Un nouveau type de matière active explique la formation d’agrégats bactériens et leur impact sur l’infection à méningocoque

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Développement de la morphogenèse et de la polarité d’une cellule unique

How cells establish their proper shapes and organization is a fundamental biological problem. In this thesis, I investigated the dynamic development of cellular form and polarity in the rod-shape fission yeast cell. These studies are based on monitoring how small symmetric fission yeast spores grow and self-organize to break symmetry for the definition of their very first polarity axis. In a first part, I studied interplays between surface mechanics of the spore cell wall and the stability of Cdc42-based polarity domains which control spatio-temporal aspects of spore symmetry breaking. In a second part, I studied mechanisms by which these polarity domains control their width and adapt it to cell surface geometry, a process likely relevant to understand how functional cortical domains scale to cell size. Overall these novel investigations focusing on how cells dynamically develop their form and polarity de novo highlight complex feedbacks in morphogenesis that cannot be evidenced by looking at cells at “steady state” or with genetics.Comment les cellules établissent leurs formes et organisations internes est un problème biologique fondamental. Au cours de cette thèse, j’ai étudié le développement de la forme cellulaire et de la polarité chez la cellule de levure fissipare. Ces études sont fondées sur l’exploration de la façon dont les petites spores symétriques de levures se développent et s’organisent pour briser la symétrie pour la définition de leur tout premier axe de polarité. Dans une première partie, j’ai étudié les couplages entre la mécanique de surface de la paroi cellulaire des spores et la stabilité de domaines de polarité de Cdc42 qui contrôlent les aspects spatio-temporelles de la brisure de symétrie de ces spores. Dans une seconde partie, j’ai étudié les mécanismes par lesquels ces domaines de polarité contrôlent leur taille et l'adapte à la géométrie de la cellule, un processus vraisemblablement pertinents pour comprendre comment des domaines fonctionnels corticaux s’adaptent à la taille des cellules. Globalement, ces nouvelles recherches focalisant sur la façon dont les cellules développent dynamiquement leur forme et polarité de novo, permettent de mettre en évidence des couplages complexes dans la morphogenèse qui ne peuvent pas être testés en regardant les cellules à « l’état stationnaire» ou avec des outils génétiques

Measurement and manipulation of cell size parameters in fission yeast.

International audienceCells usually grow to a certain size before they divide. The fission yeast Schizosaccharomyces pombe is an established model to dissect the molecular control of cell size homeostasis and cell cycle. In this chapter, we describe two simple methods to: (1) precisely compute geometrical parameters (cell length, diameter, surface, and volume) of single growing and dividing fission yeast cells with image analysis scripts and (2) manipulate cell diameter with microfabricated chambers and assess for cell size at division. We demonstrate the strength of these approaches in the context of growing spores, which constantly change size and shape and in deriving allometric relationships between cell geometrical parameters associated with G2/M transition. We emphasize these methods to be useful to investigate problems of growth, size, and division in fungal or bacterial cells

MOLECULAR RECOGNITION OF CHIRAL CONFORMERS: A ROTATIONAL STUDY OF THE DIMERS OF GLYCIDOL

K.-M. Marstokk, H. Mollendal and Y. Stenstrom,Acta Chem. Scand. 1992, 46, 432Author Institution: Dipartimento di Chimica "G. Ciamician" dell'Universita, Via Selmi 2, I-40126 Bologna, ItalyWe report for the first time the rotational spectra of dimers deriving from the combinations of different conformers of a chiral alcohol, glycidol. Its MW spectra has been reported, and two hydrogen bonded conformers, 1 and 2 have been identified.} Conformer 1 is more stable by 3.4 (4) kJ/mol. A pure chiral species (either R or S) can form seven hydrogen bonded homo - dimers, identified with the label Hom. They can be divided into two groups: one group where the hydrogen bonds form a ring involving 8 heavy atoms and another group where this ring involves only 5 heavy atoms, called 8Hom and 5Hom. Six (three Hom and three Het) conformers have the 8-heavy-atom-frame while eight (four Hom and four Het) conformers have the 5-heavy-atom-frame. Only the rotational spectra of species 8Hom-11 and 8Hom-12 have been observed. We recorded the FTMW spectra in a supersonic expansion. The relative energies and dynamics of formation of the dimers in the supersonic expansion will be discussed

Electrochemical regulation of budding yeast polarity.

