FMP Publications

Our publications are recorded in a searchable database since 2010, updates will be added regularly.

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References
Modulation of monocarboxylate transporter 8 oligomerization by specific pathogenic mutations
Fischer(*), J., Kleinau(*), G., Müller(*), A., Kühnen(*), P., Zwanziger(*), D., Kinne, A., Rehders(*), M., Moeller(*), L. C., Führer(*), D., Grüters(*), A., Krude(*), H., Brix(*), K.; Biebermann(*), H.
Journal of molecular endocrinology, 54:39-50
(2015)

Tags: Structural Bioinformatics and Protein Design (Krause, G.)

Abstract: The monocarboxylate transporter 8 (MCT8) is a member of the major facilitator superfamily (MFS). These membrane-spanning proteins facilitate translocation of a variety of substrates, MCT8 specifically transports iodothyronines. Mutations in MCT8 are the underlying cause of severe X-linked psychomotor retardation. At the molecular level, such mutations led to deficiencies in substrate translocation due to reduced cell-surface expression, impaired substrate binding, or decreased substrate translocation capabilities. However, the causal relationships between genotypes, molecular features of mutated MCT8, and patient characteristics have not yet been comprehensively deciphered. We investigated the relationship between pathogenic mutants of MCT8 and their capacity to form dimers (presumably oligomeric structures) as a potential regulatory parameter of the transport function of MCT8. Fourteen pathogenic variants of MCT8 were investigated in vitro with respect to their capacity to form oligomers. Particular mutations close to the substrate translocation channel (S194F, A224T, L434W, and R445C) were found to inhibit dimerization of MCT8. This finding is in contrast to those for other transporters or transmembrane proteins, in which substitutions predominantly at the outer-surface inhibit oligomerization. Moreover, specific mutations of MCT8 located in transmembrane helix 2 (del230F, V235M, and ins236V) increased the capacity of MCT8 variants to dimerize. We analyzed the localization of MCT8 dimers in a cellular context, demonstrating differences in MCT8 dimer formation and distribution. In summary, our results add a new link between the functions (substrate transport) and protein organization (dimerization) of MCT8, and might be of relevance for other members of the MFS. Finally, the findings are discussed in relationship to functional data combined with structural-mechanistical insights into MCT8.

Differences in Signal Activation by LH and hCG are Mediated by the LH/CG Receptor's Extracellular Hinge Region
Grzesik, P., Kreuchwig, A., Rutz, C., Furkert, J., Wiesner, B., Schülein, R., Kleinau(*), G., Gromoll(*), J.; Krause, G.
Front Endocrinol (Lausanne), 6:140
(2015)

Tags: Structural Bioinformatics and Protein Design (Krause, G.), Protein Trafficking (Schülein), Cellular Imaging (Wiesner)

Abstract: The human lutropin (hLH)/choriogonadotropin (hCG) receptor (LHCGR) can be activated by binding two slightly different gonadotropic glycoprotein hormones, choriogonadotropin (CG) - secreted by the placenta, and lutropin (LH) - produced by the pituitary. They induce different signaling profiles at the LHCGR. This cannot be explained by binding to the receptor's leucine-rich-repeat domain (LRRD), as this binding is similar for the two hormones. We therefore speculate that there are previously unknown differences in the hormone/receptor interaction at the extracellular hinge region, which might help to understand functional differences between the two hormones. We have therefore performed a detailed study of the binding and action of LH and CG at the LHCGR hinge region. We focused on a primate-specific additional exon in the hinge region, which is located between LRRD and the serpentine domain. The segment of the hinge region encoded by exon10 was previously reported to be only relevant to hLH signaling, as the exon10-deletion receptor exhibits decreased hLH signaling, but unchanged hCG signaling. We designed an advanced homology model of the hormone/LHCGR complex, followed by experimental characterization of relevant fragments in the hinge region. In addition, we examined predictions of a helical exon10-encoded conformation by block-wise polyalanine (helix supporting) mutations. These helix preserving modifications showed no effect on hormone-induced signaling. However, introduction of a structure-disturbing double-proline mutant LHCGR-Q303P/E305P within the exon10-helix has, in contrast to exon10-deletion, no impact on hLH, but only on hCG signaling. This opposite effect on signaling by hLH and hCG can be explained by distinct sites of hormone interaction in the hinge region. In conclusion, our analysis provides details of the differences between hLH- and hCG-induced signaling that are mainly determined in the L2-beta loop of the hormones and in the hinge region of the receptor.

