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79 Publications

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    Singer Lab
    04/02/12 | Transcription goes digital.
    Lionnet T, Singer RH
    EMBO Reports. 2012 Apr 2;13(4):313-21. doi: 10.1038/embor.2012.31

    Transcription is a complex process that integrates the state of the cell and its environment to generate adequate responses for cell fitness and survival. Recent microscopy experiments have been able to monitor transcription from single genes in individual cells. These observations have revealed two striking features: transcriptional activity can vary markedly from one cell to another, and is subject to large changes over time, sometimes within minutes. How the chromatin structure, transcription machinery assembly and signalling networks generate such patterns is still unclear. In this review, we present the techniques used to investigate transcription from single genes, introduce quantitative modelling tools, and discuss transcription mechanisms and their implications for gene expression regulation.

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    Singer Lab
    02/23/12 | An unbiased analysis method to quantify mRNA localization reveals its correlation with cell motility.
    Park HY, Trcek T, Wells AL, Chao JA, Singer RH
    Cell Reports. 2012 Feb 23;1(2):179-84. doi: 10.1016/j.celrep.2011.12.009

    Localization of mRNA is a critical mechanism used by a large fraction of transcripts to restrict its translation to specific cellular regions. Although current high-resolution imaging techniques provide ample information, the analysis methods for localization have either been qualitative or employed quantification in nonrandomly selected regions of interest. Here, we describe an analytical method for objective quantification of mRNA localization using a combination of two characteristics of its molecular distribution, polarization and dispersion. The validity of the method is demonstrated using single-molecule FISH images of budding yeast and fibroblasts. Live-cell analysis of endogenous β-actin mRNA in mouse fibroblasts reveals that mRNA polarization has a half-life of ~16 min and is cross-correlated with directed cell migration. This novel approach provides insights into the dynamic regulation of mRNA localization and its physiological roles.

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    Singer Lab
    02/02/12 | Single-mRNA counting using fluorescent in situ hybridization in budding yeast.
    Trcek T, Chao JA, Larson DR, Park HY, Zenklusen D, Shenoy SM, Singer RH
    Nature Protocols. 2012 Feb 2;7(2):408-19. doi: 10.1038/nprot.2011.451

    Fluorescent in situ hybridization (FISH) allows the quantification of single mRNAs in budding yeast using fluorescently labeled single-stranded DNA probes, a wide-field epifluorescence microscope and a spot-detection algorithm. Fixed yeast cells are attached to coverslips and hybridized with a mixture of FISH probes, each conjugated to several fluorescent dyes. Images of cells are acquired in 3D and maximally projected for single-molecule analysis. Diffraction-limited labeled mRNAs are observed as bright fluorescent spots and can be quantified using a spot-detection algorithm. FISH preserves the spatial distribution of cellular RNA distribution within the cell and the stochastic fluctuations in individual cells that can lead to phenotypic differences within a clonal population. This information, however, is lost if the RNA content is measured on a population of cells by using reverse transcriptase PCR, microarrays or high-throughput sequencing. The FISH procedure and image acquisition described here can be completed in 3 d.

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    Singer Lab
    02/01/12 | Multiscale dynamics in nucleocytoplasmic transport.
    Grunwald D, Singer RH
    Current Opinion in Cell Biology. 2012 Feb;24(1):100-6. doi: 10.1016/j.ceb.2011.11.011

    The nuclear pore complex (NPC) has long been viewed as a point-like entry and exit channel between the nucleus and the cytoplasm. New data support a different view whereby the complex displays distinct spatial dynamics of variable duration ranging from milliseconds to events spanning the entire cell cycle. Discrete interaction sites outside the central channel become apparent, and transport regulation at these sites seems to be of greater importance than currently thought. Nuclear pore components are highly active outside the NPC or impact the fate of cargo transport away from the nuclear pore. The NPC is a highly dynamic, crowded environment-constantly loaded with cargo while providing selectivity based on unfolded proteins. Taken together, this comprises a new paradigm in how we view import/export dynamics and emphasizes the multiscale nature of NPC-mediated cellular transport.

