Structure formation in active networks

Structure formation and constant reorganization of the actin cytoskeleton are key requirements for the function of living cells. Here we show that a minimal reconstituted system consisting of actin filaments, crosslinking molecules and molecular-motor filaments exhibits a generic mechanism of structure formation, characterized by a broad distribution of cluster sizes. We demonstrate that the growth of the structures depends on the intricate balance between crosslinker-induced stabilization and simultaneous destabilization by molecular motors, a mechanism analogous to nucleation and growth in passive systems. We also show that the intricate interplay between force generation, coarsening and connectivity is responsible for the highly dynamic process of structure formation in this heterogeneous active gel, and that these competing mechanisms result in anomalous transport, reminiscent of intracellular dynamics

Renewal processes and fluctuation analysis of molecular motor stepping

We model the dynamics of a processive or rotary molecular motor using a renewal processes, in line with the work initiated by Svoboda, Mitra and Block. We apply a functional technique to compute different types of multiple-time correlation functions of the renewal process, which have applications to bead-assay experiments performed both with processive molecular motors, such as myosin V and kinesin, and rotary motors, such as F1-ATPase

Phoretic Motion of Spheroidal Particles Due To Self-Generated Solute Gradients

We study theoretically the phoretic motion of a spheroidal particle, which generates solute gradients in the surrounding unbounded solvent via chemical reactions active on its surface in a cap-like region centered at one of the poles of the particle. We derive, within the constraints of the mapping to classical diffusio-phoresis, an analytical expression for the phoretic velocity of such an object. This allows us to analyze in detail the dependence of the velocity on the aspect ratio of the polar and the equatorial diameters of the particle and on the fraction of the particle surface contributing to the chemical reaction. The particular cases of a sphere and of an approximation for a needle-like particle, which are the most common shapes employed in experimental realizations of such self-propelled objects, are obtained from the general solution in the limits that the aspect ratio approaches one or becomes very large, respectively.Comment: 18 pages, 5 figures, to appear in European Physical Journal

Droplet ripening in concentration gradients

Living cells use phase separation and concentration gradients to organize chemical compartments in space. Here, we present a theoretical study of droplet dynamics in gradient systems. We derive the corresponding growth law of droplets and fi nd that droplets exhibit a drift velocity and position dependent growth. As a consequence, the dissolution boundary moves through the system, thereby segregating droplets to one end. We show that for steep enough gradients, the ripening leads to a transient arrest of droplet growth that is induced by a narrowing of the droplet size distribution

Physics of active emulsions

Phase separating systems that are maintained away from thermodynamic equilibrium via molecular processes represent a class of active systems, which we call \textit{ active emulsions}. These systems are driven by external energy input for example provided by an external fuel reservoir. The external energy input gives rise to novel phenomena that are not present in passive systems. For instance, concentration gradients can spatially organise emulsions and cause novel droplet size distributions. Another example are active droplets that are subject to chemical reactions such that their nucleation and size can be controlled and they can spontaneously divide. In this review we discuss the physics of phase separation and emulsions and show how the concepts that governs such phenomena can be extended to capture the physics of active emulsions. This physics is relevant to the spatial organisation of the biochemistry in living cells, for the development novel applications in chemical engineering and models for the origin of life

Les tumeurs mélanocytaires cutanées du chat

Les tumeurs mélanocytaires cutanées sont rares dans l'espèce féline. Elles se situent essentiellement sur la tête et le cou de chats âgés. Aucune prédisposition de sexe ou de race n'a pu être mise en évidence. Malgré leur apparence macroscopique fréquemment pigmentée, certaines sont achromiques, et seule l'analyse histopathologique approfondie permet le diagnostic. Cinq types cellulaires sont reconnus : épithélioïde, fusiforme, mixte, à cellules ballonnisantes et à cellules rhabdoïdes. Une des particularité, chez le chat, repose sur la grande difficulté à établir une classification fiable, selon le caractère bénin/malin de ces tumeurs. Le pronostic est cependant, le plus souvent réservé. A l'heure actuelle, l'exérèse chirurgicale, lorsqu'elle s'avère possible, reste le traitement de choix. Les protocoles de chimiothérapie, de radiothérapie, d'immunothérapie et leur efficacité sont encore au stade d'étude chez le chat.TOULOUSE3-BU Santé-Centrale (315552105) / SudocTOULOUSE-EN Vétérinaire (315552301) / SudocSudocFranceF

Plasma Troponin-I levels in both study cohorts.

<p>Mean values and standard deviation of fast troponin I plasma levels (ng/ml) of the immobilization (top panel) and recovery (bottom panel) cohorts of mice. Definition of abbreviations: ng, nanogram; ml, milliliter; I, immobilization; R, recovery. Statistical significance is represented as follows: **, p≤0.01, ***, p≤0.001, and n.s., non-significant differences between any of the immobilized animals and the non-immobilized controls; §§, p≤0.01, and n.s., non-significant differences between any of the R groups of animals and the 7-day I mice.</p