Diffusional spread and confinement of newly exocytosed synaptic vesicle proteins
Gimber, N., Tadeus, G., Maritzen, T., Schmoranzer, J.; Haucke, V.
Nat Commun, 6:8392
(2015)

Tags: Molecular Pharmacology and Cell Biology (Haucke), Membrane Traffic and Cell Motility (Maritzen)

Abstract: Neurotransmission relies on the calcium-triggered exocytic fusion of non-peptide neurotransmitter-containing small synaptic vesicles (SVs) with the presynaptic membrane at active zones (AZs) followed by compensatory endocytic retrieval of SV membranes. Here, we study the diffusional fate of newly exocytosed SV proteins in hippocampal neurons by high-resolution time-lapse imaging. Newly exocytosed SV proteins rapidly disperse within the first seconds post fusion until confined within the presynaptic bouton. Rapid diffusional spread and confinement is followed by slow reclustering of SV proteins at the periactive endocytic zone. Confinement within the presynaptic bouton is mediated in part by SV protein association with the clathrin-based endocytic machinery to limit diffusional spread of newly exocytosed SV proteins. These data suggest that diffusion, and axonal escape of newly exocytosed vesicle proteins, are counteracted by the clathrin-based endocytic machinery together with a presynaptic diffusion barrier.

Vesicular Synaptobrevin/VAMP2 Levels Guarded by AP180 Control Efficient Neurotransmission
Koo, S. J., Kochlamazashvili, G., Rost(*), B., Puchkov, D., Gimber, N., Lehmann, M., Tadeus, G., Schmoranzer, J., Rosenmund(*), C., Haucke, V.; Maritzen, T.
Neuron, 88:330-344
(2015)

Tags: Membrane Traffic and Cell Motility (Maritzen), Molecular Pharmacology and Cell Biology (Haucke)

Abstract: Neurotransmission depends on synaptic vesicle (SV) exocytosis driven by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex formation of vesicular synaptobrevin/VAMP2 (Syb2). Exocytic fusion is followed by endocytic SV membrane retrieval and the high-fidelity reformation of SVs. Syb2 is the most abundant SV protein with 70 copies per SV, yet, one to three Syb2 molecules appear to be sufficient for basal exocytosis. Here we demonstrate that loss of the Syb2-specific endocytic adaptor AP180 causes a moderate activity-dependent reduction of vesicular Syb2 levels, defects in SV reformation, and a corresponding impairment of neurotransmission that lead to excitatory/inhibitory imbalance, epileptic seizures, and premature death. Further reduction of Syb2 levels in AP180(-/-)/Syb2(+/-) mice results in perinatal lethality, whereas Syb2(+/-) mice partially phenocopy loss of AP180, indicating that reduced vesicular Syb2 levels underlie the observed defects in neurotransmission. Thus, a large vesicular Syb2 pool maintained by AP180 is crucial to sustain efficient neurotransmission and SV reformation.

Peptide-polymer ligands for a tandem WW-domain, an adaptive multivalent protein-protein interaction: lessons on the thermodynamic fitness of flexible ligands
Koschek, K., Durmaz(*), V., Krylova, O., Wieczorek, M., Gupta(*), S., Richter, M., Bujotzek(*), A., Fischer(*), C., Haag(*), R., Freund, C., Weber(*), M.; Rademann, J.
Beilstein J Org Chem, 11:837-847
(2015)

Tags: Medicinal Chemistry (Rademann), Protein Engineering (Freund), Peptide-Lipid-Interaction/ Peptide Transport (Dathe)

Abstract: Three polymers, poly(N-(2-hydroxypropyl)methacrylamide) (pHPMA), hyperbranched polyglycerol (hPG), and dextran were investigated as carriers for multivalent ligands targeting the adaptive tandem WW-domain of formin-binding protein (FBP21). Polymer carriers were conjugated with 3-9 copies of the proline-rich decapeptide GPPPRGPPPR-NH2 (P1). Binding of the obtained peptide-polymer conjugates to the tandem WW-domain was investigated employing isothermal titration calorimetry (ITC) to determine the binding affinity, the enthalpic and entropic contributions to free binding energy, and the stoichiometry of binding for all peptide-polymer conjugates. Binding affinities of all multivalent ligands were in the microM range, strongly amplified compared to the monovalent ligand P1 with a K D > 1 mM. In addition, concise differences were observed, pHPMA and hPG carriers showed moderate affinity and bound 2.3-2.8 peptides per protein binding site resulting in the formation of aggregates. Dextran-based conjugates displayed affinities down to 1.2 microM, forming complexes with low stoichiometry, and no precipitation. Experimental results were compared with parameters obtained from molecular dynamics simulations in order to understand the observed differences between the three carrier materials. In summary, the more rigid and condensed peptide-polymer conjugates based on the dextran scaffold seem to be superior to induce multivalent binding and to increase affinity, while the more flexible and dendritic polymers, pHPMA and hPG are suitable to induce crosslinking upon binding.