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    Singer Lab
    01/15/12 | IGF2BP1 promotes cell migration by regulating MK5 and PTEN signaling.
    Stöhr N, Köhn M, Lederer M, Glass M, Reinke C, Singer RH, Hüttelmaier S
    Genes & Development. 2012 Jan 15;26(2):176-89. doi: 10.1101/gad.177642.111

    In primary neurons, the oncofetal RNA-binding protein IGF2BP1 (IGF2 mRNA-binding protein 1) controls spatially restricted β-actin (ACTB) mRNA translation and modulates growth cone guidance. In cultured tumor-derived cells, IGF2BP1 was shown to regulate the formation of lamellipodia and invadopodia. However, how and via which target mRNAs IGF2BP1 controls the motility of tumor-derived cells has remained elusive. In this study, we reveal that IGF2BP1 promotes the velocity and directionality of tumor-derived cell migration by determining the cytoplasmic fate of two novel target mRNAs: MAPK4 and PTEN. Inhibition of MAPK4 mRNA translation by IGF2BP1 antagonizes MK5 activation and prevents phosphorylation of HSP27, which sequesters actin monomers available for F-actin polymerization. Consequently, HSP27-ACTB association is reduced, mobilizing cellular G-actin for polymerization in order to promote the velocity of cell migration. At the same time, stabilization of the PTEN mRNA by IGF2BP1 enhances PTEN expression and antagonizes PIP(3)-directed signaling. This enforces the directionality of cell migration in a RAC1-dependent manner by preventing additional lamellipodia from forming and sustaining cell polarization intrinsically. IGF2BP1 thus promotes the velocity and persistence of tumor cell migration by controlling the expression of signaling proteins. This fine-tunes and connects intracellular signaling networks in order to enhance actin dynamics and cell polarization.

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    Singer Lab
    01/12/12 | Transgenic expression of ZBP1 in neurons suppresses cocaine-associated conditioning.
    Lapidus KA, Nwokafor C, Scott D, Baroni TE, Tenenbaum SA, Hiroi N, Singer RH, Czaplinski K
    Learning & Memory. 2012 Feb;19(2):35-42. doi: 10.1101/lm.024471.111

    To directly address whether regulating mRNA localization can influence animal behavior, we created transgenic mice that conditionally express Zipcode Binding Protein 1 (ZBP1) in a subset of neurons in the brain. ZBP1 is an RNA-binding protein that regulates the localization, as well as translation and stability of target mRNAs in the cytoplasm. We took advantage of the absence of ZBP1 expression in the mature brain to examine the effect of expressing ZBP1 on animal behavior. We constructed a transgene conditionally expressing a GFP-ZBP1 fusion protein in a subset of forebrain neurons and compared cocaine-cued place conditioning in these mice versus noninduced littermates. Transgenic ZBP1 expression resulted in impaired place conditioning relative to nonexpressing littermates, and acutely repressing expression of the transgene restored normal cocaine conditioning. To gain insight into the molecular changes that accounted for this change in behavior, we identified mRNAs that specifically immunoprecipitated with transgenic ZBP1 protein from the brains of these mice. These data suggest that RNA-binding proteins can be used as a tool to identify the post-transcriptional regulation of gene expression in the establishment and function of neural circuits involved in addiction behaviors.

