HAL Portal ESPCI (Ecole Supérieure de Physique et de Chimie Industrielles)
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Fatty acid metabolism and the oxidative stress response support bacterial predation
No full textInternational audienceDespite growing awareness of their importance in soil ecology, the genetic and physiological traits of bacterial predators are still relatively poorly understood. In the course of a Myxococcus xanthus predator evolution experiment, we identified a class of genotypes leading to enhanced predation against diverse species. RNA-seq analysis demonstrated that this phenotype is linked to the constitutive activation of a predation-specific program. Functional analysis of the mutations accumulated across the evolutionary time in a two-component system and Acyl-CoA-manipulating enzymes revealed the critical roles of fatty acid metabolism and antioxidant gene induction. The former likely adapts the predator to metabolites derived from the prey while the latter protects predatory cells from reactive oxygen species generated by prey cells under stress and released upon lysis during predation. These findings reveal interesting parallels between bacterial predator–prey dynamics and pathogen–host cell interactions
Self-organization and emergence of memory in a cyclically driven elastoplastic model of an amorphous solid
No full textInternational audienceThe mechanical behavior of disordered materials, such as dense suspensions, glasses, and granular materials, depends on their thermal and mechanical history. Under periodic driving, these materials can evolve into states that encode a memory of their annealing. Such memory effects have been observed experimentally in systems ranging from sheared non-Brownian suspensions to crumpled elastic sheets and in atomistic simulations. Here, we show that a quenched mesoscopic elastoplastic model of a sheared amorphous solid reproduces the phenomena of self-organization and memory formation under mechanical annealing. We analyze sample-to-sample fluctuations under readout protocols and demonstrate their connection to the irreversibility transition. Our model allows us to understand in detail the mechanical processes underlying memory formation. We find that annealing by cyclic shear leads to the self-organization of plasticity, which can be characterized by a density of local mechanical stress thresholds. These thresholds exhibit anisotropy, depending on their alignment with the driving direction. The interplay between these thresholds dictates how the driving history—particularly its direction and amplitude—is imprinted into the material's local structure. We develop readout protocols capable of accessing both the amplitude and the direction of the mechanical training. Our findings can be understood within the framework of return point memory which emerges as a result of mechanical annealing. Building on this, we develop a Preisach-like model of directional memory that describes well our numerical results. We conclude with a discussion of similarities of the evolution of plasticity under mechanical annealing and adaptive evolution in changing environments
Unfitted finite element modelling of surface-bulk viscous flows in animal cells
No full textThis work presents a novel unfitted finite element framework to simulate coupled surface-bulk problems in time-dependent domains, focusing on fluid-fluid interactions in animal cells between the actomyosin cortex and the cytoplasm. The cortex, a thin layer beneath the plasma membrane, provides structural integrity and drives shape changes by generating surface contractile forces akin to tension. Cortical contractions generate Marangoni-like surface flows and induce intracellular cytoplasmic flows that are essential for processes such as cell division, migration, and polarization, particularly in large animal cells. Despite its importance, the spatiotemporal regulation of cortex-cytoplasm interactions remains poorly understood and computational modelling can be very challenging because surface-bulk dynamics often lead to large cell deformations. To address these challenges, we propose a sharp-interface framework that uniquely combines the trace finite element method for surface flows with the aggregated finite element method for bulk flows. This approach enables accurate and stable simulations on fixed Cartesian grids without remeshing. The model also incorporates mechanochemical feedback through the surface transport of a molecular regulator of active tension. We solve the resulting mixed-dimensional system on a fixed Cartesian grid using a level-set-based method to track the evolving surface. Numerical experiments validate the accuracy and stability of the method, capturing phenomena such as self-organised pattern formation, curvature-driven relaxation, and cell cleavage. This novel framework offers a powerful and extendable tool for investigating increasingly complex morphogenetic processes in animal cells
