Biomedical subjects

B Senger

Publications and source records attributed to B Senger.

At least 19 recordsLinked to original sources

PubMed2003

Modeling of the detachment of a molecule from a surface: illustration of the "Bell-Evans effect".

This article deals with the modeling of the detachment of a molecule initially adsorbed on a surface and submitted to an external force whose strength increases with time. By means of an atomic force microscope (AFM), it is possible to measure the force when the molecule separates from the substrate. However, it is known that this force depends to a large extend on the rate at which the pulling force is applied ("Bell-Evans effect"). Two models are described to illustrate this behavior. First, a random walk approach is suggested to reveal the fundamental principle of the escape over a time-dependent energy barrier. Second, a multi bead-and-spring model is proposed to mimic the AFM experiment and numerical simulations, based on Brownian dynamics, are performed.

Binding Sites↗

PubMed2002

Clinical relevance of a simple fragmentation model to evaluate human masticatory performance.

To explain the breakage process of food particles in human mastication, we propose a simple fragmentation model. To verify its clinical accuracy, a mechanical test with a sieving method and a natural test food (coffee) was performed on three groups of subjects. The values obtained permitted to demonstrate that the model could predict, at least qualitatively, the general trend of the experimental data, even for a very small group (two subjects). It also shows that the choice of the yielding factors used to calculate the masticatory index (MI) is relevant.

Coffee↗

PubMed2001

Yeast cytoplasmic and mitochondrial methionyl-tRNA synthetases: two structural frameworks for identical functions.

The yeast Saccharomyces cerevisiae possesses two methionyl-tRNA synthetases (MetRS), one in the cytoplasm and the other in mitochondria. The cytoplasmic MetRS has a zinc-finger motif of the type Cys-X(2)-Cys-X(9)-Cys-X(2)-Cys in an insertion domain that divides the nucleotide-binding fold into two halves, whereas no such motif is present in the mitochondrial MetRS. Here, we show that tightly bound zinc atom is present in the cytoplasmic MetRS but not in the mitochondrial MetRS. To test whether the presence of a zinc-binding site is required for cytoplasmic functions of MetRS, we constructed a yeast strain in which cytoplasmic MetRS gene was inactivated and the mitochondrial MetRS gene was expressed in the cytoplasm. Provided that methionine-accepting tRNA is overexpressed, this strain was viable, indicating that mitochondrial MetRS was able to aminoacylate tRNA(Met) in the cytoplasm. Site-directed mutagenesis demonstrated that the zinc domain was required for the stability and consequently for the activity of cytoplasmic MetRS. Mitochondrial MetRS, like cytoplasmic MetRS, supported homocysteine editing in vivo in the yeast cytoplasm. Both MetRSs catalyzed homocysteine editing and aminoacylation of coenzyme A in vitro. Thus, identical synthetic and editing functions can be carried out in different structural frameworks of cytoplasmic and mitochondrial MetRSs.

Acylation↗

PubMed2001

Semi-automatized processing of AFM force-spectroscopy data.

Atomic force microscopy operated in the force-spectroscopy mode is now a widespread technique, often used to investigate ligand-receptor interactions with the goal of measuring forces at the individual molecule level. However, in an experiment, the simultaneous interaction of several ligand/receptor pairs cannot be excluded. This may produce complicated force curves, although unambiguous ruptures are sometimes observed. In the case of the non-specific adhesion of molecules, such as fibrinogen, to a surface, it is usually difficult to identify the real events on the force curves. This can render the application of fixed rules uneasy and in addition can introduce some degree of arbitrariness if the analysis has to be performed by hand. In the present paper a computer algorithm, aimed at speeding up the processing, and at applying selection rules in a reproducible manner, is proposed. It is applied to force recordings performed at various retraction velocities, thus various loading rates. The influence on the evaluation of the rupture forces of the different parameters that can be set by the operator is discussed.

Adsorption↗

PubMed2001

The nucle(ol)ar Tif6p and Efl1p are required for a late cytoplasmic step of ribosome synthesis.

