Virgile Viasnoff

Research Interests and news

  • My research themes

  • Website of my Activities in Singapore (MBI)

    Posted by vviasnof on 11/16/2012

       click here to see our activities on cell-cell adhesion

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    Easy fabrication of thin membranes with through holes. Application to protein stencil patterning.

    Posted by vviasnof on 02/17/2012

    Master,T et al Submitted Lab on Chip.

    Abstract: Since protein patterning on 2D surfaces has emerged as an important tool in cell biology, the development of methods that can be easily implemented in biology labs has gained importance. In this paper we present a simple, rapid and reliable technique to fabricate thin layers of UV curable polymer with through holes. These membranes are as easy to fabricate as microcontact printing stamps and can be readily used for stencil patterning. We show how this microfabrication scheme allows highly reproducible and highly homogeneous protein patterning with micron sized resolution on surfaces as large as 10 cm2. Using these stencils, fragile proteins were patterned without loss of function in a fully hydrated state. We further demonstrate how intricate patterns of multiple proteins can be achieved by stacking the stencil membranes. We termed this approach microserigraphy.

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    Vers une cartographie des tensions mécaniques intracellulaires?

    Posted by vviasnof on 02/17/2012

    Viasnoff, V. (2011). “Towards mapping mechanical tensions inside cells?” Med Sci (Paris) 27(1): 14-16.

    Les stimuli biochimiques étaient considérés jusqu’à la fin des années 70 comme seuls acteurs de la régulation de l’activité cellulaire, la morphogenèse ou la différentiation cellulaire. Il est progressivement apparu que les contraintes mécaniques pouvaient elles-aussi,  jouer un rôle  de stimulus externe lors de ces processus [1-2].  Les cellules sont en effet  capables  de sonder et de réagir aux sollicitations mécaniques exercées soit par les autres cellules soit par leur  environnement (ex : matrice extra cellulaire ECM).  L’intégration de ces stimuli et leur transduction en voies d’activation biochimiques sont actuellement un domaine d’étude en pleine expansion [3]. Par exemple, certaines voies de régulation peuvent être activées lors de l’ouverture de pores membranaires mécano-sensibles consécutive à l’augmentation de la tension de membrane ou bien encore par l’enrichissement local de récepteurs et  d’effecteurs au niveau de la déformation du cortex.  Enfin l’application de contraintes locales dans le  cytoplasme conduisant à la déformation de protéines révélant  des sites catalytiques ou de recrutement constitue une autre voie possible de transduction mécano-chimique. La capacité d’une cellule à répondre efficacement à une sollicitation mécanique est largement liée à la mise en tension permanente de son cytosquelette via les réseaux dynamiques de filaments interconnectés.  L’existence de cette précontrainte permet une transmission physique (donc extrêmement rapide) des sollicitations mécaniques externes qui pourraient potentiellement être intégrées au niveau de la membrane cellulaire ou du nucléosquelette afin d’activer le programme de transcription génétique approprié.

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    Force fluctuations assist nanopore unzipping of DNA

    Posted by vviasnof on 11/03/2010

    Viasnoff et al JPCM 22 454122 2010

    We experimentally study the statistical distributions and the voltage dependence of the
    unzipping time of 45 base-pair-long double-stranded DNA through a nanopore. We then
    propose a quantitative theoretical description considering the nanopore unzipping process as a
    random walk of the opening fork through the DNA sequence energy landscape biased by a
    time-fluctuating force. To achieve quantitative agreement fluctuations need to be correlated over
    the millisecond range and have an amplitude of order kBT/bp. Significantly slower or faster
    fluctuations are not appropriate, suggesting that the unzipping process is efficiently enhanced by
    noise in the kHz range. We further show that the unzipping time of short 15 base-pair hairpins
    does not always increase with the global stability of the double helix and we theoretically study
    the role of DNA elasticity on the conversion of the electrical bias into a mechanical unzipping
    force.