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    Singer Lab
    01/01/12 | Spatial arrangement of an RNA zipcode identifies mRNAs under post-transcriptional control.
    Patel VL, Mitra S, Harris R, Buxbaum AR, Lionnet T, Brenowitz M, Girvin M, Levy M, Almo SC, Singer RH, Chao JA
    Genes & Development. 2012 Jan 1;26(1):43-53. doi: 10.1101/gad.177428.111

    How RNA-binding proteins recognize specific sets of target mRNAs remains poorly understood because current approaches depend primarily on sequence information. In this study, we demonstrate that specific recognition of messenger RNAs (mRNAs) by RNA-binding proteins requires the correct spatial positioning of these sequences. We characterized both the cis-acting sequence elements and the spatial restraints that define the mode of RNA binding of the zipcode-binding protein 1 (ZBP1/IMP1/IGF2BP1) to the β-actin zipcode. The third and fourth KH (hnRNP K homology) domains of ZBP1 specifically recognize a bipartite RNA element comprised of a 5' element (CGGAC) followed by a variable 3' element (C/A-CA-C/U) that must be appropriately spaced. Remarkably, the orientation of these elements is interchangeable within target transcripts bound by ZBP1. The spatial relationship of this consensus binding site identified conserved transcripts that were verified to associate with ZBP1 in vivo. The dendritic localization of one of these transcripts, spinophilin, was found to be dependent on both ZBP1 and the RNA elements recognized by ZBP1 KH34.

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    Singer Lab
    12/23/11 | Single-molecule mRNA decay measurements reveal promoter- regulated mRNA stability in yeast.
    Trcek T, Larson DR, Moldón A, Query CC, Singer RH
    Cell. 2011 Dec 23;147(7):1484-97. doi: 10.1016/j.cell.2011.11.051

    Messenger RNA decay measurements are typically performed on a population of cells. However, this approach cannot reveal sufficient complexity to provide information on mechanisms that may regulate mRNA degradation, possibly on short timescales. To address this deficiency, we measured cell cycle-regulated decay in single yeast cells using single-molecule FISH. We found that two genes responsible for mitotic progression, SWI5 and CLB2, exhibit a mitosis-dependent mRNA stability switch. Their transcripts are stable until mitosis, when a precipitous decay eliminates the mRNA complement, preventing carryover into the next cycle. Remarkably, the specificity and timing of decay is entirely regulated by their promoter, independent of specific cis mRNA sequences. The mitotic exit network protein Dbf2p binds to SWI5 and CLB2 mRNAs cotranscriptionally and regulates their decay. This work reveals the promoter-dependent control of mRNA stability, a regulatory mechanism that could be employed by a variety of mRNAs and organisms.

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    Singer Lab
    12/09/11 | A date with telomerase: pick you up at S phase.
    Hocine S, Singer RH
    Molecular Cell. 2011 Dec 9;44(5):685-6. doi: 10.1016/j.molcel.2011.11.013

    Using the MS2 system for labeling mRNA, in this issue, Gallardo et al. (2011) find that telomere lengthening depends on a stable accumulation of multiple telomerase complexes in late S phase and that this process is temporally regulated by Rif1/2 proteins.

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    Singer Lab
    12/01/11 | Cotranscriptional effect of a premature termination codon revealed by live-cell imaging.
    de Turris V, Nicholson P, Orozco RZ, Singer RH, Mühlemann O
    RNA. 2011 Dec;17(12):2094-107. doi: 10.1261/rna.02918111

    Aberrant mRNAs with premature translation termination codons (PTCs) are recognized and eliminated by the nonsense-mediated mRNA decay (NMD) pathway in eukaryotes. We employed a novel live-cell imaging approach to investigate the kinetics of mRNA synthesis and release at the transcription site of PTC-containing (PTC+) and PTC-free (PTC-) immunoglobulin-μ reporter genes. Fluorescence recovery after photobleaching (FRAP) and photoconversion analyses revealed that PTC+ transcripts are specifically retained at the transcription site. Remarkably, the retained PTC+ transcripts are mainly unspliced, and this RNA retention is dependent upon two important NMD factors, UPF1 and SMG6, since their depletion led to the release of the PTC+ transcripts. Finally, ChIP analysis showed a physical association of UPF1 and SMG6 with both the PTC+ and the PTC- reporter genes in vivo. Collectively, our data support a mechanism for regulation of PTC+ transcripts at the transcription site.

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