Determination of DNA persistence length by cryo-electron microscopy. Separation of the static and dynamic contributions to the apparent persistence length of DNA.

Axial deflection of DNA molecules in solution results from thermal motion and intrinsic curvature related to the DNA sequence. In order to measure directly the contribution of thermal motion we constructed intrinsically straight DNA molecules and measured their persistence length by cryo-electron microscopy. The persistence length of such intrinsically straight DNA molecules suspended in thin layers of cryo-vitrified solutions is about 80 nm. In order to test our experimental approach, we measured the apparent persistence length of DNA molecules with natural "random" sequences. The result of about 45 nm is consistent with the generally accepted value of the apparent persistence length of natural DNA sequences. By comparing the apparent persistence length to intrinsically straight DNA with that of natural DNA, it is possible to determine both the dynamic and the static contributions to the apparent persistence length.

Electron cryo-microscopy reveals mechanism of action of propranolol on artificial membranes.

The pharmacological activity of several amphiphilic drugs is often related to their ability to interact with biological membranes. Propranolol is an efficient multidrug resistance (MDR) modulator; it is a nonselective beta-blocker and is thought to reduce hypertension by decreasing the cardiac frequency and thus blood pressure. It is used in drug delivery studies in order to treat systemic hypertension. We are interested in the interaction of propranolol with artificial membranes, as liposomes of controllable size are used as biocompatible and protective structures to encapsulate labile molecules, such as proteins, nucleic acids or drugs, for pharmaceutical, cosmetic or chemical applications. We present here a study of the interaction of propranolol, a cationic surfactant, with pure egg phosphatidylcholine (EPC) vesicles. The gradual transition from liposome to micelle of EPC vesicles in the presence of propranolol was monitored by time-resolved electron cryo-microscopy (cryo-EM) under different experimental conditions. The liposome-drug interaction was studied with varying drug/lipid (D/L) ratios and different stages were captured by direct thin-film vitrification. The time-series cryo-EM data clearly illustrate the mechanism of action of propranolol on the liposome structure: the drug disrupts the lipid bilayer by perturbing the local organization of the phospholipids. This is followed by the formation of thread-like micelles, also called worm-like micelles (WLM), and ends with the formation of spherical (globular) micelles. The overall reaction is slow, with the process taking almost two hours to be completed. The effect of a monovalent salt was also investigated by repeating the lipid-surfactant interaction experiments in the presence of KCl as an additive to the lipid/drug suspension. When KCl was added in the presence of propranolol the overall reaction was the same but with slower kinetics, suggesting that this monovalent salt affects the general lipid-to-micelle transition by stabilizing the membrane, presumably by binding to the carbonyl chains of the phosphatidylcholine.

Why do scientists migrate? A diffusion model

The article is intended to improve our understanding of the reasons underlying the intellectual migration of scientists from well-known cognitive domains to nascent scientific fields. To that purpose we present, first, a number of findings from the sociology of science that give different insights about this phenomenon. We then attempt to bring some of these insights together under the conceptual roof of an actor-based approach linking expected utility and diffusion theory. Intellectual migration is regarded as the rational choice of scientists who decide under uncertainty and on the base of a number of decision-making variables, which define probabilities, costs, and benefits of the migration.

Struggles for meaning and struggles for control: the diffusion of bandwagon technology in two institutional environments

In this thesis, I examine the diffusion process for a complex medical technology, the PET scanner, in two different health care systems, one of which is more market-oriented (Switzerland) and the other more centrally managed by a public agency (Quebec). The research draws on institutional and socio-political theories of the diffusion of innovations to examine how institutional contexts affect processes of diffusion. I find that diffusion proceeds more rapidly in Switzerland than in Quebec, but that processes in both jurisdictions are characterized by intense struggles among providers and between providers and public agencies. I show that the institutional environment influences these processes by determining the patterns of material resources and authority available to actors in their struggles to strategically control the technology, and by constituting the discursive resources or institutional logics on which actors may legitimately draw in their struggles to give meaning to the technology in line with their interests and values. This thesis illustrates how institutional structures and meanings manifest themselves in the context of specific decisions within an organizational field, and reveals the ways in which governance structures may be contested and realigned when they conflict with interests that are legitimized by dominant institutional logics. It is argued that this form of contestation and readjustment at the margins constitutes one mechanism by which institutional frameworks are tested, stretched and reproduced or redefined.

