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Abstract
The ability to investigate cellular processes in vitro permits detailed analysis of the process and its molecular components. Eukaryotic translation and expression is one system that has been well studied. This overview describes the development of in vitro systems, including such approaches as continuous-flow systems, coupled transcription/translation, and the incorporation of non-natural amino acids. It also discusses molecular and genetic studies to probe translation, including post-translational fate of the synthesized proteins.
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Affiliation(s)
- Rosemary Jagus
- Center of Marine and Biotechnology and Greenebaum Cancer Center, Baltimore, Maryland, USA
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2
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Abstract
The type IIa Na(+)/P(i) cotransporter (NaPi-IIa) plays a key role in the reabsorption of inorganic phosphate (P(i)) in the renal proximal tubule. The rat NaPi-IIa isoform is a protein of 637 residues for which different algorithms predict 8-12 transmembrane domains (TMDs). Epitope tagging experiments demonstrated that both the N and the C termini of NaPi-IIa are located intracellularly. Site-directed mutagenesis revealed two N-glycosylation sites in a large putative extracellular loop. Results from structure-function studies suggested the assembly of two similar opposed regions that possibly constitute part of the substrate translocation pathway for one phosphate ion together with three sodium ions. Apart from these topological aspects, other structural features of NaPi-IIa are not known. In this study, we have addressed the topology of NaPi-IIa using in vitro transcription/translation of HK-M0 and HK-M1 fusion vectors designed to test membrane insertion properties of cDNA sequences encoding putative NaPi-IIa TMDs. Based on the results of in vitro transcription/translation analyses, we propose a model of NaPi-IIa comprising 12 TMDs, with both N and C termini orientated intracellularly and a large hydrophilic extracellular loop between the fifth and sixth TMDs. The proposed model is in good agreement with the prediction of the NaPi-IIa structure obtained by the hidden Markov algorithm HMMTOP.
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Affiliation(s)
- Tamara Radanovic
- Institute of Physiology and Center for Integrative Human Physiology, University of Zurich, Winterthurerstrasse 190, CH-8057, Zurich, Switzerland
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3
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Abstract
We demonstrated previously that calreticulin (CRT) interacts with the lumenal COOH-terminal sequence of sarco endoplasmic reticulum (ER) calcium ATPase (SERCA) 2b to inhibit Ca2+ oscillations. Work from other laboratories demonstrated that CRT also interacts with the ER oxidoreductase, ER protein 57 (also known as ER-60, GRP58; ERp57) during folding of nascent glycoproteins. In this paper, we demonstrate that ERp57 overexpression reduces the frequency of Ca2+ oscillations enhanced by SERCA 2b. In contrast, overexpression of SERCA 2b mutants defective in cysteines located in intralumenal loop 4 (L4) increase Ca2+ oscillation frequency. In vitro, we demonstrate a Ca2+-dependent and -specific interaction between ERp57 and L4. Interestingly, ERp57 does not affect the activity of SERCA 2a or SERCA 2b mutants lacking the CRT binding site. Overexpression of CRT domains that disrupt the interaction of CRT with ERp57 behave as dominant negatives in the Ca2+ oscillation assay. Our results suggest that ERp57 modulates the redox state of ER facing thiols in SERCA 2b in a Ca2+-dependent manner, providing dynamic control of ER Ca2+ homeostasis.
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Affiliation(s)
- Yun Li
- Dept. of Physiology, MSC 7756, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
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4
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Abstract
The ten-transmembrane Na,K-ATPase alpha-subunit exposes very few amino acids to the extra membrane space except for an approximately 408 residue-long loop between transmembrane segments four and five. The present paper focuses on the role of this loop in biosynthesis of functional Na,K-ATPase. Expression of 39 mutations in this loop to phylogenetically conserved as well as nonconserved residues showed that only two could be expressed at 30 degrees C. By contrast, only five could not be produced in a functional form at 15 degrees C. A detailed analysis showed that a number of these mutants are temperature-sensitive folding mutants, as they induce the unfolded protein response at 30 degrees C but not at 15 degrees C. We used an algorithm to predict that residues (868)ENGFLIPIHLL(878) in the L78 loop exposed to the endoplasmic reticulum lumen constitute the most likely BiP binding site. Correct folding of this sequence may be important in the endoplasmic reticulum quality control, as the same loop is responsible for the alpha-beta-associations required to leave this compartment. On the basis of the Ca-ATPase crystal structure and the presented data, we propose a model to account for the role of the TM4-TM5 loop in Na,K-ATPase biosynthesis.
