Biomedical subjects

Michel Thellier

Publications and source records attributed to Michel Thellier.

8 recordsLinked to original sources

PubMed2004

Plant sensitivity to low intensity 105 GHz electromagnetic radiation.

Exposing seedlings of the flax, Linum usitatissimum L., to a variety of weak environmental stresses followed by a 2 day calcium deprivation, triggers the common response of production of epidermal meristems (actively dividing groups of cells) in the hypocotyl, which is the part of the stem between the root and the cotyledons (the pre-existing leaves in the embryo). This production reaches a plateau of 10-20 meristems after a month in the case of mechanical stimulation and cold shock. Recently, we have shown that radiation from a global system for mobile communication (GSM) telephone also triggers production of meristems with a plateau of around six meristems. Here, we show that a single 2 h exposure to radiation emitted at 105 GHz at non-thermal levels by a Gunn oscillator induces meristem production with kinetics similar to that induced by weak environmental stimuli and radiation from GSM telephone.

Calcium↗

PubMed2004

Ion condensation and signal transduction.

Many abiotic and other signals are transduced in eukaryotic cells by changes in the level of free calcium via pumps, channels and stores. We suggest here that ion condensation should also be taken into account. Calcium, like other counterions, is condensed onto linear polymers at a critical value of the charge density. Such condensation resembles a phase transition and has a topological basis in that it is promoted by linear as opposed to spherical assemblies of charges. Condensed counterions are delocalised and can diffuse in the so-called near region along the polymers. It is generally admitted that cytoskeletal filaments, proteins colocalised with these filaments, protein filaments distinct from cytoskeletal filaments, and filamentous assemblies of other macromolecules, constitute an intracellular macromolecular network. Here we draw attention to the fact that this network has physicochemical characteristics that enable counterion condensation. We then propose a model in which the feedback relationships between the condensation/decondensation of calcium and the activation of calcium-dependent kinases and phosphatases control the charge density of the filaments of the intracellular macromolecular network. We show how condensation might help mediate free levels of calcium both locally and globally. In this model, calcium condensation/decondensation on the macromolecular network creates coherent patterns of protein phosphorylation that integrate signals. This leads us to hypothesize that the process of ion condensation operates in signal transduction, that it can have an integrative role and that the macromolecular network serves as an integrative receptor.

Animals↗

PubMed2003

Thermodynamic parameters monitoring the equilibrium shift of enzyme-catalyzed hydrolysis/synthesis reactions in favor of synthesis in mixtures of water and organic solvent.

The main strategy developed to shift the equilibrium state of a hydrolase-catalyzed hydrolysis/synthesis reaction consists in reducing water activity by addition of organic solvents in the reaction medium. We have used several mixtures of water and 1,4-butanediol, ranging from pure water to pure 1,4-butanediol, to study the hydrolysis/synthesis reaction of the N-Cbz-L-tryptophanyl-glycineamide dipeptide, catalyzed by alpha-chymotrypsin. In the presence of 1,4-butanediol, alpha-chymotrypsin also catalyzed the esterification reaction between this diol and N-Cbz-L-tryptophan; this ester hydrolysis/synthesis reaction has thus also been examined. The dipeptide and ester equilibrium concentrations increase when the water content of the reaction medium is decreased. Using our experimental data, we have determined the equilibrium constants of the hydrolysis/synthesis equilibria involving the nonionized forms of the protected amino acids, the estimated values of which are Ksp = 8 10(5) for the dipeptide and Kse = 78 for the ester respectively. They are true thermodynamic equilibrium constants, each related to a single, well-defined reaction equilibrium and with water activity being taken into account. If an organic solvent is added to the reaction medium these equilibria can be shifted towards synthesis by decreasing the water activity but also by modifying the ionization/neutralization equilibrium constant of the ionizable groups. These two effects depend both on the water content and on the nature of the organic solvent used, and, in particular, on its dielectric constant. Because of the importance of this parameter in our study, we discuss using it as an indicator to select an appropriate organic solvent to perform an enzyme-catalyzed synthesis.

Bioreactors↗

PubMed2003

Networks as constrained thermodynamic systems.

We show how a network of interconnections between nodes can be constructed to have a specified distribution of nodal degrees. This is achieved by treating the network as a thermodynamic system subject to constraints and then rewiring the system to maintain the constraints while increasing the entropy. The general construction is given and illustrated by the simple example of an exponential network. By considering the constraints as a cost function analogous to an internal energy, we obtain a characterisation of the correspondence between the intensive and extensive variables of the network. Applied to networks in living organisms, this approach may lead to macroscopic variables useful in characterising living systems.