Deletion of elongation factor-like 1 (Efl1p), a cytoplasmic GTPase homologous to the ribosomal translocases EF-G/EF-2, results in nucle(ol)ar pre-rRNA processing and pre-60S subunits export defects. Efl1p interacts genetically with Tif6p, a nucle(ol)ar protein stably associated with pre-60S subunits and required for their synthesis and nuclear exit. In the absence of Efl1p, 50% of Tif6p is relocated to the cytoplasm. In vitro, the GTPase activity of Efl1p is stimulated by 60S, and Efl1p promotes the dissociation of Tif6p-60S complexes. We propose that Tif6p binds to the pre-60S subunits in the nucle(ol)us and escorts them to the cytoplasm where the GTPase activity of Efl1p triggers a late structural rearrangement, which facilitates the release of Tif6p and its recycling to the nucle(ol)us.

Biological Transport↗

PubMed2001

Coffee beans as a natural test food for the evaluation of the masticatory efficiency.

UNLABELLED: A lot of test foods have been used during this century to evaluate the masticatory ability of human subjects. Nevertheless, none has been universally admitted. If the test food by itself is of importance, attention should also be paid to its behaviour during the chewing test procedure. Therefore, we analysed step by step coffee beans through the processing of the chewing test and a dry sieving METHOD: The development of a compression test and a computer simulation have shown that groups of 11 coffee beans give satisfactory results and deserve to be used in mastication studies.

Coffee↗

PubMed2000

Unbinding process of adsorbed proteins under external stress studied by atomic force microscopy spectroscopy.

We report the study of the dynamics of the unbinding process under a force load f of adsorbed proteins (fibrinogen) on a solid surface (hydrophilic silica) by means of atomic force microscopy spectroscopy. By varying the loading rate r(f), defined by f = r(f) t, t being the time, we find that, as for specific interactions, the mean rupture force increases with r(f). This unbinding process is analyzed in the framework of the widely used Bell model. The typical dissociation rate at zero force entering in the model lies between 0. 02 and 0.6 s(-1). Each measured rupture is characterized by a force f(0), which appears to be quantized in integer multiples of 180-200 pN.

Fibrinogen↗

PubMed2000

Protein interactions with polyelectrolyte multilayers: interactions between human serum albumin and polystyrene sulfonate/polyallylamine multilayers.

The interactions between polystyrenesulfonate (PSS)/polyallylamine (PAH) multilayers with human serum albumin (HSA) were investigated by means of scanning angle reflectometry (SAR). We find that albumin adsorbs both on multilayers terminating with PSS (negatively charged) or PAH (positively charged) polyelectrolytes. On films terminating with PSS only, an albumin equivalent monolayer is found whereas when PAH constitutes the outer layer, albumin interacts with the multilayer in such a way as to form a protein film that extends over thicknesses that can be as high as four times the largest dimension of the native albumin molecule. Once the protein film is formed, it is found that when the albumin solution is replaced by a pure buffer solution of same ionic strength as the adsorption solution almost no desorption takes place. On the other hand, when a buffer solution of higher ionic strength is brought in contact with the albumin film, a significant amount of adsorbed proteins is released. One also observes that, for albumin solutions of a given protein concentration, the adsorbed protein amount depends on the ionic strength of the adsorption solution. On surfaces terminating with PAH, the adsorbed protein amount first increases rapidly but passes through a maximum and decreases with the ionic strength. The ionic strength corresponding to the maximum of the adsorbed albumin amount itself depends on the albumin concentration. On the other hand, on films terminating with PSS the adsorbed amount increases with the salt concentration before leveling-off. These results show that the underlying complexity of concentration and pH dependent adsorption/desorption equilibria often simply termed "protein adsorption" is the result of antagonist competing interactions that are mainly of electrostatic origin. We also propose two microscopic models, that are compatible with our experimental observations.

Adsorption↗

PubMed2000

[Leukocyte adhesion on a fibrinogen-coated surface under static conditions: experimentation and creation of a model].