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Affiliation(s)
- Jesper R Jørgensen
- Biomembrane Research Centre, August Krogh Institute, University of Copenhagen, Denmark
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5
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Chami M, Gozuacik D, Lagorce D, Brini M, Falson P, Peaucellier G, Pinton P, Lecoeur H, Gougeon ML, le Maire M, Rizzuto R, Bréchot C, Paterlini-Bréchot P. SERCA1 truncated proteins unable to pump calcium reduce the endoplasmic reticulum calcium concentration and induce apoptosis. J Cell Biol 2001; 153:1301-14. [PMID: 11402072 PMCID: PMC2192035 DOI: 10.1083/jcb.153.6.1301] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
By pumping calcium from the cytosol to the ER, sarco/endoplasmic reticulum calcium ATPases (SERCAs) play a major role in the control of calcium signaling. We describe two SERCA1 splice variants (S1Ts) characterized by exon 4 and/or exon 11 splicing, encoding COOH terminally truncated proteins, having only one of the seven calcium-binding residues, and thus unable to pump calcium. As shown by semiquantitative RT-PCR, S1T transcripts are differentially expressed in several adult and fetal human tissues, but not in skeletal muscle and heart. S1T proteins expression was detected by Western blot in nontransfected cell lines. In transiently transfected cells, S1T homodimers were revealed by Western blot using mildly denaturing conditions. S1T proteins were shown, by confocal scanning microscopy, to colocalize with endogenous SERCA2b into the ER membrane. Using ER-targeted aequorin (erAEQ), we have found that S1T proteins reduce ER calcium and reverse elevation of ER calcium loading induced by SERCA1 and SERCA2b. Our results also show that SERCA1 variants increase ER calcium leakage and are consistent with the hypothesis of a cation channel formed by S1T homodimers. Finally, when overexpressed in liver-derived cells, S1T proteins significantly induce apoptosis. These data reveal a further mechanism modulating Ca(2+) accumulation into the ER of nonmuscle cells and highlight the relevance of S1T proteins to the control of apoptosis.
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Affiliation(s)
- Mounia Chami
- The French Institute of Health and Medical Research Institut National de la Santé et de la Recherche Médicale (INSERM/Pasteur U370)/Necker Faculty Institute of Medicine, 75015 Paris, France
| | - Devrim Gozuacik
- The French Institute of Health and Medical Research Institut National de la Santé et de la Recherche Médicale (INSERM/Pasteur U370)/Necker Faculty Institute of Medicine, 75015 Paris, France
| | - David Lagorce
- The French Institute of Health and Medical Research Institut National de la Santé et de la Recherche Médicale (INSERM/Pasteur U370)/Necker Faculty Institute of Medicine, 75015 Paris, France
| | - Marisa Brini
- Department of Biochemistry and Center for the Study of Biomembranes of the National Research Council (CNR), University of Padova, 35121 Padova, Italy
| | - Pierre Falson
- URA Centre National de Recherche Scientifique (CNRS) 2096, CEA Saclay, 91191 Gif sur Yvette, France
| | - Gérard Peaucellier
- National Center Scientific Research, URA 2156, Arago Laboratory, F66651 Banyuls sur mer, France
| | - Paolo Pinton
- Department of Experimental and Diagnostic Medicine, Section of General Pathology, 44100 Ferrara, Italy
| | - Hervé Lecoeur
- Pasteur Institute, Unit of Viral Oncology, SIDA Department of Retrovirus, 75015 Paris, France
| | - Marie-Lyse Gougeon
- Pasteur Institute, Unit of Viral Oncology, SIDA Department of Retrovirus, 75015 Paris, France
| | - Marc le Maire
- URA Centre National de Recherche Scientifique (CNRS) 2096, CEA Saclay, 91191 Gif sur Yvette, France
| | - Rosario Rizzuto
- Department of Experimental and Diagnostic Medicine, Section of General Pathology, 44100 Ferrara, Italy
| | - Christian Bréchot
- The French Institute of Health and Medical Research Institut National de la Santé et de la Recherche Médicale (INSERM/Pasteur U370)/Necker Faculty Institute of Medicine, 75015 Paris, France