Postsynaptic D2 dopamine receptor supersensitivity in the striatum of mice lacking TAAR1
Espinoza(*), S., Ghisi, V., Emanuele(*), M., Leo(*), D., Sukhanov(*), I., Sotnikova(*), T. D., Chieregatti(*), E.; Gainetdinov(*), R. R.
Neuropharmacology, 93:308-313
(2015)

Tags: Molecular Neuroscience and Biophysics (Plested)

Abstract: Trace Amine-Associated Receptor 1 (TAAR1) is a G protein-coupled receptor (GPCR) known to modulate dopaminergic system through several mechanisms. Mice lacking this receptor show a higher sensitivity to dopaminergic stimuli, such as amphetamine; however, it is not clear whether D1 or D2 dopamine receptors and which associated intracellular signaling events are involved in this modulation. In the striatum of TAAR1 knock out (TAAR1-KO mice) we found that D2, but not D1, dopamine receptors were over-expressed, both in terms of mRNA and protein levels. Moreover, the D2 dopamine receptor-related G protein-independent AKT/GSK3 signaling pathway was selectively activated, as indicated by the decrease of phosphorylation of AKT and GSK3beta. The decrease in phospho-AKT levels, suggesting an increase in D2 dopamine receptor activity in basal conditions, was associated with an increase of AKT/PP2A complex, as revealed by co-immunoprecipitation experiments. Finally, we found that the locomotor activation induced by the D2 dopamine receptor agonist quinpirole, but not by the full D1 dopamine receptor agonist SKF-82958, was increased in TAAR1-KO mice. These data demonstrate pronounced supersensitivity of postsynaptic D2 dopamine receptors in the striatum of TAAR1-KO mice and indicate that a close interaction of TAAR1 and D2 dopamine receptors at the level of postsynaptic structures has important functional consequences.

Redox Regulation of Cell Contacts by Tricellulin and Occludin: Redox-Sensitive Cysteine Sites in Tricellulin Regulate Both Tri- and Bicellular Junctions in Tissue Barriers as Shown in Hypoxia and Ischemia
Cording, J., Günther, R., Vigolo(*), E., Tscheik, C., Winkler, L., Schlattner, I., Lorenz, D., Haseloff, R. F., Schmidt-Ott(*), K. M., Wolburg(*), H.; Blasig, I. E.
Antioxid Redox Signal, 23:1035-1049
(2015)

Tags: Molecular Cell Physiology (Blasig, I.E.)

Abstract: UNLABELLED: Tight junctions (TJs) seal paracellular clefts in epithelia/endothelia and form tissue barriers for proper organ function. TJ-associated marvel proteins (TAMPs; tricellulin, occludin, marvelD3) are thought to be relevant to regulation. Under normal conditions, tricellulin tightens tricellular junctions against macromolecules. Traces of tricellulin occur in bicellular junctions. AIMS: As pathological disturbances have not been analyzed, the structure and function of human tricellulin, including potentially redox-sensitive Cys sites, were investigated under reducing/oxidizing conditions at 3- and 2-cell contacts. RESULTS: Ischemia, hypoxia, and reductants redistributed tricellulin from 3- to 2-cell contacts. The extracellular loop 2 (ECL2; conserved Cys321, Cys335) trans-oligomerized between three opposing cells. Substitutions of these residues caused bicellular localization. Cys362 in transmembrane domain 4 contributed to bicellular heterophilic cis-interactions along the cell membrane with claudin-1 and marvelD3, while Cys395 in the cytosolic C-terminal tail promoted homophilic tricellullar cis-interactions. The Cys sites included in homo-/heterophilic bi-/tricellular cis-/trans-interactions contributed to cell barrier tightness for small/large molecules. INNOVATION: Tricellulin forms TJs via trans- and cis-association in 3-cell contacts, as demonstrated electron and quantified fluorescence microscopically; it tightens 3- and 2-cell contacts. Tricellulin's ECL2 specifically seals 3-cell contacts redox dependently; a structural model is proposed. CONCLUSIONS: TAMP ECL2 and claudins' ECL1 share functionally and structurally similar features involved in homo-/heterophilic tightening of cell-cell contacts. Tricellulin is a specific redox sensor and sealing element at 3-cell contacts and may compensate as a redox mediator for occludin loss at 2-cell contacts in vivo and in vitro. Molecular interaction mechanisms were proposed that contribute to tricellulin's function. In conclusion, tricellulin is a junctional redox regulator for ischemia-related alterations.