Immunocytochemical localization of the glucose transporter 2 (GLUT2) in the adult rat brain. II. Electron microscopic study.

Following a former immunohistochemical study in the rat brain [Arluison, M., Quignon, M., Nguyen, P., Thorens, B., Leloup, C., Penicaud, L. Distribution and anatomical localization of the glucose transporter 2 (GLUT2) in the adult rat brain. I. Immunohistochemical study. J. Chem. Neuroanat., in press], we have analyzed the ultrastructural localization of GLUT2 in representative and/or critical areas of the forebrain and hindbrain. In agreement with previous results, we observe few oligodendrocyte and astrocyte cell bodies discretely labeled for GLUT2 in large myelinated fibre bundles and most brain areas examined, whereas the reactive glial processes are more numerous and often localized in the vicinity of nerve terminals and/or dendrites or dendritic spines forming synaptic contacts. Only some of them appear closely bound to unlabeled nerve cell bodies and dendrites. Furthermore, the nerve cell bodies prominently immunostained for GLUT2 are scarce in the brain nuclei examined, whereas the labeled dendrites and dendritic spines are relatively numerous and frequently engaged in synaptic junctions. In conformity with the observation of GLUT2-immunoreactive rings at the periphery of numerous nerve cell bodies in various brain areas (see previous paper), we report here that some neuronal perikarya of the dorsal endopiriform nucleus/perirhinal cortex exhibit some patches of immunostaining just below the plasma membrane. However, the presence of many GLUT2-immunoreactive nerve terminals and/or astrocyte processes, some of them being occasionally attached to nerve cell bodies and dendrites, could also explain the pericellular labeling observed. The results here reported support the idea that GLUT2 may be expressed by some cerebral neurones possibly involved in glucose sensing, as previously discussed. However, it is also possible that this transporter participate in the regulation of neurotransmitter release and, perhaps, in the release of glucose by glial cells.

Representing diffusion MRI in 5D for segmentation of white matter tracts with a level set method.

We present a method for segmenting white matter tracts from high angular resolution diffusion MR. images by representing the data in a 5 dimensional space of position and orientation. Whereas crossing fiber tracts cannot be separated in 3D position space, they clearly disentangle in 5D position-orientation space. The segmentation is done using a 5D level set method applied to hyper-surfaces evolving in 5D position-orientation space. In this paper we present a methodology for constructing the position-orientation space. We then show how to implement the standard level set method in such a non-Euclidean high dimensional space. The level set theory is basically defined for N-dimensions but there are several practical implementation details to consider, such as mean curvature. Finally, we will show results from a synthetic model and a few preliminary results on real data of a human brain acquired by high angular resolution diffusion MRI.

Variabilité intra/interobservateur du coefficient de diffusion apparente dans les lésions hépatiques malignes traitées

Objectifs : Le coefficient de diffusion apparente (ADC) est utilisé pour le suivi des lésions hépatiques malignes traitées. Cependant, l'ADC est généralement mesuré dans la lésion entière, alors que cela devrait être réalisé dans la zone la plus restreinte (ZLPR), cette dernière représentant potentiellement du résidu tumoral. Notre objectif était d'évaluer la variabilité inter/intraobservateur de l'ADC dans la tumeur entière et dans la ZLPR. Matériels et méthodes : Quarante patients traités par chimioembolisation ou radiofréquence ont été évalués. Après consensus, deux lecteurs ont indépendamment mesuré l'ADC de la lésion entière et de la ZLPR. Les mêmes mesures ont été répétées deux semaines plus tard. Le test de Spearman et la méthode de Bland-Altman ont été utilisées. Résultats : La corrélation interobservateur de l'ADC dans la lésion entière et dans la ZLPR était de 0,962 et de 0,884. La corrélation intraobservateur était de 0,992 et de 0,979, respectivement. Les limites de variabilité interobservateur (mm2/sec*10 - 3) étaient entre -0,25/+0,28 dans la lésion entière et entre -0,51/+0,46 dans la ZLPR. Les limites de variabilité intraobservateur étaient respectivement : -0,25/+0,24 et -0,43/+0,47. Conclusion : La corrélation inter/intraobservateur dans les mesures d'ADC est bonne. Toutefois, une variabilité limitée existe et doit être considérée lors de l'interprétation des valeurs d'ADC des tumeurs hépatiques.