| | - Patrizia Paterlini-Bréchot
- The French Institute of Health and Medical Research Institut National de la Santé et de la Recherche Médicale (INSERM/Pasteur U370)/Necker Faculty Institute of Medicine, 75015 Paris, France
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6
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Chami M, Gozuacik D, Saigo K, Capiod T, Falson P, Lecoeur H, Urashima T, Beckmann J, Gougeon ML, Claret M, le Maire M, Bréchot C, Paterlini-Bréchot P. Hepatitis B virus-related insertional mutagenesis implicates SERCA1 gene in the control of apoptosis. Oncogene 2000; 19:2877-86. [PMID: 10871838 DOI: 10.1038/sj.onc.1203605] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
We have used the Hepatitis B Virus DNA genome as a probe to identify genes clonally mutated in vivo, in human liver cancers. In a tumor, HBV-DNA was found to be integrated into the gene encoding Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA), which pumps calcium, an important intracellular messenger for cell viability and growth, from the cytosol to the endoplasmic reticulum. The HBV X gene promoter cis-activates chimeric HBV X/SERCA1 transcripts, with splicing of SERCA1 exon 11, encoding C-terminally truncated SERCA1 proteins. Two chimeric HBV X/SERCA1 proteins accumulate in the tumor and form dimers. In vitro analyses have demonstrated that these proteins localize to the ER, determine its calcium depletion and induce cell death. We have also shown that these biological effects are related to expression of the SERCA, rather than of the viral moiety. This report involves for the first time the expression of mutated SERCA proteins in vivo in a tumor cell proliferation and in vitro in the control of cell viability. Oncogene (2000).
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Affiliation(s)
- M Chami
- U-370 INSERM, Necker Institute, 75015 Paris, France
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7
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Béguin P, Hasler U, Staub O, Geering K. Endoplasmic reticulum quality control of oligomeric membrane proteins: topogenic determinants involved in the degradation of the unassembled Na,K-ATPase alpha subunit and in its stabilization by beta subunit assembly. Mol Biol Cell 2000; 11:1657-72. [PMID: 10793142 PMCID: PMC14874 DOI: 10.1091/mbc.11.5.1657] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
The molecular nature of determinants that mediate degradation of unassembled, polytopic subunits of oligomeric membrane proteins and their stabilization after partner subunit assembly is largely unknown. Expressing truncated Na,K-ATPase alpha subunits alone or together with beta subunits, we find that in unassembled alpha subunits neither the four N-terminal transmembrane segments acting as efficient alternating signal anchor-stop transfer sequences nor the large, central cytoplasmic loop exposes any degradation signal, whereas poor membrane insertion efficiency of C-terminal membrane domains M5, M7, and M9 coincides with the transient exposure of degradation signals to the cytoplasmic side. beta assembly with an alpha domain comprising at least D902 up to Y910 in the extracytoplasmic M7/M8 loop is necessary to stabilize Na,K-ATPase alpha subunits by favoring M7/M8 membrane pair formation and by protecting a degradation signal recognized from the endoplasmic reticulum (ER) lumenal side. Thus our results suggest that ER degradation of Na,K-ATPase alpha subunits is 1) mainly mediated by folding defects caused by inefficient membrane insertion of certain membrane domains, 2) a multistep process, which involves proteolytic and/or chaperone components acting from the ER lumenal side in addition to cytosolic, proteasome-related factors, and 3) prevented by partner subunit assembly because of direct protection and retrieval of degradation signals from the cytoplasm to the ER lumenal side. These results likely represent a paradigm for the ER quality control of unassembled, polytopic subunits of oligomeric membrane proteins.
