HAL Portal ESPCI (Ecole Supérieure de Physique et de Chimie Industrielles)
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Towards High-Speed and High-Resolution Real-Time Optical Flow Particle Image Velocimetry
No full textInternational audienceOne of the major advantages of Optical Flow PIV (Particle Image Velocimetry) algorithms over Cross-Correlation PIV is their scalability leading to potentially very high computational speeds. This is confirmed in this study using different GPUs (Graphics Processor Unit) and different image sizes. The other advantage is the possibility of obtaining dense velocity fields of up to one vector per pixel. It is well known that particle seeding plays a crucial role in the results of standard particle image velocimetry based on cross-correlation algorithms. Its influence on the quality of the optical flow algorithm is not as well established. In this article the influence of particle concentration is quantified by introducing a criterion taking into account the proportion of "active" pixels in a snapshot. It is shown that it is possible to optimize particle concentration to maximize the percentage of active pixels, leading to better spatial resolution, down to one vector per pixel. The principle is validated on a vortex-free flow and applied to the complex 3D flow downstream a backward-facing step
Ca 2+ regulation of Myosin II and Myosin VI during rupture of the Shigella -containing vacuole
No full textShigella , the causative agent of bacillary dysentery, invades epithelial cells to colonize the intestinal mucosa. Following invasion, Shigella is enclosed in a vacuole that needs to rupture for bacterial intra-cytosolic replication. We show here that rupture of the Shigella vacuole requires Ca 2+ influx leading to long lasting local Ca 2+ increases that regulate actin dynamics affecting the Shigella vacuole integrity. These Ca 2+ increases promote vacuolar rupture by activating myosin II associated with actin filaments in membrane ruffles distant from the vacuole, while tethering myosin VI at the actin coat-surrounded vacuole. Ca 2+ depletion and myosin II inhibition impair formation of the actin coat and vacuole rupture. Inhibition of myosin VI also delays rupture of vacuoles but lead to their tumbling. These findings highlight a role for Ca 2+ in coordinating actin–based forces and constraints during early rupture steps of bacterial vacuole, that pull on vacuolar membranes and tether them to the actin cortex via myosin II and VI, respectively, a process relevant to intracellular pathogen and endomembrane trafficking
Sedimentation of a single soluble particle at low Reynolds and high Péclet numbers
No full textInternational audienceWe investigate experimentally the dissolution of an almost spherical butyramide particle during its sedimentation, in the low Reynolds high Péclet regime. The particle sediments in a quiescent aqueous solution, and its shape and position are measured simultaneously by a camera attached to a translation stage. The particle is tracked in real time, and the translation stage moves accordingly to keep the particle in the field of the camera. The measurements from the particle image show that the radius shrinking rate is constant with time, and independent of the initial radius of the particle. We explain this with a simple model, based on the sedimentation law in the Stokes' regime and the mass transfer rate at low Reynolds and high Péclet numbers. The theoretical and experimental results are consistent within 20%. We introduce two correction factors to take into account the non-sphericity of the particle and the inclusions of air bubbles inside the particle, and reach quantitative agreement. With these corrections, the indirect measurement of the radius shrinking rate deduced from the position measurement is also in agreement with the model. We discuss other correction factors, and explain why there are negligible in the present experiment. We also compute the effective Sherwood number as a function of an effective Péclet number
Long range mutual activation establishes Rho and Rac polarity during cell migration
No full textIn migrating cells, the GTPase Rac organizes a protrusive front, whereas Rho organizes a contractile back. How these GTPases are appropriately positioned at the opposite poles of a migrating cell is unknown. Here we leverage optogenetics, manipulation of cell mechanics, and mathematical modeling to reveal a surprising long-range mutual activation of the front and back polarity programs that complements their well-known local mutual inhibition. This long-range activation is rooted in two distinct modes of mechanochemical crosstalk. Local Rac-based protrusion stimulates Rho activation at the opposite side of the cell via membrane tension-based activation of mTORC2. Conversely, local Rho-based contraction induces cortical-flow-based remodeling of membrane-to-cortex interactions leading to PIP2 release, PIP3 generation, and Rac activation at the opposite side of the cell. We develop a minimal unifying mechanochemical model of the cell to explain how this long-range mechanical facilitation complements local biochemical inhibition to enable robust global Rho and Rac partitioning. Finally, we validate the importance of this long-range facilitation in the context of chemoattractant-based cell polarization and migration in primary human lymphocytes. Our findings demonstrate that the actin cortex and plasma membrane function as an integrated mechanochemical system for long-range partitioning of Rac and Rho during cell migration and likely other cellular contexts
Development of an analytical method for the simultaneous determination of 22 Polycyclic Aromatic Hydrocarbons (PAHs) in maternal and umbilical cord blood