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    On Sabbatical

    Posted by vviasnof on 09/09/2010

    I am currently on sabbatical at the National University of Singapore working on the mechanobiology of the cells.
    My email has not changed. My Phone number is 65 6601 1400 (+6h from Paris time)

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    My thesis as an Ebook

    Posted by vviasnof on 05/19/2010

    You can now read my thesis as an e-book. Just for fun.

    [issuu layout=http%3A%2F%2Fskin.issuu.com%2Fv%2Fdarkicons%2Flayout.xml showflipbtn=true documentid=100519092139-7e002a62b4d84da8a0d2fc55de2962da docname=these username=viasnoff loadinginfotext=These%20doctorat showhtmllink=true tag=rheology width=420 height=297 unit=px]

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    Is the in vitro ejection of bacteriophage DNA quasi-static? A bulk to single virus study.

    Posted by vviasnof on 04/29/2010

    Newly Accepted paper in Biophysical journal

    N.Chiaruttini et al Biophys J, 99 (2) 2010.
    Collaboration L.Letellier, M.de Frutos, P.Boulanger IBBMC Paris Sud

    ejectphage

    Time lapse determination of DNA ejection from a bacteriophage T5. The ejection display pauses that we attribute to rearragement of DNA inside the capsid

    Abstract:

    Bacteriophage T5 DNA ejection is a complex process that occurs on several time scales in vitro. Using a combination of bulk and single phage measurements we quantitatively study the three steps of the ejection: binding to the host receptor, channel opening step and DNA release. Each step is separately addressed and its kinetics parameters evaluated. We reconstruct the bulk kinetics from the distribution of single phage events following individual DNA molecules with unprecedented time resolution. We show that at the single phage level the ejection kinetics of the DNA happens by rapid transient bursts that are not correlated to any genome sequence defects. We speculate that these transient pauses are due to local phase transitions of the DNA inside the capsid. We predict that such pauses should be seen for other phages with similar DNA packing ratio.

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    Localized Joule heating produced by ion current focusing through micron-size holes

    Posted by vviasnof on 04/29/2010

    Newly published article in Applied Physics Letters

    V.Viasnoff et al Appl Phys Lett, 96,163701, 2010
    Collaboration Y.Tsori, H.Isambert, A.Meller


    Local heating

    Focusing ionic current through a micron size hole separating 2 chambers may lead to a high local heating (up to 100°C). We exploit this effect to construct a set up that allows spatial determination of DNA melting profiles

    Abstract: We provide an experimental demonstration that the focusing of ionic currents in a micron size hole
    connecting two chambers can produce local temperature increases of up to 100 °C with gradients as large as 1 °K /micron. We find a good agreement between the measured temperature profiles and a finite elements-based numerical calculation. We show how the thermal gradients can be used to measure the full melting profile of DNA duplexes within a region of 40 microns. The possibility to produce even larger gradients using submicron pores is discussed.

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    DNA translocation and unzipping through a nanopore: some geometrical effects.

    Posted by vviasnof on 04/28/2010

    Newly published Paper in Biophysical journal

    J.Muzard et al Biophysical Journal, 98,10 2010,2170-2179

    Collaboration U.Bockelmann, J.Mathé

    DNA in alphahemolysinDNA Duplexes can enter alphahemolysin pores from both sides.

    However when the vestibule is entered first the DNA is hold in front of the stem entrance. Unzipping is then faster.

    Abstract: This paper explores the role of some geometrical factors on the electrophoretically driven translocations of macromolecules through nanopores. In the case of asymmetric pores, we show how the entry requirements and the direction of translocation can modify the information content of the blocked ionic current as well as the transduction of the electrophoretic drive into a mechanical force. To address these effects we studied the translocation of single stranded DNA through an asymmetric alphahemolysin pore. Depending on the direction of the translocation, we measure the capacity of the pore to discriminate between both DNA orientations. By unzipping DNA hairpins from both sides of the pores we show that the presence of single strand or double strand in the pore can be discriminated based on ionic current levels. We also show that the  transduction of the electrophoretic drive into a denaturing mechanical force depends on the local geometry of the pore entrance. Eventually we discuss the application of this work to the measurement of energy barriers for DNA unzipping as well as for protein binding and unfolding.

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