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Affiliation(s)
- P Béguin
- Institute of Pharmacology and Toxicology, University of Lausanne, CH-1005 Lausanne, Switzerland
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8
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Abstract
Integral membrane proteins are found in all cellular membranes and carry out many of the functions that are essential to life. The membrane-embedded domains of integral membrane proteins are structurally quite simple, allowing the use of various prediction methods and biochemical methods to obtain structural information about membrane proteins. A critical step in the biosynthetic pathway leading to the folded protein in the membrane is its insertion into the lipid bilayer. Understanding of the fundamentals of the insertion and folding processes will significantly improve the methods used to predict the three-dimensional membrane protein structure from the amino acid sequence. In the first part of this review, biochemical approaches to elucidate membrane protein topology are reviewed and evaluated, and in the second part, the use of similar techniques to study membrane protein insertion is discussed. The latter studies search for signals in the polypeptide chain that direct the insertion process. Knowledge of the topogenic signals in the nascent chain of a membrane protein is essential for the evaluation of membrane topology studies.
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Affiliation(s)
- M van Geest
- Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, 9751 NN Haren, The Netherlands
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9
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Abstract
X-linked recessive nephrogenic diabetes insipidus is caused by mutations in the gene encoding the V2 vasopressin receptor (V2R), the mediator of the antidiuretic effect of arginine vasopressin (AVP) in mammalian kidneys. Upon binding to AVP, the receptor activates the G protein Gs, stimulating a phosphorylation cascade that promotes translocation of presynthesized water channels to the apical surface of the principal cells lining the last segments of the nephron. The presence of these channels allows the flow of water from the hypotonic lumen of the nephron into the hypertonic interstitium. More than 100 different mutations have been identified since the receptor gene was characterized--in most cases one per family, although some families bear two and three mutations in the same gene. The frequency of the de novo mutations identified suggests that the DNA at the end of the long arm of the X chromosome is very susceptible to alteration. The mutations are scattered within the coding region, not confined to a particular segment of the receptor protein, and in most cases confined to a single amino acid change that significantly reduces the number of receptors present on the plasma membrane. Some mutations do not affect protein synthesis but significantly reduce the coupling efficiency between the receptor and G protein. Analysis of the biochemical impact of the mutations has provided valuable information about the synthesis and regulation of the receptor.
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Affiliation(s)
- M Birnbaumer
- Department of Anesthesiology, University of California, Los Angeles School of Medicine 90095, USA.
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10
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Rulli SJ, Horiba MN, Skripnikova E, Rabon EC. Glu-857 moderates K+-dependent stimulation and SCH 28080-dependent inhibition of the gastric H,K-ATPase. J Biol Chem 1999; 274:15245-50. [PMID: 10329734 DOI: 10.1074/jbc.274.21.15245] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The rabbit H,K-ATPase alpha- and beta-subunits were transiently expressed in HEK293 T cells. The co-expression of the H,K-ATPase alpha- and beta-subunits was essential for the functional H,K-ATPase. The K+-stimulated H,K-ATPase activity of 0.82 +/- 0.2 micromol/mg/h saturated with a K0.5 (KCl) of 0.6 +/- 0.1 mM, whereas the 2-methyl-8-(phenylmethoxy)imidazo[1,2a]pyridine-3-acetonitrile (SCH 28080)-inhibited ATPase of 0.62 +/- 0.07 micromol/mg/h saturated with a Ki (SCH 28080) of 1.0 +/- 0.3 microM. Site mutations were introduced at the N,N-dicyclohexylcarbodiimide-reactive residue, Glu-857, to evaluate the role of this residue in ATPase function. Variations in the side chain size and charge of this residue did not inhibit the specific activity of the H,K-ATPase, but reversal of the side chain charge by substitution of Lys or Arg for Glu produced a reciprocal change in the sensitivity of the H,K-ATPase to K+ and SCH 28080. The K0.5 for K+stimulated ATPase was decreased to 0.2 +/-.05 and 0.2 +/-.03 mM, respectively, in Lys-857 and Arg-857 site mutants, whereas the Ki for SCH 28080-dependent inhibition was increased to 6.5 +/- 1.4 and 5.9 +/- 1.5 microM, respectively. The H,K-ATPase kinetics were unaffected by the introduction of Ala at this site, but Leu produced a modest reciprocal effect. These data indicate that Glu-857 is not an essential residue for cation-dependent activity but that the residue influences the kinetics of both K+ and SCH 28080-mediated functions. This finding suggests a possible role of this residue in the conformational equilibrium of the H,K-ATPase.
