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7 Janelia Publications

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    04/01/20 | 3D ATAC-PALM: super-resolution imaging of the accessible genome.
    Xie L, Dong P, Chen X, Hsieh TS, Banala S, De Marzio M, English BP, Qi Y, Jung SK, Kieffer-Kwon K, Legant WR, Hansen AS, Schulmann A, Casellas R, Zhang B, Betzig E, Lavis LD, Chang HY, Tjian R, Liu Z
    Nature Methods. 2020 Apr 01;17(4):430-6. doi: 10.1038/s41592-020-0775-2

    To image the accessible genome at nanometer scale in situ, we developed three-dimensional assay for transposase-accessible chromatin-photoactivated localization microscopy (3D ATAC-PALM) that integrates an assay for transposase-accessible chromatin with visualization, PALM super-resolution imaging and lattice light-sheet microscopy. Multiplexed with oligopaint DNA–fluorescence in situ hybridization (FISH), RNA–FISH and protein fluorescence, 3D ATAC-PALM connected microscopy and genomic data, revealing spatially segregated accessible chromatin domains (ACDs) that enclose active chromatin and transcribed genes. Using these methods to analyze genetically perturbed cells, we demonstrated that genome architectural protein CTCF prevents excessive clustering of accessible chromatin and decompacts ACDs. These results highlight 3D ATAC-PALM as a useful tool to probe the structure and organizing mechanism of the genome.

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    08/03/16 | Real-time imaging of Huntingtin aggregates diverting target search and gene transcription.
    Li L, Liu H, Dong P, Li D, Legant WR, Grimm JB, Lavis LD, Betzig E, Tjian R, Liu Z
    eLife. 2016 Aug 03;5:. doi: 10.7554/eLife.17056

    The presumptive altered dynamics of transient molecular interactions in vivo contributing to neurodegenerative diseases have remained elusive. Here, using single-molecule localization microscopy, we show that disease-inducing Huntingtin (mHtt) protein fragments display three distinct dynamic states in living cells - 1) fast diffusion, 2) dynamic clustering and 3) stable aggregation. Large, stable aggregates of mHtt exclude chromatin and form 'sticky' decoy traps that impede target search processes of key regulators involved in neurological disorders. Functional domain mapping based on super-resolution imaging reveals an unexpected role of aromatic amino acids in promoting protein-mHtt aggregate interactions. Genome-wide expression analysis and numerical simulation experiments suggest mHtt aggregates reduce transcription factor target site sampling frequency and impair critical gene expression programs in striatal neurons. Together, our results provide insights into how mHtt dynamically forms aggregates and disrupts the finely-balanced gene control mechanisms in neuronal cells.

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    03/07/16 | High-density three-dimensional localization microscopy across large volumes.
    Legant WR, Shao L, Grimm JB, Brown TA, Milkie DE, Avants BB, Lavis LD, Betzig E
    Nature Methods. 2016 Mar 7:. doi: 10.1038/nmeth.3797

    Extending three-dimensional (3D) single-molecule localization microscopy away from the coverslip and into thicker specimens will greatly broaden its biological utility. However, because of the limitations of both conventional imaging modalities and conventional labeling techniques, it is a challenge to localize molecules in three dimensions with high precision in such samples while simultaneously achieving the labeling densities required for high resolution of densely crowded structures. Here we combined lattice light-sheet microscopy with newly developed, freely diffusing, cell-permeable chemical probes with targeted affinity for DNA, intracellular membranes or the plasma membrane. We used this combination to perform high-localization precision, ultrahigh-labeling density, multicolor localization microscopy in samples up to 20 μm thick, including dividing cells and the neuromast organ of a zebrafish embryo. We also demonstrate super-resolution correlative imaging with protein-specific photoactivable fluorophores, providing a mutually compatible, single-platform alternative to correlative light-electron microscopy over large volumes.

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    05/21/15 | Imaging live-cell dynamics and structure at the single-molecule level.
    Liu Z, Lavis LD, Betzig E
    Molecular Cell. 2015 May 21;58(4):644-59. doi: 10.1016/j.molcel.2015.02.033

    Observation of molecular processes inside living cells is fundamental to a quantitative understanding of how biological systems function. Specifically, decoding the complex behavior of single molecules enables us to measure kinetics, transport, and self-assembly at this fundamental level that is often veiled in ensemble experiments. In the past decade, rapid developments in fluorescence microscopy, fluorescence correlation spectroscopy, and fluorescent labeling techniques have enabled new experiments to investigate the robustness and stochasticity of diverse molecular mechanisms with high spatiotemporal resolution. This review discusses the concepts and strategies of structural and functional imaging in living cells at the single-molecule level with minimal perturbations to the specimen.

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    12/24/14 | 3D imaging of Sox2 enhancer clusters in embryonic stem cells.
    Liu Z, Legant WR, Chen B, Li L, Grimm JB, Lavis LD, Betzig E, Tjian R
    eLife. 2014 Dec 24;3:. doi: 10.7554/eLife.04236

    Combinatorial cis-regulatory networks encoded in animal genomes represent the foundational gene expression mechanism for directing cell-fate commitment and maintenance of cell identity by transcription factors (TFs). However, the 3D spatial organization of cis-elements and how such sub-nuclear structures influence TF activity remain poorly understood. Here, we combine lattice light-sheet imaging, single-molecule tracking, numerical simulations, and ChIP-exo mapping to localize and functionally probe Sox2 enhancer-organization in living embryonic stem cells. Sox2 enhancers form 3D-clusters that are segregated from heterochromatin but overlap with a subset of Pol II enriched regions. Sox2 searches for specific binding targets via a 3D-diffusion dominant mode when shuttling long-distances between clusters while chromatin-bound states predominate within individual clusters. Thus, enhancer clustering may reduce global search efficiency but enables rapid local fine-tuning of TF search parameters. Our results suggest an integrated model linking cis-element 3D spatial distribution to local-versus-global target search modalities essential for regulating eukaryotic gene transcription.

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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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    11/18/11 | Facile and general synthesis of photoactivatable xanthene dyes.
    Wysocki LM, Grimm JB, Tkachuk AN, Brown TA, Betzig E, Lavis LD
    Angewandte Chemie. 2011 Nov 18;50:11206-9. doi: 10.1002/anie.201104571

    Despite the apparent simplicity of the xanthene fluorophores, the preparation of caged derivatives with free carboxy groups remains a synthetic challenge. A straightforward and flexible strategy for preparing rhodamine and fluorescein derivatives was developed using reduced, “leuco” intermediates.

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