Cutting artefacts and cutting process in vitreous sections for cryo-electron microscopy.

Cryo-electron microscopy of vitreous sections (CEMOVIS) has recently been shown to provide images of biological specimens with unprecedented quality and resolution. Cutting the sections remains however the major difficulty. Here, we examine the parameters influencing the quality of the sections and analyse the resulting artefacts. They are in particular: knife marks, compression, crevasses, and chatter. We propose a model taking into account the interplay between viscous flow and fracture. We confirm that crevasses are formed on only one side of the section, and define conditions by which they can be avoided. Chatter is an effect of irregular compression due to friction of the section of the knife edge and conditions to prevent this are also explored. In absence of crevasses and chatter, the bulk of the section is compressed approximately homogeneously. Within this approximation, it is possible to correct for compression by a simple linear transformation for the bulk of the section. A research program is proposed to test and refine our understanding of the sectioning process.

Visibilité et diffusion de la recherche qualitative en psychologie à partir des sites Internet : une étude préliminaire.

Les méthodes de recherche qualitative en psychologie connaissent une diffusion de plus en plus importante, soutenue notamment par le développement phénoménal des réseaux de l'Internet. Nos investigations mettent en lumière diverses catégories de sites liés à la recherche qualitative, pour la plupart issues de grandes institutions nord-américaines. Cependant, la présence d'autant d'informa- tions sur le Web pose la question de la crédibilité des sites disponibles et de leur visibilité : en effet, si une certaine qualité peut être attendue des sites émanant d'institutions officielles, en termes de qualité d'information et de facilité d'accès, en revanche, il s'avère plus difficile d'évaluer certaines pages se réclamant pourtant de la recherche scientifique. Dans cet article, nous présentons quelques sites et liens Internet susceptibles de séduire les chercheurs intéressés par les méthodes qualitatives en psychologie.

The Institutional Foundations of Regulatory Capitalism: The Diffusion of Independent Regulatory Agencies in Western Europe

This article studies the diffusion of the main institutional feature of regulatory capitalism, namely, independent regulatory agencies. While only a few such authorities existed in Europe in the early 1980s, by the end of the twentieth century they had spread impressively across countries and sectors. The analysis finds that three classes of factors (bottom-up, top-down, and horizontal) explain this trend. First, the establishment of independent regulatory agencies was an attempt to improve credible commitment capacity when liberalizing and privatizing utilities and to alleviate the political uncertainty problem, namely, the risk to a government that its policies will be changed when it loses power. Second, Europeanization favored the creation of independent regulators. Third, individual decisions were interdependent, as governments were influenced by the decisions of others in an emulation process where the symbolic properties of independent regulators mattered more than the functions they performed.

Granular layer in the periplasmic space of gram-positive bacteria and fine structures of Enterococcus gallinarum and Streptococcus gordonii septa revealed by cryo-electron microscopy of vitreous sections.

High-resolution structural information on optimally preserved bacterial cells can be obtained with cryo-electron microscopy of vitreous sections. With the help of this technique, the existence of a periplasmic space between the plasma membrane and the thick peptidoglycan layer of the gram-positive bacteria Bacillus subtilis and Staphylococcus aureus was recently shown. This raises questions about the mode of polymerization of peptidoglycan. In the present study, we report the structure of the cell envelope of three gram-positive bacteria (B. subtilis, Streptococcus gordonii, and Enterococcus gallinarum). In the three cases, a previously undescribed granular layer adjacent to the plasma membrane is found in the periplasmic space. In order to better understand how nascent peptidoglycan is incorporated into the mature peptidoglycan, we investigated cellular regions known to represent the sites of cell wall production. Each of these sites possesses a specific structure. We propose a hypothetic model of peptidoglycan polymerization that accommodates these differences: peptidoglycan precursors could be exported from the cytoplasm to the periplasmic space, where they could diffuse until they would interact with the interface between the granular layer and the thick peptidoglycan layer. They could then polymerize with mature peptidoglycan. We report cytoplasmic structures at the E. gallinarum septum that could be interpreted as cytoskeletal elements driving cell division (FtsZ ring). Although immunoelectron microscopy and fluorescence microscopy studies have demonstrated the septal and cytoplasmic localization of FtsZ, direct visualization of in situ FtsZ filaments has not been obtained in any electron microscopy study of fixed and dehydrated bacteria.