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Lee Tzumin Lab / Publications
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12 Publications

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    04/30/13 | Coordinated elevation of mitochondrial oxidative phosphorylation and autophagy help drive hepatocyte polarization.
    Fu D, Mitra K, Sengupta P, Jarnik M, Lippincott-Schwartz J, Arias IM
    Proceedings of the National Academy of Sciences of the United States of America. 2013 Apr 30;110(18):7288-93. doi: 10.1073/pnas.1304285110

    Cell polarization requires increased cellular energy and metabolic output, but how these energetic demands are met by polarizing cells is unclear. To address these issues, we investigated the roles of mitochondrial bioenergetics and autophagy during cell polarization of hepatocytes cultured in a collagen sandwich system. We found that as the hepatocytes begin to polarize, they use oxidative phosphorylation to raise their ATP levels, and this energy production is required for polarization. After the cells are polarized, the hepatocytes shift to become more dependent on glycolysis to produce ATP. Along with this central reliance on oxidative phosphorylation as the main source of ATP production in polarizing cultures, several other metabolic processes are reprogrammed during the time course of polarization. As the cells polarize, mitochondria elongate and mitochondrial membrane potential increases. In addition, lipid droplet abundance decreases over time. These findings suggest that polarizing cells are reliant on fatty acid oxidation, which is supported by pharmacologic inhibition of β-oxidation by etomoxir. Finally, autophagy is up-regulated during cell polarization, with inhibition of autophagy retarding cell polarization. Taken together, our results describe a metabolic shift involving a number of coordinated metabolic pathways that ultimately serve to increase energy production during cell polarization.

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    04/24/13 | Carbofluoresceins and carborhodamines as scaffolds for high-contrast fluorogenic probes.
    Grimm JB, Sung AJ, Legant WR, Hulamm P, Matlosz SM, Betzig E, Lavis LD
    ACS Chemical Biology. 2013 Apr 24;8(6):1303-10. doi: 10.1021/cb4000822

    Fluorogenic molecules are important tools for advanced biochemical and biological experiments. The extant collection of fluorogenic probes is incomplete, however, leaving regions of the electromagnetic spectrum unutilized. Here, we synthesize green-excited fluorescent and fluorogenic analogues of the classic fluorescein and rhodamine 110 fluorophores by replacement of the xanthene oxygen with a quaternary carbon. These anthracenyl "carbofluorescein" and "carborhodamine 110" fluorophores exhibit excellent fluorescent properties and can be masked with enzyme- and photolabile groups to prepare high-contrast fluorogenic molecules useful for live cell imaging experiments and super-resolution microscopy. Our divergent approach to these red-shifted dye scaffolds will enable the preparation of numerous novel fluorogenic probes with high biological utility.

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    04/22/13 | Automated alignment of imperfect EM images for neural reconstruction.
    Scheffer LK, Karsh B, Vitaladevun S
    arXiv. 2013 Apr-22:arXiv:1304.6034 [q-bio.QM]

    The most established method of reconstructing neural circuits from animals involves slicing tissue very thin, then taking mosaics of electron microscope (EM) images. To trace neurons across different images and through different sections, these images must be accurately aligned, both with the others in the same section and to the sections above and below. Unfortunately, sectioning and imaging are not ideal processes - some of the problems that make alignment difficult include lens distortion, tissue shrinkage during imaging, tears and folds in the sectioned tissue, and dust and other artifacts. In addition the data sets are large (hundreds of thousands of images) and each image must be aligned with many neighbors, so the process must be automated and reliable. This paper discusses methods of dealing with these problems, with numeric results describing the accuracy of the resulting alignments.

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    04/22/13 | Clonal development and organization of the adult Drosophila central brain.
    Yu H, Awasaki T, Schroeder MD, Long F, Yang JS, He Y, Ding P, Kao J, Wu GY, Peng H, Myers G, Lee T
    Current biology : CB. 2013 Apr 22;23:633-43. doi: 10.1016/j.cub.2013.02.057

    BACKGROUND: The insect brain can be divided into neuropils that are formed by neurites of both local and remote origin. The complexity of the interconnections obscures how these neuropils are established and interconnected through development. The Drosophila central brain develops from a fixed number of neuroblasts (NBs) that deposit neurons in regional clusters. RESULTS: By determining individual NB clones and pursuing their projections into specific neuropils, we unravel the regional development of the brain neural network. Exhaustive clonal analysis revealed 95 stereotyped neuronal lineages with characteristic cell-body locations and neurite trajectories. Most clones show complex projection patterns, but despite the complexity, neighboring clones often coinnervate the same local neuropil or neuropils and further target a restricted set of distant neuropils. CONCLUSIONS: These observations argue for regional clonal development of both neuropils and neuropil connectivity throughout the Drosophila central brain.