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Affiliation(s)
- S J Rulli
- Department of Physiology, Tulane University Medical Center and the Department of Veterans Affairs, New Orleans, Louisiana 70112, USA
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11
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Beggah AT, Béguin P, Bamberg K, Sachs G, Geering K. beta-subunit assembly is essential for the correct packing and the stable membrane insertion of the H,K-ATPase alpha-subunit. J Biol Chem 1999; 274:8217-23. [PMID: 10075726 DOI: 10.1074/jbc.274.12.8217] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The alpha-subunits of H,K-ATPase (HKAalpha) and Na,K-ATPase require a beta-subunit for maturation. We investigated the role of the beta-subunit in the membrane insertion and stability of the HKAalpha expressed in Xenopus oocytes. Individual membrane segments M1, M2, M3, M4, and M9 linked to a glycosylation reporter act as signal anchor (SA) motifs, and M10 acts as a partial stop transfer motif. In combined HKAalpha constructs, M2 acts as an efficient stop transfer sequence, and M3 acts as a SA sequence. However, M5 and M9 have only partial SA function, and M7 has no SA function. Consistent with the membrane insertion properties of segments in combined alpha constructs, M1-3 alpha-proteins are resistant to cellular degradation, and M1-5 up to M1-10 alpha-proteins are not resistant to cellular degradation. However, co-expression with beta-subunits increases the membrane insertion of M9 in a M1-9 alpha-protein and completely protects M1-10 alpha-proteins against cellular degradation. Our results indicate that HKAalpha N-terminal (M1-M4) membrane insertion and stabilization are mediated by intrinsic molecular characteristics; however, the C-terminal (M5-M10) membrane insertion and thus the stabilization of the entire alpha-subunit depend on intramolecular and intermolecular beta-subunit interactions that are similar but not identical to data obtained for the Na,K-ATPase alpha-subunit.
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Affiliation(s)
- A T Beggah
- Institut de Pharmacologie et de Toxicologie de l'Université, Rue du Bugnon 27, CH-1005 Lausanne, Switzerland
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12
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Abstract
The control of membrane insertion of polytopic proteins is still poorly understood. We carried out in vivo translation/insertion experiments in Xenopus oocytes with combined wild type or mutant membrane segments of the alpha-subunit of the heterodimeric Na, K-ATPase linked to a glycosylation reporter sequence. We confirm that the four N-terminal hydrophobic segments of the alpha-subunit behave as alternating signal anchor/stop transfer motifs necessary for two lipid-inserted membrane pairs. For the six C-terminal membrane segments, however, proper packing depends on specific sequence information and association with the beta-subunit. M5 is a very inefficient signal anchor sequence due to the presence of prolines and polar amino acids. Its correct membrane insertion is probably mediated by posttranslational hairpin formation with M6, which is favored by a proline pair in the connecting loop. M7 has partial signal anchor function, which may be mediated by the presence of glycine and glutamine residues. The formation of a transmembrane M7/M8 pair requires the association of the beta-subunit, which induces a conformational change in the connecting extracytoplasmic loop that favors M7/M8 packing. The formation of the M9/M10 pair appears to be predominantly mediated by the efficient stop transfer function of M10. Mutations that provide signal anchor function to M5, M7, and M9 abolish or impede the transport activity of the enzyme. These data illustrate the importance of specific amino acids near or within hydrophobic regions as well as of subunit oligomerization for correct topographical alignment that is necessary for proper folding and/or activity of oligomeric membrane proteins.
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Affiliation(s)
- P Béguin
- Institut de Pharmacologie et de Toxicologie de l'Université, Rue du Bugnon 27, CH-1005 Lausanne, Switzerland
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