The adhesion of polymorphonuclear leukocytes (PMNs) on the vascular endothelium is a complex process that occurs during different biological and pathological events and involves numerous molecules. The adhesion cascade is induced after PMN stimulation by various molecular or cellular signals. Fibrinogen is one of the substrates for CD11b/CD18 B2-integrins expressed at the PMN surface; fibrinogen-neutrophil binding is induced by inflammatory reactions. In order to understand this process, we have carried out studies on the basis of preliminary experiments on red blood cells and synthetic particles. The modelization of quiescent PMNs adhesion on a fibrinogen substrate was investigated with a sedimentation cell chamber. Two different physiological conditions were tested: the activated state of PMN by a synthetic pro-inflammatory activator (FMLP). The activated state of PMNs was both quantified by flow cytometry and controlled by fluorescence microscopy. The results suggest that quiescent neutrophils deposit in accordance with the ballistic deposition model. This random adsorption model differs from random sequential adsorption (RSA) in that the cells arriving at the surface are able to roll along cells previously adsorbed introducing the notion of gravitational attraction of cells. The preliminary results obtained with stimulated PMN do not allow to choose between one of this two deposition models. Nevertheless, the qualitative and quantitative effects of FMLP on neutrophils were demonstrated by modifications of adhesion molecules expression.

Antigens, CD↗

PubMed1999

Extended random sequential adsorption model of irreversible deposition processes: from simulations to experiments.

An experimental study of the irreversible deposition of colloidal particles of various radii R on a solid surface is presented over a wide range of the Péclet number, Pe, or reduced radius R* (Pe = R*(4)). The experimental data are analyzed by means of a new generalized random sequential adsorption model that takes explicitly the diffusion of the particles during the deposition into account. It allows description of the continuous transition from a random sequential adsorption-like to a ballistic-like deposition behavior. It depends on three parameters: d(s), related to the diffusion of the particles before adhesion; n(s), related to the number of allowed adhesion trials of a particle; and R(e), representing the effective particle radius. The model allows accounting for all of the experimental observations relative to the radial distribution functions and the number density fluctuations over the whole coverage range and all investigated values of R*. In addition, it is found that d(s)/R is proportional to R*(-2) as expected for a diffusional process. Moreover, the parameters d(s) and n(s) appear to be connected through the empirical relation (d(s)/R)n(s)(2/3) = C, where C is found to be of the order of 50. This unique statistical model allows an accurate description of the irreversible deposition process, whatever the influence of gravity with respect to diffusion.

Journal Article↗

PubMed1999

Adhesion Probability of Colloidal Particles with Repulsive Soft Interaction.

Computer simulations of the irreversible adhesion of charged colloidal particles at a solid/liquid interface are performed to determine whether the distribution of particles in the vicinity of a preadsorbed (also charged) one follows the Boltzmann law applied to an a priori uniform adhesion probability, as first assumed by Adamczyk et al. (J. Colloid Interface Sci. 140, 123 (1990)). If true, this would indicate that the whole information on the deposition process is contained in the potential energy distribution on the adsorbing surface. In general, diffusion in a field of force and the irreversibility of the process induce significant deviations from the Boltzmann-weighted uniform adhesion density. Nevertheless, it is shown that for particles characterized by a small gravitational energy this procedure leads to a reasonable first approximation of the distribution of the particles over the adsorbing surface. This observation thus demonstrates the validity of Adamczyk's assumption and extends its range of applicability to the case of a weak gravitational field. Copyright 1999 Academic Press.

Journal Article↗

PubMed1999

Dynamic and cyclic fatigue of engine-driven rotary nickel-titanium endodontic instruments.

The absence of adequate testing standards for engine-driven nickel-titanium (NiTi) instruments necessitates further study of these instruments in all areas. This study examined three groups of engine-driven rotary NiTi endodontic instruments (Profile, Hero, and Quantec) and assessed the times for dynamic fracture in relation to the radius of curvature to which the instruments were subjected during preparation, with the instrument diameter determined by size and taper and the mode by which the fracture occurred. Ten instruments were randomly selected representing each size and taper for each group and for each radius of curvature: 600 in total. The instruments were rotated at 350 rpm and introduced into a tempered steel curve that simulated a canal. Two radii of curvature of canals were used: 5 and 10 mm. Time at fracture was noted for all files, and the fracture faces of each file were analyzed with scanning electron microscopy. Radius of curvature was found to be the most significant factor in determining the fatigue resistance of the files. As radius of curvature decreased, fracture time decreased. Taper of files was found to be significant in determining fracture time. As diameter increased, fracture time decreased. In all cases, fracture was found to be of a ductile nature, thus implicating cyclic fatigue as a major cause of failure and necessitating further analyses and setting of standards in this area.

