structural stability, transmembrane signaling, and ion/water homeostasis. Extensive infoldings of the plasma-facing basal labyrinth together with lush microvilli at the CSF-facing membrane afford surface area, as great as that at BBB, for epithelial solute and water exchange. CSF formation occurs by basolateral carrier-mediated uptake of Na+, Cl-, and HCO3-, followed by apical release via ion channel conductance and osmotic flow of water through AQP1 channels. Transcellular epithelial active transport and secretion are energized and channeled via a highly dense organelle network of mitochondria, endoplasmic reticulum, and Golgi; bleb formation occurs at the CSF surface. Claudin-2 in tight junctions helps to modulate the lower electrical resistance and greater permeability in CP than at BBB. Still, ratio analyses of influx coefficients (Kin) for radiolabeled solutes indicate that paracellular diffusion of small nonelectrolytes (e.g., urea and mannitol) through tight junctions is restricted; molecular sieving is proportional to solute size. Protein/peptide movement across BCSFB is greatly limited, occurring by paracellular leaks through incomplete tight junctions and low-capacity transcellular pinocytosis/exocytosis. Steady-state concentration ratios, CSF/plasma, ranging from 0.003 for IgG to 0.80 for urea, provide insight on plasma solute penetrability, barrier permeability, and CSF sink action to clear substances from CNS.
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Affiliation(s)
- Conrad E Johanson
- Department of Clinical Neuroscience, Alpert Medical School at Brown University, Providence, RI, USA
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4
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The transthyretin gene is expressed in human and rodent dorsal root ganglia. Neurosci Lett 2008; 436:335-9. [DOI: 10.1016/j.neulet.2008.03.063] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2008] [Revised: 03/19/2008] [Accepted: 03/20/2008] [Indexed: 11/20/2022]
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Kassem NA, Deane R, Segal MB, Preston JE. Role of transthyretin in thyroxine transfer from cerebrospinal fluid to brain and choroid plexus. Am J Physiol Regul Integr Comp Physiol 2006; 291:R1310-5. [PMID: 16825415 DOI: 10.1152/ajpregu.00789.2005] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The transport of 125I-labeled thyroxine (T4) from the cerebrospinal fluid (CSF) into brain and choroid plexus (CP) was measured in anesthetized rabbit [0.5 mg/kg medetomidine (Domitor) and 10 mg/kg pentobarbitonal sodium (Sagatal) iv] using the ventriculocisternal (V-C) perfusion technique. 125I-labeled T4 contained in artificial CSF was continually perfused into the lateral ventricles for up to 4 h and recovered from the cisterna magna. The %recovery of 125I-labeled T4 from the aCSF was 47.2+/-5.6% (n=10), indicating removal of 125I-labeled T4 from the CSF. The recovery increased to 53.2+/-6.3% (n=4) and 57.8+/-14.8% (n=3), in the presence of 100 and 200 microM unlabeled-T4, respectively (P<0.05), indicating a saturable component to T4 removal from CSF. There was a large accumulation of 125I-labeled T4 in the CP, and this was reduced by 80% in the presence of 200 microM unlabeled T4, showing saturation. In the presence of the thyroid-binding protein transthyretin (TTR), more 125I-labeled T4 was recovered from CSF, indicating that the binding protein acted to retain T4 in CSF. However, 125I-labeled T4 uptake into the ependymal region (ER) of the frontal cortex also increased by 13 times compared with control conditions. Elevation was also seen in the hippocampus (HC) and brain stem. Uptake was significantly inhibited by the presence of endocytosis inhibitors nocodazole and monensin by >50%. These data suggest that the distribution of T4 from CSF into brain and CP is carrier mediated, TTR dependent, and via RME. These results support a role for TTR in the distribution of T4 from CSF into brain sites around the ventricular system, indicating those areas involved in neurogenesis (ER and HC).
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Affiliation(s)
- Nouhad A Kassem
- King's College London, Institute of Gerontology and Wolfson Centre for Age Related Diseases, Hodgkin Bldg., Guy's Hospital Campus, London SE1 1UL, UK.