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    Svoboda Lab
    04/17/13 | The mechanical variables underlying object localization along the axis of the whisker.
    Pammer L, O’Connor DH, Hires SA, Clack NG, Huber D, Myers EW, Svoboda K
    The Journal of Neuroscience. 2013 Apr 17;33(16):6726-41. doi: 10.1523/JNEUROSCI.4316-12.2013

    Rodents move their whiskers to locate objects in space. Here we used psychophysical methods to show that head-fixed mice can localize objects along the axis of a single whisker, the radial dimension, with one-millimeter precision. High-speed videography allowed us to estimate the forces and bending moments at the base of the whisker, which underlie radial distance measurement. Mice judged radial object location based on multiple touches. Both the number of touches (1-17) and the forces exerted by the pole on the whisker (up to 573 μN; typical peak amplitude, 100 μN) varied greatly across trials. We manipulated the bending moment and lateral force pressing the whisker against the sides of the follicle and the axial force pushing the whisker into the follicle by varying the compliance of the object during behavior. The behavioral responses suggest that mice use multiple variables (bending moment, axial force, lateral force) to extract radial object localization. Characterization of whisker mechanics revealed that whisker bending stiffness decreases gradually with distance from the face over five orders of magnitude. As a result, the relative amplitudes of different stress variables change dramatically with radial object distance. Our data suggest that mice use distance-dependent whisker mechanics to estimate radial object location using an algorithm that does not rely on precise control of whisking, is robust to variability in whisker forces, and is independent of object compliance and object movement. More generally, our data imply that mice can measure the amplitudes of forces in the sensory follicles for tactile sensation.

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    04/12/13 | Large scale structural rearrangement of a serine hydrolase from Francisella tularensis facilitates catalysis.
    Filippova EV, Weston LA, Kuhn ML, Geissler B, Gehring AM, Armoush N, Adkins CT, Minasov G, Dubrovska I, Shuvalova L, Winsor JR, Lavis LD, Satchell KJ, Becker DP, Anderson WF, Johnson RJ
    J Biol Chem. 2013 Apr 12;288(15):10522-35. doi: 10.1074/jbc.M112.446625

    Tularemia is a deadly, febrile disease caused by infection by the gram-negative bacterium, Francisella tularensis. Members of the ubiquitous serine hydrolase protein family are among current targets to treat diverse bacterial infections. Herein we present a structural and functional study of a novel bacterial carboxylesterase (FTT258) from F. tularensis, a homologue of human acyl protein thioesterase (hAPT1). The structure of FTT258 has been determined in multiple forms, and unexpectedly large conformational changes of a peripheral flexible loop occur in the presence of a mechanistic cyclobutanone ligand. The concomitant changes in this hydrophobic loop and the newly exposed hydrophobic substrate binding pocket suggest that the observed structural changes are essential to the biological function and catalytic activity of FTT258. Using diverse substrate libraries, site-directed mutagenesis, and liposome binding assays, we determined the importance of these structural changes to the catalytic activity and membrane binding activity of FTT258. Residues within the newly exposed hydrophobic binding pocket and within the peripheral flexible loop proved essential to the hydrolytic activity of FTT258, indicating that structural rearrangement is required for catalytic activity. Both FTT258 and hAPT1 also showed significant association with liposomes designed to mimic bacterial or human membranes, respectively, even though similar structural rearrangements for hAPT1 have not been reported. The necessity for acyl protein thioesterases to have maximal catalytic activity near the membrane surface suggests that these conformational changes in the protein may dually regulate catalytic activity and membrane association in bacterial and human homologues.