Dental High-Speed Technique↗

PubMed1999

Extended (n/v)-Stillinger cluster for use in the theory of homogeneous nucleation.

In this paper we develop a theory for an extended version of the (n/v)-Stillinger cluster that has been used in nucleation theory, where n means the number of particles constituting the cluster characterized by the volume v. The "extended cluster" incorporates some of the surrounding supersaturated vapor. This cluster, although requiring more extensive simulation than the original (n/v)-Stillinger cluster, is almost devoid of approximation. It maintains the non-ad-hoc nature of the original (n/v)-Stillinger cluster implicitly. The theory of the cluster is also applicable to clusters which avoid redundancy by some other means than the so-called "connectivity requirement." Simulation of the extended cluster is now being implemented and will be used in the theory of the homogeneous nucleation rate.

Journal Article↗

PubMed1999

Initial features of the inner dental epithelium histo-morphogenesis in the first lower molar in mouse.

First lower molar development in the mouse was investigated from the cap to early bell stage using histology, morphometry, TEM and 3D reconstructions. This period was characterized by the histogenesis of the enamel organ (EO), folding of the epithelio-mesenchymal junction and growth of the tooth. The histogenesis of the EO and appearance of the enamel knot (EK) were initiated at the early cap stage (ED14). From ED14 to ED15, the anterior and posterior extension of the EK was very prominent whilst the length of the enamel organ did not substantially change. The EK appeared as a dynamic and transitory histological structure including dying and replacement cells. At ED16, the folding of the IDE, which extended over the anterior two thirds of the molar, was the first sign of cuspidogenesis. It was accompanied by a local remodeling of the basement membrane (BM): IDE cells involved in this folding transitorily lost contact with the BM which formed a loop in the mesenchyme. During this period, the growth of the lower M1 along the antero-posterior axis was restricted to the posterior part of the molar. Histogenesis occurred in the whole EO, whilst initial cuspidogenesis was limited to the anterior part of the tooth. Distinct cell populations were thus involved in different contemporary processes leading to changes in the cell density in the mesenchyme, in the mitotic activity, in cell-shape, and cell-matrix interactions in the IDE, and remodeling of the BM where both epithelium and mesenchyme might participate.

Animals↗

PubMed1999

Mechanical effects and volatile sulfur compound-reducing effects of chewing gums: comparison between test and base gums and a control group.

OBJECTIVE: Chewing gum may act as a masking or a therapeutic agent against the different chemical compounds that are responsible for oral malodor. An open-label exploratory study investigated the effect of mastication and aromatic components of chewing gum on reducing oral volatile sulfur compounds. METHOD AND MATERIALS: Twelve dental students (5 males and 7 females) acted as their own controls. Toothbrushing stopped 12 hours before observations. Measurements included organoleptic and volatile sulfur compound scores and the pH of the anterior and posterior zones of the dorsal tongue. Measurements were made at 9 AM and 12 PM on 1 day for 3 successive weeks; week 1, no gum (control); week 2, test gum; week 3, unsweetened gum base. This open-label study was then completed by an observer-blind study, according to the same schedule; the recorded measurement was the plaque index. RESULTS: The pH, volatile sulfur compounds, and organoleptic scores were similar for all groups. The pH was more basic in the posterior part than in the anterior zone of the dorsal tongue, irrespective of time and presence or absence of chewing gum. In addition, the volatile sulfur compound score rose transiently immediately after the test gum, and the organoleptic score fell in the first hour only after the test gum. The two chewing gum groups seemed to have a greater reduction in plaque index than did the control (no gum) group. CONCLUSION: Chewing gum may have a valuable mechanical role in cleaning dental surfaces, and the test gum may temporarily control bad breath. After 3 hours, similar volatile sulfur compound scores were observed for subjects who chewed either test or unsweetened gum base and control subjects.

Analysis of Variance↗

PubMed1999

Modelisation of leukocyte adhesion on a fibrinogen coated surface in static conditions.

