Pattern Formation of Ion Channels with State Dependent Electrophoretic Charges and Diffusion Constants in Fluid Membranes

A model of mobile, charged ion channels in a fluid membrane is studied. The channels may switch between an open and a closed state according to a simple two-state kinetics with constant rates. The effective electrophoretic charge and the diffusion constant of the channels may be different in the closed and in the open state. The system is modeled by densities of channel species, obeying simple equations of electro-diffusion. The lateral transmembrane voltage profile is determined from a cable-type equation. Bifurcations from the homogeneous, stationary state appear as hard-mode, soft-mode or hard-mode oscillatory transitions within physiologically reasonable ranges of model parameters. We study the dynamics beyond linear stability analysis and derive non-linear evolution equations near the transitions to stationary patterns.Comment: 10 pages, 7 figures, will be submitted to Phys. Rev.

Astrocytic Ion Dynamics: Implications for Potassium Buffering and Liquid Flow

We review modeling of astrocyte ion dynamics with a specific focus on the implications of so-called spatial potassium buffering, where excess potassium in the extracellular space (ECS) is transported away to prevent pathological neural spiking. The recently introduced Kirchoff-Nernst-Planck (KNP) scheme for modeling ion dynamics in astrocytes (and brain tissue in general) is outlined and used to study such spatial buffering. We next describe how the ion dynamics of astrocytes may regulate microscopic liquid flow by osmotic effects and how such microscopic flow can be linked to whole-brain macroscopic flow. We thus include the key elements in a putative multiscale theory with astrocytes linking neural activity on a microscopic scale to macroscopic fluid flow.Comment: 27 pages, 7 figure

Bioelectrical signals and ion channels in the modeling of multicellular patterns and cancer biophysics

Bioelectrical signals and ion channels are central to spatial patterns in cell ensembles, a problem of fundamental interest in positional information and cancer processes. We propose a model for electrically connected cells based on simple biological concepts: i) the membrane potential of a single cell characterizes its electrical state; ii) the long-range electrical coupling of the multicellular ensemble is realized by a network of gap junction channels between neighboring cells; and iii) the spatial distribution of an external biochemical agent can modify the conductances of the ion channels in a cell membrane and the multicellular electrical state. We focus on electrical effects in small multicellular ensembles, ignoring slow diffusional processes. The spatio-temporal patterns obtained for the local map of cell electric potentials illustrate the normalization of regions with abnormal cell electrical states. The effects of intercellular coupling and blocking of specific channels on the electrical patterns are described. These patterns can regulate the electrically-induced redistribution of charged nanoparticles over small regions of a model tissue. The inclusion of bioelectrical signals provides new insights for the modeling of cancer biophysics because collective multicellular states show electrical coupling mechanisms that are not readily deduced from biochemical descriptions at the individual cell level

On biomembrane electrodiffusive models

Two models are used in the literature, to study the electric behaviour of cellular membranes such as in protein aggregates, excitable media or ionic currents for examples. The first one is the Electroneutral Model based on Nernst-Planck and Poisson equations with a specific condition of microscopic electroneutrality. The second one is the Cable Model valid for long wavelengths based on an analogy between an electric cable and a cell. Convincing experiments have justified the Cable equation. First, we show that these two models are in contradiction. More precisely the assumption of electroneutrality is not considered in the Cable Model. The main difference between the two models is highlighted by the analysis of the well known voltage instability due to a negative differential conductance. Then, we derive a new semi-microscopic model (the Biomembrane Electrodiffusive Model, called BEM) valid for phenomena at any wavelength. The BEM is based on Nernst-Planck and Poisson equations but, doesn't imply microscopic electroneutrality. It reveals the capacitive behaviour of the membrane. In the limit of long wavelengths, one recovers the behaviour described within the Cable framework, as shown precisely in the study of the negative differential conductance analysis. Finally, we demonstrate the intimate link between the last models: the Cable Model appears as the limit of the BEM for large wavelengths with some prerequisites which are discussed. The effects of geometry and asymmetrical media are introduced

Ion transport in marine algae: from uniform to self-organized processes.