International audienceCells are naturally surrounded by organized electrical signals in the form of local ion fluxes, membrane potential, and electric fields (EFs) at their surface. Although the contribution of electrochemical elements to cell polarity and migration is beginning to be appreciated, underlying mechanisms are not known. Here we show that an exogenous EF can orient cell polarization in budding yeast (Saccharomyces cerevisiae) cells, directing the growth of mating projections towards sites of hyperpolarized membrane potential, while directing bud emergence in the opposite direction, towards sites of depolarized potential. Using an optogenetic approach, we demonstrate that a local change in membrane potential triggered by light is sufficient to direct cell polarization. Screens for mutants with altered EF responses identify genes involved in transducing electrochemical signals to the polarity machinery. Membrane potential, which is regulated by the potassium transporter Trk1p, is required for polarity orientation during mating and EF response. Membrane potential may regulate membrane charges through negatively charged phosphatidylserines (PSs), which act to position the Cdc42p-based polarity machinery. These studies thus define an electrochemical pathway that directs the orientation of cell polarization

Symmetry breaking in spore germination relies on an interplay between polar cap stability and spore wall mechanics.

International audienceThe morphogenesis of single cells depends on their ability to coordinate surface mechanics and polarity. During germination, spores of many species develop a polar tube that hatches out of a rigid outer spore wall (OSW) in a process termed outgrowth. However, how these awakening cells reorganize to stabilize this first growth axis remains unknown. Here, using quantitative experiments and modeling, we reveal the mechanisms underlying outgrowth in fission yeast. We find that, following an isotropic growth phase during which a single polarity cap wanders around the surface, outgrowth occurs when spores have doubled their volume, concomitantly with the stabilization of the cap and a singular rupture in the OSW. This rupture happens when OSW mechanical stress exceeds a threshold, releases the constraints of the OSW on growth, and stabilizes polarity. Thus, outgrowth exemplifies a self-organizing morphogenetic process in which reinforcements between growth and polarity coordinate mechanics and internal organization

An optogenetic assay shows that asymmetries in membrane potential can direct polarity.

<p>(A) Optogenetic assay to generate asymmetries in membrane potential and assess for effect on polarity. Schematic representation of the experimental setup: a yellow laser (λ = 535 nm) is used to photoactivate Halorhodopsin (Halo) in selected regions of <i>rsr1</i>Δ cells. θ is the final angle of shmoo or bud emergence with respect to the direction of the photoactivated region. (B) <i>rsr1</i>Δ (left) and Halorhodopsin-GFP-expressing <i>rsr1</i>Δ (right) cells in the presence of α-factor (αF) and retinal are continuously photoactivated from time 0 to 20 min at the indicated yellow region. After 2 h, shmoos grow and polarity orientation can be quantified with respect to the photoactivated region. White arrowheads indicate sites of shmoo formation. (C) Quantification of optogenetic experiments: radial histogram of polarized growth orientation with respect to photoactivation angle in <i>rsr1</i>Δ and <i>rsr1</i>Δ + Halorhodopsin-GFP cells treated with α-factor. (D) Average orientation of polarized growth in budding and shmooing cells after 2 h of growth following local photoactivation for a population of <i>rsr1</i>Δ, <i>rsr1</i>Δ Hxt3-GFP, and <i>rsr1</i>Δ + Halorhodopsin-GFP cells (<i>n</i>>70 cells gathered from four independent datasets for all conditions and <i>n</i> = 166 cells gathered from seven independent experiments for <i>rsr1</i>Δ + Halorhodopsin-GFP + α-factor). **Student's <i>t</i> test, <i>p</i><0.05. Error bars represent standard deviations.</p

Membrane hyperpolarization orients polarity through local phosphatidylserine accumulation.