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6
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Chen RL, Kassem NA, Preston JE. Dose-dependent transthyretin inhibition of T4 uptake from cerebrospinal fluid in sheep. Neurosci Lett 2006; 396:7-11. [PMID: 16325339 DOI: 10.1016/j.neulet.2005.11.003] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2005] [Revised: 11/02/2005] [Accepted: 11/02/2005] [Indexed: 11/21/2022]
Abstract
Transthyretin (TTR), synthesized by the choroid plexuses (CP) has an important role in transporting thyroxine from blood to cerebrospinal fluid (CSF). However, the role of TTR on thyroxine transport from CSF to either blood or brain is not clear. By using the incubated isolated ovine brain tissues technique, we found the CP accumulated most 125I-T4 compared to ventricular ependymal, frontal cortex or cerebellum. The accumulation was higher in the young CP than the old. There was dose-dependent inhibition by TTR on 125I-T4 accumulation in the brain tissues, and kinetics of T4 accumulation in presence of TTR was obtained by plotting a double reciprocal of B (bound) versus TTR concentration curve. The KD of 125I-T4 binding to TTR was higher in the CP compared to other tissues, suggesting that CP competes with TTR for T4 binding to a greater extent than the other tissues. Using the isolated perfused CP preparation, TTR significantly inhibited 125I-T4 efflux across CP from the CSF to blood side. Bovine serum albumin (BSA) was also able to inhibit 125I-T4 accumulation in the incubated tissues, but required higher concentrations to reach the level of inhibition seen with TTR. In conclusion, this study found a significant role for CSF TTR in preventing T4 loss to blood across the CP, and TTR inhibited both CP and selected brain tissue uptake in a dose-dependent manner. The physiological relevance of TTR may relate to preventing T4 loss from CSF and encouraging redistribution of hormone around the brain in CSF.
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Affiliation(s)
- R L Chen
- Institute of Gerontology, Franklin-Wilkins Building, King's College London, 150 Stamford Street, London SE1 1UL, UK.
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Devarajan K, Marchant EG, Rusak B. Circadian and light regulation of oxytocin and parvalbumin protein levels in the ciliated ependymal layer of the third ventricle in the C57 mouse. Neuroscience 2005; 134:539-47. [PMID: 15964694 DOI: 10.1016/j.neuroscience.2005.04.034] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2004] [Revised: 04/05/2005] [Accepted: 04/15/2005] [Indexed: 10/25/2022]
Abstract
The walls of the third ventricle have been proposed to serve as a bidirectional conduit for exchanges between the neural parenchyma and the cerebrospinal fluid. In immunohistochemical studies of mice, we observed that light exposure and circadian phase affected peptide staining surrounding the third ventricle at the level of the suprachiasmatic nuclei. Under high magnification, we observed robust staining for the neurohormone oxytocin and the calcium-binding protein parvalbumin associated with cilia extending into the third ventricle from the surrounding ventricular wall; no similar staining was observed for vasopressin or calbindin. Retinal illumination had opposite effects on levels of parvalbumin and oxytocin in the cilia: light exposure during late subjective night increased oxytocin staining, but decreased parvalbumin staining in the cilia. Preventing cellular transport with colchicine eliminated immunohistochemical staining for oxytocin in the cilia. There was also a significant daily rhythm of oxytocin immunostaining in the third ventricle wall, and in magnocellular neurons in the anterior hypothalamus. The results suggest that environmental lighting and circadian rhythms regulate levels of oxytocin in the cerebrospinal fluid, possibly by regulating movement of oxytocin through the third ventricle wall.