No full textInternational audienceLes femmes enceintes et leurs fœtus, une population très fragile, sont exposés aux hydrocarbures aromatiques polycycliques (HAP) et soumis à d'éventuels risques sanitaires graves. Par conséquent, la détermination des HAP dans le sérum maternel et le sang du cordon ombilical est nécessaire mais représente un défi en raison de leurs propriétés physico-chimiques très variées et leur présence à l’état de traces. Ce travail visait à développer une méthode permettant pour la première fois la détermination simultanée de 22 HAP répertoriés à partir de faibles volumes (100 µL) de sérum maternel et de sang de cordon ombilical, incluant un prétraitement de l'échantillon réduisant autant que possible la quantité de solvants utilisés. Ainsi, une étape extraction sur phase solide sur silice greffée C18 a été optimisée en étudiant l'effet de différents paramètres (nature et proportion du solvant organique ajouté à l'échantillon, conditions de lavage et d'élution) sur les rendements d’extraction. L'étape d'évaporation a été soigneusement contrôlée pour limiter la perte des HAP les plus volatils. Ensuite, pour rompre toutes les interactions entre les HAP et les protéines sériques, une étape de précipitation avant l’extraction sur phase solide a été optimisée à l'aide d'un plan d'expériences. La procédure finale de traitement des échantillons a conduit à des rendements d'extraction compris entre 27 et 57% et entre 34 et 69% pour 22 HAP dans le sérum et le sang de cordon ombilical, respectivement, avec des valeurs d'écart-type inférieures à 11%. Des limites de quantification comprises entre 0,2 et 3,1 µg/L ont été atteintes pour les HAP qui fluorescent dans des échantillons réels dopés, ce qui est proche des concentrations attendues dans les échantillons biologiques
Getting the best out of capillary electrophoresis and capillary electrophoresis–mass spectrometry by quantifying sources of peak broadening for proteins using polyelectrolyte multilayer coated fused silica capillaries
No full textInternational audienceCapillary electrophoresis (CE) has emerged as a relevant technique for protein and biopharmaceutical analysis, as it combines high separation efficiency, sensitivity, and versatility. The use of capillary coatings, including successive multiple ionic-polymer layers (SMILs), reduces interactions between analytes and the capillary, further improving the CE performance. Nevertheless, separations done on SMIL coatings rarely surpass 500 × 103 plates/m. To obtain the best out of the CE, it is interesting to have a detailed look at the sources of peak dispersion. Separations of a mix of model proteins were performed on (poly(diallyldimethylammonium chloride)/poly(styrenesulfonate))2.5-coated capillaries at different electrical field strengths, leading to plate height H against migration velocity u plots that enabled a quantitative analysis of each contribution. Using this model, capillary lengths and injected volumes were systematically varied. For the first time, the contribution of sample electrophoretic heterogeneity to the total peak dispersion was deciphered for model proteins and a monoclonal antibody. Dispersion due to electromigration was seen to have an impact on plate heights in the case of triangular peaks of small molecules but not for proteins under the present conditions. UV and mass spectrometry detections were compared on the same capillary, providing valuable information on the impact of the detection type on separation efficiency. Close to 1 million plates/m were reached in the best conditions
Striatal projection neurons coexpressing dopamine D1 and D2 receptors modulate the motor function of D1- and D2-SPNs
No full textInternational audienc
Striatal projection neurons coexpressing dopamine D1 and D2 receptors modulate the motor function of D1- and D2-SPNs
No full textInternational audienc
Transient contacts between filaments impart its elasticity to branched actin
No full textBranched actin networks exert pushing forces in eukaryotic cells, and adapt their stiffness to their environment. The physical basis for their mechanics and adaptability is however not understood. Indeed, here we show that their high density and low connectivity place them outside the scope of standard elastic network models for actin. We combine high-precision mechanical experiments, molecular dynamics simulations and a mean-field elastic theory to show that they are instead dominated by the proliferation of interfilament contacts under compression. This places branched actin in the same category as undercoordinated, fibrous materials such as sheep's wool. When the network is grown under force, filaments entangle as if knitted together and trap contacts in their structure. Trapped contacts play a similar role as crosslinkers in rigidifying the network, and are thus key to its active adaptive mechanics
Four-electron reduction of CO<sub>2</sub>: from formaldehyde and acetal synthesis to complex transformations
No full textInternational audienceThe expansive and dynamic field of the CO2 Reduction Reaction (CO2RR) seeks to harness CO2 as a sustainable carbon source or energy carrier. While significant progress has been made in two, six, and eight-electron reductions of CO2, the four-electron reduction remains understudied. This review fills this gap, comprehensively exploring CO2 reduction into formaldehyde (HCHO) or acetal-type compounds (EOCH2OE, with E = [Si], [B], [Zr], [U], [Y], [Nb], [Ta] or –R) using various CO2RR systems. These encompass (photo)electro-, bio-, and thermal reduction processes with diverse reductants. Formaldehyde, a versatile C1 product, is challenging to synthesize and isolate from the CO2RR. The review also discusses acetal compounds, emphasizing their significance as pathways to formaldehyde with distinct reactivity. Providing an overview of the state of four-electron CO2 reduction, this review highlights achievements, challenges, and the potential of the produced compounds – formaldehyde and acetals – as sustainable sources for valuable product synthesis, including chiral compounds