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    04/12/13 | Structure, activity, and substrate selectivity of the Orf6 thioesterase from Photobacterium profundum.
    Rodríguez-Guilbe M, Oyola-Robles D, Schreiter ER, Baerga-Ortiz A
    Journal of Biological Chemistry. 2013 Apr 12;288(15):10841-8. doi: 10.1074/jbc.M112.446765

    Thioesterase activity is typically required for the release of products from polyketide synthase enzymes, but no such enzyme has been characterized in deep-sea bacteria associated with the production of polyunsaturated fatty acids. In this work, we have expressed and purified the Orf6 thioesterase from Photobacterium profundum. Enzyme assays revealed that Orf6 has a higher specific activity toward long-chain fatty acyl-CoA substrates (palmitoyl-CoA and eicosapentaenoyl-CoA) than toward short-chain or aromatic acyl-CoA substrates. We determined a high resolution (1.05 Å) structure of Orf6 that reveals a hotdog hydrolase fold arranged as a dimer of dimers. The putative active site of this structure is occupied by additional electron density not accounted for by the protein sequence, consistent with the presence of an elongated compound. A second crystal structure (1.40 Å) was obtained from a crystal that was grown in the presence of Mg(2+), which reveals the presence of a binding site for divalent cations at a crystal contact. The Mg(2+)-bound structure shows localized conformational changes (root mean square deviation of 1.63 Å), and its active site is unoccupied, suggesting a mechanism to open the active site for substrate entry or product release. These findings reveal a new thioesterase enzyme with a preference for long-chain CoA substrates in a deep-sea bacterium whose potential range of applications includes bioremediation and the production of biofuels.

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    04/09/13 | Structural foundations of resting-state and task-based functional connectivity in the human brain.
    Hermundstad AM, Bassett DS, Brown KS, Aminoff EM, Clewett D, Freeman S, Frithsen A, Johnson A, Tipper CM, Miller MB, Grafton ST, Carlson JM
    Proceedings of the National Academy of Sciences of the United States of America. 2013 Apr 9;110(15):6169-74. doi: 10.1073/pnas.1219562110

    Magnetic resonance imaging enables the noninvasive mapping of both anatomical white matter connectivity and dynamic patterns of neural activity in the human brain. We examine the relationship between the structural properties of white matter streamlines (structural connectivity) and the functional properties of correlations in neural activity (functional connectivity) within 84 healthy human subjects both at rest and during the performance of attention- and memory-demanding tasks. We show that structural properties, including the length, number, and spatial location of white matter streamlines, are indicative of and can be inferred from the strength of resting-state and task-based functional correlations between brain regions. These results, which are both representative of the entire set of subjects and consistently observed within individual subjects, uncover robust links between structural and functional connectivity in the human brain.

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    04/08/13 | The ENCODE project: missteps overshadowing a success.
    Eddy SR
    Current Biology. 2013 Apr 8;23(7):R259-61. doi: 10.1016/j.cub.2013.03.023

    Two clichés of science journalism have now played out around the ENCODE project. ENCODE’s publicity first presented a misleading "all the textbooks are wrong" narrative about noncoding human DNA. Now several critiques of ENCODE’s narrative have been published, and one was so vitriolic that it fueled "undignified academic squabble" stories that focused on tone more than substance. Neither story line does justice to our actual understanding of genomes, to ENCODE’s results, or to the role of big science in biology.

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    Gonen Lab
    04/02/13 | Overview of electron crystallography of membrane proteins: crystallization and screening strategies using negative stain electron microscopy.
    Nannenga BL, Iadanza MG, Vollmar BS, Gonen T
    Current Protocols in Protein Science . 2013 Apr 2;Chapter 17:Unit 17.15. doi: 10.1002/0471140864.ps1715s72

    Electron cryomicroscopy, or cryoEM, is an emerging technique for studying the three-dimensional structures of proteins and large macromolecular machines. Electron crystallography is a branch of cryoEM in which structures of proteins can be studied at resolutions that rival those achieved by X-ray crystallography. Electron crystallography employs two-dimensional crystals of a membrane protein embedded within a lipid bilayer. The key to a successful electron crystallographic experiment is the crystallization, or reconstitution, of the protein of interest. This unit describes ways in which protein can be expressed, purified, and reconstituted into well-ordered two-dimensional crystals. A protocol is also provided for negative stain electron microscopy as a tool for screening crystallization trials. When large and well-ordered crystals are obtained, the structures of both protein and its surrounding membrane can be determined to atomic resolution.

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