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Affiliation(s)
- K Devarajan
- Department of Psychology, Life Sciences Centre, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1
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Johanson CE, Duncan JA, Stopa EG, Baird A. Enhanced Prospects for Drug Delivery and Brain Targeting by the Choroid Plexus–CSF Route. Pharm Res 2005; 22:1011-37. [PMID: 16028003 DOI: 10.1007/s11095-005-6039-0] [Citation(s) in RCA: 98] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2005] [Accepted: 04/12/2005] [Indexed: 02/07/2023]
Abstract
The choroid plexus (CP), i.e., the blood-cerebrospinal fluid barrier (BCSFB) interface, is an epithelial boundary exploitable for drug delivery to brain. Agents transported from blood to lateral ventricles are convected by CSF volume transmission (bulk flow) to many periventricular targets. These include the caudate, hippocampus, specialized circumventricular organs, hypothalamus, and the downstream pia-glia and arachnoid membranes. The CSF circulatory system normally provides micronutrients, neurotrophins, hormones, neuropeptides, and growth factors extensively to neuronal networks. Therefore, drugs directed to CSF can modulate a variety of endocrine, immunologic, and behavioral phenomema; and can help to restore brain interstitial and cellular homeostasis disrupted by disease and trauma. This review integrates information from animal models that demonstrates marked physiologic effects of substances introduced into the ventricular system. It also recapitulates how pharmacologic agents administered into the CSF system prevent disease or enhance the brain's ability to recover from chemical and physical insults. In regard to drug distribution in the CNS, the BCSFB interaction with the blood-brain barrier is discussed. With a view toward translational CSF pharmacotherapy, there are several promising innovations in progress: bone marrow cell infusions, CP encapsulation and transplants, neural stem cell augmentation, phage display of peptide ligands for CP epithelium, CSF gene transfer, regulation of leukocyte and cytokine trafficking at the BCSFB, and the purification of neurotoxic CSF in degenerative states. The progressively increasing pharmacological significance of the CP-CSF nexus is analyzed in light of treating AIDS, multiple sclerosis, stroke, hydrocephalus, and Alzheimer's disease.
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Affiliation(s)
- Conrad E Johanson
- Department of Clinical Neurosciences, Rhode Island Hospital, Brown Medical School, Providence, Rhode Island 02912, USA.
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9
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Gard AL, Gavin E, Solodushko V, Pennica D. Cardiotrophin-1 in choroid plexus and the cerebrospinal fluid circulatory system. Neuroscience 2004; 127:43-52. [PMID: 15219667 DOI: 10.1016/j.neuroscience.2004.03.065] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/29/2004] [Indexed: 01/29/2023]
Abstract
There is a growing recognition of choroid plexus functioning as a source of neuropeptides, cytokines and growth factors in cerebrospinal fluid (CSF) with diffusional access into brain parenchyma. In this study, choroid plexus and other components of the CSF circulatory system were investigated by Western blotting, reverse transcriptase polymerase chain reaction and immunohistochemistry for production of interleukin-6-related cytokines characterized by neuroactivity [cardiotrophin-1 (CT-1), ciliary neurotrophic factor, leukemia inhibitory factor, oncostatin M] and signaling through the gp130/leukemia inhibitory factor receptor-beta receptor heterodimer. Western blot analysis showed that CT-1 was the only cytokine family member detectable in adult rat choroid plexus, as in leptomeninges. The specificity of detection was verified with blots of the same tissues from CT-1-deficient mice. Levels of both CT-1 mRNA and protein were constitutively high in rat from birth through adulthood in choroid plexus, up-regulated postnatally in leptomeninges and undetectable in brain parenchyma. Using antigen retrieval, CT-1 immunolocalized to choroid epithelial cells in all choroid plexuses in addition to leptomeninges (arachnoid and pial-glial membranes). Ependymal cells lining the ventricular neuroaxis, unlike the central canal, were also CT-1-immunoreactive. Western blots indicated rat choroid epithelial cells express and release CT-1 immunoreactivity under defined culture conditions and also revealed the presence of a CT-1-like protein in human choroid plexus and CSF. Previously, CT-1 has been conceptualized to function as a target-derived factor for PNS neurons. Our study clearly demonstrates production of CT-1 in the postnatal and adult CNS, specifically by cell types comprising the blood-CSF barrier, and its accumulation in ventricular ependyma. This finding has broad implications for CT-1 functioning apart from other leukemia inhibitory factor receptor ligands as a CSF-borne signal of brain homeostasis, one possibly involving regulation of the barrier itself, the ependyma or target cells in the surrounding parenchyma, including the subventricular zone. A rationale for studies examining CT-1-deficient mice in these respects is provided by the data.
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Affiliation(s)
- A L Gard
- Department of Cell Biology and Neuroscience, 2038 Medical Sciences Building, University of South Alabama College of Medicine, Mobile, AL 36688-0002, USA.