<p>(A) Average shmoo orientation in the absence or presence of an EF for a population of WT, <i>cho1</i>Δ, <i>dnf1-2</i>Δ, and <i>lem3</i>Δ cells treated with α-factor (αF) (<i>n</i>>50 cells). (B) Average bud orientation after 3 h in the absence and in the presence of an EF for a population of <i>rsr1</i>Δ, <i>rsr1</i>Δ <i>cho1</i>Δ, <i>rsr1</i>Δ <i>dnf1-2</i>Δ, and <i>rsr1</i>Δ <i>lem3</i>Δ cells (<i>n</i>>50 cells). (C) Sixteen-color epifluorescence time lapses of shmooing and budding cells polarizing in EFs and expressing GFP-Lact-C2 probe (a marker for PS). White arrowheads point at sites of PS accumulation. (D) Quantification of PS localization in EFs. The ratio of anodal versus cathodal signal is computed by measuring the total amount at the membrane on both facing sides of the cell. Left: ratio evolution for the depicted sequences in (C). The black arrows indicate the moment when shmoo tip or bud was first visible. Right: average ratio of anodal versus cathodal PS signal for shmooing and budding cells. **Student's <i>t</i> test, <i>p</i><0.001. Error bars represent standard deviations.</p

Influence of electrochemical asymmetries on polarity.

<p>(A) During normal cell polarization, electrochemical layers segregate to the front and the back of the cell and may influence polarization processes, for instance during mating. (B) In an EF, the anode-facing side has hyperpolarized membrane potential, which drives anodal growth of the shmoos, in a Trk1-, Cho1<i>-</i>, and Far1-dependent manner. The secondary default orientation mode appears to be the cathodal orientation, which drives bud emergence and shmoo growth in <i>trk1</i>Δ, <i>cho1</i>Δ, and <i>far1-s</i> mutants by a yet unknown mechanism. (C) Optogenetic experiments directly suggest that local hyperpolarization of cell membrane potential can drive shmoo polarized growth but not bud site emergence. αF, α-factor.</p

EF response involves Cdc42p polarization.

<p>(A) Percentage of new bud formation after 2 h in the absence or in the presence of an EF for a population of WT, <i>rsr1</i>Δ, and <i>cdc42-118 rsr1</i>Δ (at restrictive temperature, 36°C). (B) Percentage of shmoo formation after 2 h in the absence or in the presence of an EF for a population of WT, <i>cdc42-118</i> (at restrictive temperature), <i>bem1-s1</i>, and <i>bni1</i>Δ cells treated with α-factor (αF). (C) Confocal single plane time-lapse images of GFP-Cdc42 and Cdc24-GFP expressed in <i>rsr1</i>Δ cells grown under an EF, in the absence or in the presence of α-factor. White arrowheads indicate the successive positions of the protein polar caps. (D) Confocal single plane time-lapse images of Bem1-GFP in control and LatA-treated <i>rsr1</i>Δ cells grown in the absence and in the presence of an EF. White arrowheads indicate the successive positions of Bem1-GFP polar caps. (E) Confocal single plane time-lapse images of Bem1-GFP in control and LatA-treated <i>rsr1</i>Δ cells grown with or without an EF in the presence of α-factor. Note that LatA treatment induces rapid dispersion of the Bem1-GFP signal at the cap, with or without EF. White arrowheads indicate the successive positions of Bem1-GFP polar caps. (F) Temporal evolution of the average orientation of Bem1-GFP caps with respect to the applied EF in a population of <i>rsr1</i>Δ cells, treated with and without LatA or α-factor (top) (<i>n</i> = 13 cells for budding [blue], <i>n</i> = 9 cells for budding + LatA [green], <i>n</i> = 4 cells for shmooing [red]). Half-time (<i>t</i>_1/2) corresponding to the mean orientation of Bem1-GFP polar caps to the cathode or anode of the EF is shown at the bottom. (G) Average shmoo orientation after 3 h in the absence or in the presence of an EF for a population of WT, <i>rsr1</i>Δ, <i>cdc24-m</i>, <i>far1-s</i>, and <i>rsr1</i>Δ <i>far1-s</i> cells treated with α-factor. <i>n</i>>50 cells for each condition. Error bars represent standard deviations. Scale bars: 2 µm.</p