Exploring the limits to quantitative elastography: supersonic shear imaging in stretched soft strips
No full textInternational audienceAbstract Objective. Shear wave elastography has enriched ultrasound medical imaging with quantitative tissue stiffness measurements. We aim to explore the limitations that persist related to viscoelasticity, guiding geometry or static deformation. Approach. A nearly-incompressible soft elastomer strip is chosen to mimic the mechanical behaviour of an elongated tissue. A supersonic shear wave scanner measures the propagation of shear waves within the strip. It provides a wide range of shear wave velocities, from 2 to 6 m s −1 , depending on the frequency, the static strain as well as the orientation of the strip. Main results. To explain these different measurements, the guided wave effect is highlighted and analysed from the dispersion diagrams provided by the spatio-temporal Fourier transform of the raw data. The guided waves are then described using a material model that accounts for both the rheology and the hyperelastic behaviour, and allows to extract the mechanical parameters of the sample. Significance. To overcome some limitations of current elastography, we propose a theoretical framework which allows the simultaneous characterization of the viscoelastic and hyperelastic properties of soft tissues, paving the way for robust quantitative elastography of elongated tissues
Design of a Novel Class of N-Heterocyclic Carbene Cycloplatinated Complexes Containing Pyrene Chromophores
No full textInternational audienceCycloplatinated complexes incorporating pyrene chromophores of the formulae (CˆC*)Pt(acac) (3, 4), (CˆC* = Pyrenyl-NHC, acac = acetylacetonate) were prepared and fully characterized. For comparison, two regioisomeric complexes were prepared following synthetic procedures developed by us. One isomer has the Pt(II) center attached to the 2-position of the pyrene chromophore, while the other regioisomer has the metal center attached at the 1-position of the organic chromophore. The molecular structures of 3 and 4 were ascertained by X-ray diffraction, and they prove the identity of the targeted compounds. Both complexes are emissive at room temperature in the red part of the spectrum in poly(methyl methacrylate) (PMMA), as well as at 77 K in 2-methyltetrahydrofuran (2-MeTHF). The regioisomer containing the Pt(II) at the 1-position shows enhanced emissive properties compared to the other regioisomer
Nonlocal Dielectric Properties of Water: The Role of Electronic Delocalization
No full textInternational audienceThe nonlocal dielectric properties of liquid water are studied in the context of ab initio molecular dynamics (AIMD) simulations based on the density functional theory. We calculate the dielectric response from the charge structure factor of the liquid using the fluctuation–dissipation theorem. We show that the dielectric response function of ab initio simulations differs significantly from that of classical force-fields, both qualitatively and quantitatively. In particular, it exhibits a larger amplitude and a wider range of responding wave numbers. We suggest that the difference is due to the localization of the electronic charge density inherent in both classical force files and Wannier post-treatment of DFT densities. The localized charge models do not reproduce the shape of the response function even for q corresponding to intermolecular distances, and could lead to a significant underestimation of the dielectric response of the liquid by a factor of 10
Social hierarchy and behavioral individuality in colonies of isogenic female mice
No full textThe mechanisms underlying social organization in mice have predominantly been studied in male colonies, where sociability predicts higher rank and high-ranked individuals show greater anxiety. Here, we demonstrate that groups of isogenic female mice also form stable social hierarchies. Our data indicate that females destined for high rank already exhibit greater sociability and possibly higher anxiety before group formation, and these traits remain consistent afterwards. We further investigated the influence of sex by creating mixed-sex colonies, which revealed a similar hierarchical structure, with both males and females having equal chances of becoming high- or low-ranked. We previously found reduced dopamine neuron activity in high-ranked males; in contrast, high-ranked females show the opposite pattern. Furthermore, while glucocorticoid receptor signaling in dopaminoceptive neurons restricts high rank in males, this effect is absent in females. Overall, these results highlight sex-specific mechanisms that contribute to social ranking and related behavioral traits in mice