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Yamauchi K, Prapunpoj P, Richardson SJ. Effect of diethylstilbestrol on thyroid hormone binding to amphibian transthyretins. Gen Comp Endocrinol 2000; 119:329-39. [PMID: 11017780 DOI: 10.1006/gcen.2000.7528] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Transthyretin (TTR) is responsible for a major part of the binding of thyroid hormone to proteins in the plasma in amphibian tadpoles. To characterize the binding properties of amphibian TTRs, the effects of 17 hydrophobic signaling molecules, including 6 endocrine disruptors, on 3,5,3'-l-[(125)I]triiodothyronine ([(125)I]T(3)) binding to plasma proteins were examined in bullfrog Rana catesbeiana tadpoles. T(3) was the most potent competitive inhibitor among the 11 natural biological ligands studied, with an ID(50) of 8 nM. Diethylstilbestrol (DES) was the most powerful inhibitor among the 6 endocrine disruptors studied, with an ID(50) of 20 nM. Similar inhibitions of [(125)I]T(3) binding by these compounds were obtained when purified recombinant Xenopus and Rana TTRs were analyzed. Scatchard analysis revealed that Xenopus and Rana TTRs each possessed a single class of binding site for T(3), with a K(d) of 262 and 1.9 nM, respectively, at 0 degrees C. DES, at a concentration of 200 nM, induced the uptake of [(125)I]T(3) into Rana red blood cells suspended in Rana plasma from prometamorphic stages XIII-XV, when TTR is present in plasma. DES induced the uptake of [(125)I]T(3) into red blood cells to a lesser extent when they were suspended in Rana plasma from metamorphic climax stage XXIV, in which the level of TTR was lower than in plasma from the prometamorphic tadpoles. These results indicate that amphibian TTRs have the ability to bind DES with similar affinity to T(3), the natural ligand, and raise the possibility that DES binding to TTR might induce the temporary elevation of the free concentration of plasma T(3) followed by acceleration of cellular T(3) uptake.
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Affiliation(s)
- K Yamauchi
- Department of Biology, Shizuoka University, Shizuoka 422-8529, Japan.
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11
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Kuchler-Bopp S, Dietrich JB, Zaepfel M, Delaunoy JP. Receptor-mediated endocytosis of transthyretin by ependymoma cells. Brain Res 2000; 870:185-94. [PMID: 10869517 DOI: 10.1016/s0006-8993(00)02413-6] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
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.
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MESH Headings
- Animals
- Biological Transport/physiology
- Blotting, Northern
- Brain Neoplasms
- Cell Line, Transformed/chemistry
- Cell Line, Transformed/metabolism
- Cell Line, Transformed/ultrastructure
- Endocytosis/physiology
- Ependyma/cytology
- Ependymoma
- Gene Expression Regulation, Neoplastic
- Humans
- Iodine Radioisotopes
- Mice
- Mice, Transgenic
- Microscopy, Electron
- Prealbumin/genetics
- Prealbumin/pharmacokinetics
- RNA, Messenger/analysis
- Rats
- Receptors, Albumin/analysis
- Receptors, Albumin/metabolism
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Affiliation(s)
- S Kuchler-Bopp
- Laboratoire de Neurobiologie Moléculaire des Interactions Cellulaires, 11, rue Humann, 67085 cedex, Strasbourg, France.
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12
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Abstract
The slow clearance, prolonged half-life, and high serum concentration of thyroxine (T4) are largely due to strong binding by the principal plasma thyroid hormone-binding proteins, thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin. These proteins, which shield the hydrophobic thyroid hormones from their aqueous environment, buffer a stable free T4 concentration for cell uptake. Free rather than bound T4 is subject to homeostatic control by the hypothalamic-pituitary thyroid axis. Although it is not a protease inhibitor, sequence analysis identifies TBG as a member of the serine protease inhibitor (serpin) family of proteins. Proteolytic cleavage of TBG appears to be a mechanism for site-specific release of T4 independently of homeostatic control. TBG probably facilitates the transport of maternal T4 and iodide to the fetus, although this remains to be proven. High-affinity cellular binding sites for TTR have been described; however, their function and that of choroid plexus synthesis of TTR and transport of T4 into the cerebrospinal fluid remain unclear. Albumin, with the lowest T4 affinity and fastest T4 release of the major T4-binding proteins may promote quick exchange of T4 with tissue sites. The affinity of albumin for T4 is increased by histidine substitution for arginine 218 in the most common form of dysalbuminemic hyperthyroxinemia. However, proline and alanine substitutions at the same site have a similar effect, suggesting that arginine 218 interferes with T4 binding.
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Affiliation(s)
- G C Schussler
- State University of New York Health Science Center, Brooklyn 11203, USA.
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