Redox-sensitive structure and function of the first extracellular loop of the cell-cell contact protein claudin-1: lessons from molecular structure to animals
Dabrowski, S., Staat, C., Zwanziger, D., Sauer(*), R. S., Bellmann, C., Günther, R., Krause, E., Haseloff, R. F., Rittner(*), H.; Blasig, I. E.
Antioxid Redox Signal, 22:1-14
(2015)

Tags: Molecular Cell Physiology (Blasig, I.E.), Mass Spectrometry (Krause, E.)

Abstract: UNLABELLED: The paracellular cleft within epithelia/endothelia is sealed by tight junction (TJ) proteins. Their extracellular loops (ECLs) are assumed to control paracellular permeability and are targets of pathogenes. We demonstrated that claudin-1 is crucial for paracellular tightening. Its ECL1 is essential for the sealing and contains two cysteines conserved throughout all claudins. AIMS: We prove the hypothesis that this cysteine motif forms a redox-sensitive intramolecular disulfide bridge and, hence, the claudin-1-ECL1 constitutes a functional structure which is associated to ECLs of this and other TJ proteins. RESULTS: The structure and function of claudin-1-ECL1 was elucidated by investigating sequences of this ECL as synthetic peptides, C1C2, and as recombinant proteins, and exhibited a beta-sheet binding surface flanked by an alpha-helix. These sequences bound to different claudins, their ECL1, and peptides with nanomolar binding constants. C-terminally truncated C1C2 (-4aaC) opened cellular barriers and the perineurium. Recombinant ECL1 formed oligomers, and bound to claudin-1 expressing cells. Oligomerization and claudin association were abolished by reducing agents, indicating intraloop disulfide bridging and redox sensitivity. INNOVATION: The structural and functional model based on our in vitro and in vivo investigations suggested that claudin-1-ECL1 constitutes a functional and ECL-binding beta-sheet, stabilized by a shielded and redox-sensitive disulfide bond. CONCLUSION: Since the beta-sheet represents a consensus sequence of claudins and further junctional proteins, a general structural feature is implied. Therefore, our model is of general relevance for the TJ assembly in normal and pathological conditions. C1C2-4aaC is a new drug enhancer that is used to improve pharmacological treatment through tissue barriers.

Depletion of highly abundant proteins from human cerebrospinal fluid: a cautionary note
Günther, R., Krause, E., Schümann, M., Blasig, I. E.; Haseloff, R. F.
Mol Neurodegener, 10:53
(2015)

Tags: Molecular Cell Physiology (Blasig, I.E.), Mass Spectrometry (Krause, E.)

Abstract: Affinity-based techniques, both for enrichment or depletion of proteins of interest, suffer from unwanted interactions between the bait or matrix material and molecules different from the original target. This effect was quantitatively studied by applying two common procedures for the depletion of albumin/gamma immunoglobulin to human cerebrospinal fluid. Proteins of the depleted and the column-bound fraction were identified by mass spectrometry, employing (18)O labeling for quantitation of their abundance. To different extents, the depletion procedures caused the loss of proteins previously suggested as biomarker candidates for neurological diseases. This is an important phenomenon to consider when quantifying protein levels in biological fluids.

Perspectives for sensitivity enhancement in proton-detected solid-state NMR of highly deuterated proteins by preserving water magnetization
Chevelkov, V., Xiang(*), S. Q., Giller(*), K., Becker(*), S., Lange, A.; Reif(*), B.
J. Biomol. NMR, 61:151-160
(2015)

Tags: Molecular Biophysics (Lange, A.)

Abstract: In this work, we show how the water flip-back approach that is widely employed in solution-state NMR can be adapted to proton-detected MAS solid-state NMR of highly deuterated proteins. The scheme allows to enhance the sensitivity of the experiment by decreasing the recovery time of the proton longitudinal magnetization. The method relies on polarization transfer from non-saturated water to the protein during the inter-scan delay.

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Leibniz-Forschungsinstitut für Molekulare Pharmakologie im Forschungsverbund Berlin e.V. (FMP)
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