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S Portet

Publications and source records attributed to S Portet.

7 recordsLinked to original sources
PubMed2005

Elastic vibrations in seamless microtubules.

Parameters characterizing elastic properties of microtubules, measured in several recent experiments, reflect an anisotropic character. We describe the microscopic dynamical properties of microtubules using a discrete model based on an appropriate lattice of dimers. Adopting a harmonic approximation for the dimer-dimer interactions and estimating the lattice elastic constants, we make predictions regarding vibration dispersion relations and vibration propagation velocities. Vibration frequencies and velocities are expressed as functions of the elastic constants and of the geometrical characteristics of the microtubules. We show that vibrations which propagate along the protofilament do so significantly faster than those along the helix.

Anisotropy↗
PubMed2005

Anisotropic elastic properties of microtubules.

We review and model the experimental parameters which characterize elastic properties of microtubules. Three macroscopic estimates are made of the anisotropic elastic moduli, accounting for the molecular forces between tubulin dimers: for a longitudinal compression of a microtubule, for a lateral force and for a shearing force. These estimates reflect the anisotropies in these parameters observed in several recent experiments.

Animals↗
PubMed2004

Ionic wave propagation along actin filaments.

We investigate the conditions enabling actin filaments to act as electrical transmission lines for ion flows along their lengths. We propose a model in which each actin monomer is an electric element with a capacitive, inductive, and resistive property due to the molecular structure of the actin filament and viscosity of the solution. Based on Kirchhoff's laws taken in the continuum limit, a nonlinear partial differential equation is derived for the propagation of ionic waves. We solve this equation in two different regimes. In the first, the maximum propagation velocity wave is found in terms of Jacobi elliptic functions. In the general case, we analyze the equation in terms of Fisher-Kolmogoroff modes with both localized and extended wave characteristics. We propose a new signaling mechanism in the cell, especially in neurons.

Actin Cytoskeleton↗
PubMed2003

Models of spatial and orientational self-organization of microtubules under the influence of gravitational fields.

Tabony and co-workers [C. Papaseit, N. Pochon, and J. Tabony, Proc. Natl. Acad. Sci. U.S.A. 97, 8364 (2000)] showed that the self-organization of microtubules from purified tubulin solutions is sensitive to gravitational conditions. In this paper, we propose two models of spatial and orientational self-organization of microtubules in a gravitational field. First, the spatial model is based on the dominant chemical kinetics. The pattern formation of microtubule concentration is obtained (1) in terms of a moving kink in the limit when the disassembly rate is negligible, and (2) for the case of no free tubulin and only assembled microtubules present. Second, the orientational pattern of striped microtubule domains is consistent with predictions from a phenomenological Landau-Ginzburg free energy expansion in terms of an orientational order parameter.

Animals↗
PubMed2003

Weightlessness acts on human breast cancer cell line MCF-7.

Because cells are sensitive to mechanical forces, weightlessness might act on stress-dependent cell changes. Human breast cancer cells MCF-7, flown in space in a Photon capsule, were fixed after 1.5, 22 and 48 h in orbit. Cells subjected to weightlessness were compared to 1 g in-flight and ground controls. Post-flight, fluorescent labeling was performed to visualize cell proliferation (Ki-67), three cytoskeleton components and chromatin structure. Confocal microscopy and image analysis were used to quantify cycling cells and mitosis, modifications of the cytokeratin network and chromatin structure. Several main phenomena were observed in weightlessness: The perinuclear cytokeratin network and chromatin structure were looser; More cells were cycling and mitosis was prolonged. Finally, cell proliferation was reduced as a consequence of a cell-cycle blockade; Microtubules were altered in many cells. The results reported in the first point are in agreement with basic predictions of cellular tensegrity. The prolongation of mitosis can be explained by an alteration of microtubules. We discuss here the different mechanisms involved in weightlessness alteration of microtubules: i) alteration of their self-organization by reaction-diffusion processes, and a mathematical model is proposed, ii) activation or deactivation of microtubules stabilizing proteins, acting on both microtubule and microfilament networks in cell cortex.

Actin Cytoskeleton↗
PubMed1999

Quantitative analysis of cytokeratin network topology in the MCF7 cell line.

BACKGROUND: In the MCF7 human breast cancer cell line, several patterns of cytokeratin networks are observed, depending on the intracellular localization. Our hypothesis is that architectural variations of cytokeratin networks depend on local tensions or forces appearing spontaneously in the cytoplasm. The aim of this work was to discriminate between the different patterns and to quantitate these variations. MATERIALS AND METHODS: Image analysis procedures were developed to extract cytokeratin filament networks visualized by immunofluorescence and confocal microscopy. Two methods were used to segment sets of curvilinear objects. The first, the "mesh-approach," based on classical methods of mathematical morphology, takes into account global network topology. The second, the "filament-approach" (novel), is meant to account for individual element morphology. These methods and their combination allow the computation of several features at two levels of geometry: global (network topology) and local (filament morphology). RESULTS: Variations in cytokeratin networks are characterized by their connectivity, density, mesh structure, and filament shape. The connectivity and the density of a network describe its location in a local "stress-force" zone or in a "relaxed" zone. The mesh structure characterizes the intracellular localization of the network. Moreover, the filament shape reflects the intracellular localization and the occurrence of a "stress-force" zone. CONCLUSIONS: These features permitted the quantitation of differences within the network patterns and within the specific filament shapes according to the intracellular localization. Further experiments on cells submitted to external forces will test the hypothesis that the architectural variations of intermediate filaments reflect intracytoplasmic tensions.

Algorithms↗