Solvation Entropy as a Lever for Steering the Macroscopic Properties of a Functional Supramolecular Helical Polymer
No full textInternational audienceSolvent‐solute interactions are utterly important in supramolecular polymers (SPs), yet the high responsivity of SPs to solvent polarity makes it challenging to play on other solvation effects to tune their macroscopic properties in a rational manner. Herein, we report the characterization at various scales of the assembly properties of a C 2 ‐symmetric benzene‐1,3,5‐tricarboxamide monomer with two (1 S )‐methylheptyl moieties and one diphenylphosphino group as lateral chains. Our investigation reveals a highly cooperative structural transition between two SP states, which can exquisitely be tuned by solvents of similar polarities, leading to variation of the transition temperature (T*) over a range of 85 K. The structural transition was detected in 4 pure solvents and 13 toluene/cosolvent mixtures; a fair relationship is determined between T* and the solvent molar volume. The transition is only weakly favored by enthalpy (by ca. 2 kJ.mol −1 at 286 K). However, minimization of the entropic cost leads to a notable increase in the T*. This allows a fine tuning of the thermothickening and catalytic properties of the resulting SPs auguring that solvation, and notably solvation entropy, may constitute an important lever for steering SPs structure and properties
Dispersion négative et modes à vitesse de groupe nulle dans les guides d’ondes élastiques
No full textConférence plénière : Médaille Française de la SFA 1; GAPSUS - Acoustique Physique, Sous-Marine et Ultra-SonoreNational audienceLes ondes élastiques guidées résultent du couplage des ondes de cisaillement et des ondes de compression aux interfaces. Même dans les guides les plus simples tels que plaques ou cylindres homogènes, cette interaction complexe se traduit par l’existence de modes étranges, à vitesse de phase négative. Ces « backward modes », comme on les appelle en anglais, résultent de la répulsion entre deux branches d’une même famille ayant des fréquences de coupure voisines. Un point particulièrement intéressant est qu’en l'absence d'atténuation ou de pertes par rayonnement, la fréquence minimale d’une branche «backward» correspond à un mode de vitesse de phase finie et de vitesse de groupe nulle : il s’agit alors de modes ZGV (Zero Group Velocity) qui sont associés à de fines résonances locales. Au cours de cette présentation, ces phénomènes seront illustrés grâce à des mesures effectuées en immersion à l’aide de capteurs multi-éléments, ou par ultrasons-laser sans contact. Nous verrons, en particulier, le comportement de ces modes dans les plaques anisotropes et discuterons de leur application à la caractérisation de matériaux
Ultrasonic Wave Transport in Three-Dimensional Weakly Confined Disordered Granular Media
No full textOndes en milieu hétérogène; GAPSUS - Acoustique Physique, Sous-Marine et Ultra-SonoreNational audienceWe present experimental observations of ultrasonic wave transport in non-cohesive randomly close packed (RCP) granular media under low confinement pressures. The samples are 165-mm-diameter, and 6-mm-thick or 10-mm-thick circular slabs made of an equal number of 1.25-mm-diameter aluminum spheres and 0.95-mm-diameter borosilicate glass spheres, hermetically enclosed in thin plastic sheets. The confinement pressure is maintained by an inner partial vacuum (0.1 atm to 0.9 atm); our setup thus allows probing the dynamics as close as desired to the unjamming transition. Experiments are done in a water tank, using ultrasonic emitter/receiver pairs or an emitter and a hydrophone point receiver, over a wide frequency range (25 kHz to 500 kHz). In a typical experiment, an incident wave pulse is multiply scattered inside a granular slab, and from the total transmitted signal one can investigate wave transport from both the coherent ballistic part that travels straight through the medium without scattering out of the forward direction and from the incoherent multiply scattered part (Page et al., Phys. Rev. E, 52, 3106 (1995)). From the coherent part, we measure the frequency dependence of the phase and group velocities, attenuation and scattering mean free path. We find that the time-of-flight intensity profile of the multiply scattered coda is independent of frequency over a wide range of frequencies in the intermediate frequency regime (where the wavelength is comparable with the sizes of the beads), suggesting a plateau in the diffusion coefficient, as predicted by Vitelli and co-workers (Phys. Rev. E 81, 021301 (2010)). Near the upper edge of the intermediate frequency regime, the intensity profile parallel to the slab becomes progressively confined spatially, providing evidence for the approach to Anderson localization of ultrasonic waves in the medium (Hu et al., Nature Phys. 4, 945, (2008))