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Biomedical subjects

M Bier

Publications and source records attributed to M Bier.

At least 19 recordsLinked to original sources
PubMed2000

Association of yeast RNA polymerase I with a nucleolar substructure active in rRNA synthesis and processing.

A novel ribonucleoprotein complex enriched in nucleolar proteins was purified from yeast extracts and constituents were identified by mass spectrometry. When isolated from rapidly growing cells, the assembly contained ribonucleic acid (RNA) polymerase (pol) I, and some of its transcription factors like TATA-binding protein (TBP), Rrn3p, Rrn5p, Rrn7p, and Reb1p along with rRNA processing factors, like Nop1p, Cbf5p, Nhp2p, and Rrp5p. The small nucleolar RNAs (snoRNAs) U3, U14, and MRP were also found to be associated with the complex, which supports accurate transcription, termination, and pseudouridylation of rRNA. Formation of the complex did not depend on pol I, and the complex could efficiently recruit exogenous pol I into active ribosomal DNA (rDNA) transcription units. Visualization of the complex by electron microscopy and immunogold labeling revealed a characteristic cluster-forming network of nonuniform size containing nucleolar proteins like Nop1p and Fpr3p and attached pol I. Our results support the idea that a functional nucleolar subdomain formed independently of the state of rDNA transcription may serve as a scaffold for coordinated rRNA synthesis and processing.

Cell Nucleolus↗
PubMed2000

How yeast cells synchronize their glycolytic oscillations: a perturbation analytic treatment.

Of all the lifeforms that obtain their energy from glycolysis, yeast cells are among the most basic. Under certain conditions the concentrations of the glycolytic intermediates in yeast cells can oscillate. Individual yeast cells in a suspension can synchronize their oscillations to get in phase with each other. Although the glycolytic oscillations originate in the upper part of the glycolytic chain, the signaling agent in this synchronization appears to be acetaldehyde, a membrane-permeating metabolite at the bottom of the anaerobic part of the glycolytic chain. Here we address the issue of how a metabolite remote from the pacemaking origin of the oscillation may nevertheless control the synchronization. We present a quantitative model for glycolytic oscillations and their synchronization in terms of chemical kinetics. We show that, in essence, the common acetaldehyde concentration can be modeled as a small perturbation on the "pacemaker" whose effect on the period of the oscillations of cells in the same suspension is indeed such that a synchronization develops.

Glycolysis↗
PubMed2000

Nonlinearly coupled flows.

We study energy flows that are coupled at a higher than linear order. A number of examples are presented where a force brings about a flow in the perpendicular direction. In some cases the symmetry of the system is such that coupling can only take place at even orders. We apply the theory to recently proposed two-dimensional devices that separate colloidal particles by ratcheting the different particles in different directions.

Biophysics↗
PubMed1999

Kinetics of sealing for transient electropores in isolated mammalian skeletal muscle cells.

Permeabilization of the plasma membrane by electrical forces (electroporation) can be either transient or stable. Although the exact molecular mechanics have not yet been described, electroporation is believed to initiate primarily in the lipid bilayer. To better understand the kinetics of membrane permeabilization, we sought to determine the time constants for spontaneous transient pore sealing. By using isolated rat flexor digitorum brevis skeletal muscle cells and a two-compartment diffusion model, we found that pore sealing times (tau p) after transient electroporation were approximately 9 min. tau p was not significantly dependent on the imposed transmembrane potential. We also determined the transmembrane potential (delta Vm) thresholds necessary for transient and stable electroporation in the skeletal muscle cells. delta VmS ranging between 340 mV and 480 mV caused a transient influx of magnesium, indicating the existence of spontaneously sealing pores. An imposed delta Vm of 540 mV or greater led to complete equilibration of the intracellular and extracellular magnesium concentrations. This finding suggests that stable pores are created by the larger imposed transmembrane potentials. These results may be useful for understanding nerve and skeletal muscle injury after an electrical shock and for developing optimal strategies for accomplishing transient electroporation, particularly for gene transfection and cell transformation.

Animals↗
PubMed1999

Intrawell relaxation of overdamped Brownian particles.

We consider an overdamped Brownian particle in a well. When the particle escapes, it does so as an instanton, i.e., in one run and without dwelling anywhere on the way from the bottom of the well to the top of the barrier. For a sufficiently steep slope the instanton time equals the time it takes the particle to deterministically slide down the same slope. We show that the instanton time is also the relaxation time for the escape rate after the barrier changes shape.

Biophysical Phenomena↗
PubMed1998

Recycling isoelectric focusing and isotachophoresis.