The adhesion of polymorphonuclear leukocytes (PMNs) on the vascular endothelium is a complex process that occurs during biological and pathological events and involves a large family of molecules. This phenomenom could be approached by a modelisation study of the adhesion of PMNs on a biological substrate, fibrinogen. Two different physiological conditions were tested such as the activated state of PMNs with a synthetic pro-inflammatory activator (N-Formyl-Methionyl-Leucyl-Phenylalanine, FMLP). The activated state of PMNs was both quantified by flow cytometry and controlled by fluorescence microscopy. The results suggest that quiescent PMNs deposit in accordance with the ballistic deposition model. The preliminary results obtained with FMLP-stimulated PMNs show a different deposit process compared to quiescent PMNs but do not allow to determine exactly a deposition model.

Activated-Leukocyte Cell Adhesion Molecule↗

PubMed1998

Mtr10p functions as a nuclear import receptor for the mRNA-binding protein Npl3p.

MTR10, previously shown to be involved in mRNA export, was found in a synthetic lethal relationship with nucleoporin NUP85. Green fluorescent protein (GFP)-tagged Mtr10p localizes preferentially inside the nucleus, but a nuclear pore and cytoplasmic distribution is also evident. Purified Mtr10p forms a complex with Npl3p, an RNA-binding protein that shuttles in and out of the nucleus. In mtr10 mutants, nuclear uptake of Npl3p is strongly impaired at the restrictive temperature, while import of a classic nuclear localization signal (NLS)-containing protein is not. Accordingly, the NLS within Npl3p is extended and consists of the RGG box plus a short and non-repetitive C-terminal tail. Mtr10p interacts in vitro with Gsp1p-GTP, but with low affinity. Interestingly, Npl3p dissociates from Mtr10p only by incubation with Ran-GTP plus RNA. This suggests that Npl3p follows a distinct nuclear import pathway and that intranuclear release from its specific import receptor Mtr10p requires the cooperative action of both Ran-GTP and newly synthesized mRNA.

Amino Acid Sequence↗

PubMed1998

Structural requirements for enzymatic formation of threonylcarbamoyladenosine (t6A) in tRNA: an in vivo study with Xenopus laevis oocytes.

We have investigated the specificity of the eukaryotic enzymatic machinery that transforms adenosine at position 37 (3' adjacent to anticodon) of several tRNAs into threonylcarbamoyladenosine (t6A37). To this end, 28 variants of yeast initiator tRNAMet and yeast tRNAVal, devoid of modified nucleotide, were produced by in vitro transcription with T7 polymerase of the corresponding synthetic tRNA genes and microinjected into the cytoplasm of Xenopus laevis oocytes. Threonylcarbamoyl incorporation was analyzed in tRNA transcripts mutated in the anticodon loop by substitution, deletion, or Insertion of nucleotides, or in the overall 3D structure of the tRNA by altering critical tertiary interactions. Specifically, we tested the effects of altering ribonucleotides in the anticodon loop, changes of the loop size, perturbations of the overall tRNA 3D structure due to mutations disruptive of the tertiary base pairs, and truncated tRNAs. The results indicate that, in addition to the targeted A37, only U36 was absolutely required. However, A38 in the anticodon loop considerably facilitates the quantitative conversion of A37 into t6A37 catalyzed by the enzymes present in X. laevis. The anticodon positions 34 and 35 were absolutely "neutral" and can accept any of the four canonical nucleotides A, U, C, or G. The anticodon loop size may vary from six to eight nucleotides, and the anticodon stem may have one mismatch pair of the type AxC or GxU at location 30-40 without affecting the efficiency of t6A37 formation and still t6A37 is efficiently formed. Although threonylcarbamoylation of A37 occurred with tRNA having limited perturbations of 3D structure, the overall L-shaped architecture of the tRNA substrate was required for efficient enzymatic conversion of A37 to t6A37. These results favor the idea that unique enzymatic machinery located in the oocyte cytoplasm catalyzes the formation of t6A37 in all U36A37-containing tRNAs (anticodon NNU). Microinjection of the yeast tRNAMeti into the cytoplasm of X. laevis oocytes also revealed the enzymatic activities for several other nucleotide modifications, respectively m1Gg, m2G10, m(2)2G26, m7G46, D47, m5C48/49, and m1A58.

Adenosine↗