Cells and organelles are surrounded by at least one membrane that controls the exchange of energy and matter between the cytoplasm and its environment. Ion transport through membranes is an essential process for life. For instance, algae nutrition, osmotic and hydrostatic pressure regulation and cell signaling are typical cellular functions that are directly controlled by the transport of ions through the plasma membrane. The quantitative description of membrane transport through algae (and plants in general) is usually restricted to the uniform steady state case. However, spatial and temporal dynamics arising from the nonlinear properties of ion transport have recently been revealed to be of prime importance for cell signaling and developmental axis emergence. We will review the mechanisms of ion transport (active and passive transport) in marine algae and their implication in cell physiology, morphology and homeostasis. The basic principles of ion transport will be explain and we will show how the nonlinear coupling between different ion transport systems such as ATPases, channels and co-transports can give rise to self-organized spatiotemporal events such as action potentials or stationary patterns of transcellular currents observed in marine algae. © 2009 by Nova Science Publishers, Inc. All rights reserved.SCOPUS: ch.binfo:eu-repo/semantics/publishe

Influence of specific ionic diffusion on the protein self-aggregation instability

Transcellular currents are involved in the generation of spatial order during the growth of numerous biological cells. A dynamic instability triggered by the aggregation of the proteins may be at the origin of the symmetry breaking. An analysis in terms of a semi-microscopic model is performed. It reveals that the dynamics of the ion transferred by the mobile proteins, pumps or channels, is of paramount importance. The diffusion currents play a crucial role in the critical behaviour, even if the instability mechanism is of electric origin. When the diffusion coefficients of the different species are different, our results do not agree with those of the classical cable formalism

Plant VDAC Permeability: Molecular Basis and Role in Oxidative Stress

The mitochondrial voltage-dependent anion-selective channel (VDAC) is highly abundant in the mitochondrial outer membrane. It is permeable to molecules with a size up to about 5 kDa and is the main pathway for the exchange of metabolites and ions between the mitochondrial intermembrane space and the cytosol. Experimental studies performed for plant VDAC have shown that the channel displays properties reported for VDACs of other eukaryotic organisms. Firstly, it transports compounds as diverse as inorganic ions (e.g. K+ and Cl−), adenylates (e.g. ATP and AMP), and large macromolecules (tRNA and DNA). Secondly, despite its wide pore, the channel displays selectivity toward these compounds, i.e. it distinguishes between K+ and Cl− but also between ATP and AMP. The question of how VDAC can selectively transport these different compounds is addressed in this chapter based on data obtained for plant VDAC. It is well known that all organisms have at least one canonical VDAC isoform that shares similar electrophysiological properties and secondary structure with cognate VDAC of other organisms. For instance, this is the case of the mammalian VDAC1, the yeast Saccharomyces cerevisiae VDAC1 and the PcVDAC purified from the bean Phaseolus coccineus seeds. Consequently, Brownian dynamic simulations of monatomic ion permeation through the experimental three-dimensional structure of the mammalian VDAC1 and the PcVDAC modeled structure predict fairly well conductance and selectivity of both proteins. In addition, the data of molecular simulation studies performed on the mammalian VDAC1 agree with the experimental data obtained for PcVDAC, which suggests a similar permeation process for these VDAC proteins. Accordingly, both the experimental and theoretical studies indicate that the selectivity for inorganic ions is a consequence of the excess of positive charges and their distribution inside the pore and the absence of defined pathways for the permeation. In contrast, the permeation of metabolites involves a major binding site located at the N-terminal helix which folded into the pore lumen and occurs through a preferential pathway. The key residues forming the binding site are conserved in the PcVDAC pointing to the conserved permeation process. The process might be affected by VDAC interaction with other proteins. For example, it is suggested that plant VDAC is involved in the oxidative stress response which includes cytosolic hexokinase and thioredoxin binding to VDAC. This in turn may influence the exchange of molecules between the mitochondria and the cytosol.info:eu-repo/semantics/publishe

Application médico-légale de la variabilité de l'alvéolyse humaine : étude préliminaire

Le but de l'étude préliminaire était un essai de modélisation de l'âge chez l'Adulte à partir de l'alvéolyse mesurée sur radiotomodensitométrie. Compte tenu du lien entre l'alvéolyse et l'âge, 3 modèles ont été proposés, selon le sexe ou non, permettant d'approcher une estimation de l'âge grâce à une méthode simple, rapide et applicable aux populations actuelles en médecine légale et aux populations du passé en anthropologie funéraire