Entropy↗

PubMed2003

Biological processes in organised media.

Embedding a simple Michaelis-Menten enzyme in a gel slice may allow the catalysis of not only scalar processes but also vectorial ones, including uphill transport of a substrate between two compartments, and may make it seem as if two enzymes or transporters are present or as if an allosterically controlled enzyme/transporter is operating. The values of kinetic parameters of an enzyme in a partially hydrophobic environment are usually different from those actually measured in a homogeneous aqueous solution. This implies that fitting kinetic data (expressed in reciprocal co-ordinates) from in vivo studies of enzymes or transporters to two straight lines or a sigmoidal curve does not prove the existence of two different membrane mechanisms or allosteric control. In the artificial transport systems described here, a functional asymmetry was sufficient to induce uphill transport, therefore, although the active transport systems characterised so far correspond to proteins asymmetrically anchored in a membrane, the past or present existence of structurally symmetrical systems of transport in vivo cannot be excluded. The fact that oscillations can be induced in studies of the maintenance of the electrical potential of frog skin by addition of lithium allowed evaluation of several parameters fundamental to the functioning of the system in vivo (e.g., relative volumes of internal compartments, characteristic times of ionic exchanges between compartments). Hence, under conditions that approach real biological complexity, increasing the complexity of the behaviour of the system may provide information that cannot be obtained by a conventional, reductionist approach.

Allosteric Regulation↗

PubMed2002

Appraisal of SIMS applicability to boron studies in plants.

In the search for a new methodological approach applicable to the determination of the still poorly known primary role of boron in plant physiology, we have undertaken to appraise the potential of the SIMS method for the analytical imaging of the boron isotopes, (10)B and (11)B, at physiological concentrations in plants. With our own, CAMECA IMS4F SIMS ion analyser, and using O(2)(+) as primary ions for the detection of B(+) (plus (12)C(+) and (40)Ca(+)) secondary ions, we have been able to map quantitatively the two boron isotopes in control and boron-enriched plants, to evaluate boron concentrations at the level of individual cells and to determine boron isotopic ratios. This provides the opportunity to carry out the simultaneous labeling and imaging of boron, using enrichment with the stable isotopes, (10)B and (11)B. The method has also the potential for the simultaneous, quantitative detection of the boron isotopes and of the borate-binding sites in plant cells.

Boron↗

PubMed2002

Quasi-periodic behaviour in a model for the lithium-induced, electrical oscillations of frog skin.

The fact that oscillations can be induced in studies of the maintenance of the electrical potential of frog skin by addition of lithium allowed evaluation of several parameters fundamental to the functioning of the system in vivo (e.g. relative volumes of internal compartments, characteristic times of ionic exchanges between compartments). A realistic model was thus proposed under the form of a set of ordinary differential equations. In the past, numerical simulations using such a model reproduced the periodic experimental oscillations and was able to provide an explanation for the global synchronised oscillations of the whole skin. In that paper, new numerical simulations reproduce the non-periodic oscillations which were observed two decades ago, but not reproduced by the model. Moreover, the dynamical process under which all the local oscillators are synchronised is explained in terms of a tangent bifurcation.

Adaptation, Physiological↗

PubMed2002

Hyperstructures, genome analysis and I-cells.

New concepts may prove necessary to profit from the avalanche of sequence data on the genome, transcriptome, proteome and interactome and to relate this information to cell physiology. Here, we focus on the concept of large activity-based structures, or hyperstructures, in which a variety of types of molecules are brought together to perform a function. We review the evidence for the existence of hyperstructures responsible for the initiation of DNA replication, the sequestration of newly replicated origins of replication, cell division and for metabolism. The processes responsible for hyperstructure formation include changes in enzyme affinities due to metabolite-induction, lipid-protein affinities, elevated local concentrations of proteins and their binding sites on DNA and RNA, and transertion. Experimental techniques exist that can be used to study hyperstructures and we review some of the ones less familiar to biologists. Finally, we speculate on how a variety of in silico approaches involving cellular automata and multi-agent systems could be combined to develop new concepts in the form of an Integrated cell (I-cell) which would undergo selection for growth and survival in a world of artificial microbiology.

Algorithms↗