Of all electrophoretic methods, isoelectric focusing offers the highest resolution and is best suited for preparative applications. Over the years, several instruments were developed for this purpose, all operating in free fluids, in the absence of gels or other supporting matrices. In such systems, the avoidance of gravity or electrically driven convections is essential. Successful stratagems for fluid control included rapid recycling or rotation, in combination with either fine porosity screens or narrow gaps between parallel plates. The most successful apparatus so far is the Rotofor, in which fluid is stabilized by combining horizontal rotation with fine porosity screen partitioning. Recycling isotachophoresis offers the potential of separating proteins at high concentration. A new concept of tangential electrophoresis is described. To optimize the use of these devices for protein separation, low molecular weight, biologically acceptable buffers of known composition are essential. The buffering system developed for this purpose comprises a series of binary buffers that cover the pH range in steps of 1 pH unit or less. The pH gradient can be custom-designed and is of remarkable stability in operation.

Buffers↗
PubMed1996

Control analysis of glycolytic oscillations.

The principles involved in the control of the frequency of sustained metabolic oscillations are developed in the context of glycolytic oscillations in Saccharomyces cerevisiae. To this purpose, an existing mathematical model that describes the experimentally obtained oscillations was simplified to a core model. Frequency, relative phase, average concentrations and amplitudes of the oscillations were well approximated by writing the two remaining metabolic variables of the core model (representing [ATP] and [hexose]) as harmonic functions of time and by requiring them to fulfill the differential equations. The extent to which an enzyme (-conglomerate) controls the frequency in a sustained oscillation is defined as the log-log derivative of that frequency with respect to enzyme activity. In both the full model and the core model this control of frequency and the control over the average concentrations proved to be distributed over the enzymes. We identified a summation theorem, stating that the sum of such control coefficients over all processes equals unity for frequency and zero for the average concentrations.

Catalysis↗
PubMed1996

The alpha 2 adrenoreceptor agonist clonidine suppresses seizures, whereas the alpha 2 adrenoreceptor antagonist idazoxan promotes seizures: pontine microinfusion studies of amygdala-kindled kittens.

This is the first report showing that microinfusion of alpha 2 adrenoreceptor agonists and antagonists into the vicinity of the locus ceruleus (LC) have contrasting effects on evoked amygdala-kindled seizure susceptibility. Microinfusion (1 microliter) of the alpha 2 agonist clonidine (CLON) and of the alpha 2 antagonist idazoxan (IDA) were made over 1 min through cannulae in the LC ipsilateral to the kindled amygdala in 6 kittens. Order of administered drugs (CLON vs. IDA) and dosages (n = 3 each) were partly counterbalanced. Focal and convulsive seizure thresholds were evaluated 10-12 min post-infusion and compared to thresholds obtained during two, interspersed control conditions (vehicle control = 1 microliter microinfusion of sterile saline; sham control = needle insertion only). CLON significantly elevated focal and generalized seizure thresholds, whereas IDA significantly reduced seizure thresholds when compared to controls. Magnitude of effects was dose-dependent. These findings confirm that norepinephrine (NE) is a potent antiepileptic agent. Results also suggest that pontine microinfusions could eventually provide an alternative treatment option for medically refractory limbic epilepsy.

Adrenergic alpha-2 Receptor Agonists↗
PubMed1996

Mechanochemical coupling of the motion of molecular motors to ATP hydrolysis.

The typical biochemical paradigm for coupling between hydrolysis of ATP and the performance of chemical or mechanical work involves a well-defined sequence of events (a kinetic mechanism) with a fixed stoichiometry between the number of ATP molecules hydrolyzed and the turnover of the output reaction. Recent experiments show, however, that such a deterministic picture of coupling may not be adequate to explain observed behavior of molecular motor proteins in the presence of applied forces. Here we present a general model in which the binding of ATP and release of ADP serve to modulate the binding energy of a motor protein as it travels along a biopolymer backbone. The mechanism is loosely coupled--the average number of ATPs hydrolyzed to cause a single step from one binding site to the next depends strongly on the magnitude of an applied force and on the effective viscous drag force. The statistical mechanical perspective described here offers insight into how local anisotrophy along the "track" for a molecular motor, combined with an energy-releasing chemical reaction to provide a source of nonequilibrium fluctuations, can lead to macroscopic motion.

Adenosine Triphosphate↗
PubMed1996

Alteration in sensory nerve function following electrical shock.

A study of the effects of electrical shock on peripheral nerve fibres is presented. Strength and duration of the applied shocks were similar to those encountered in a typical industrial electrical accident. The purpose of this study is: (i) to identify the electrophysiological and morphological change in nerve fibres after the application of electrical current shocks; (ii) to examine the ability of the peripheral nerve fibres to spontaneously regain function and; (iii) to demonstrate the usefulness of the sensory refractory spectrum as an additional technique in assessing the damage. Three groups of animals received twelve 4-ms electric field pulses of approximately 37 V/cm (n = 5), 75 V/cm (n = 9) and 150 V/cm (n = 6), respectively. Group 4 was a control group and received a direct application of 2 per cent lidocaine over the sciatic nerve for 30 min. Thermal effects of the shocks were negligible. The sensory refractory spectrum shows that electrical shock damage was mainly to the large, fast myelinated fibres and that higher field strengths do more damage. Also in a histological examination it was found that the more heavily shocked myelinated fibres had sustained more damage.

Action Potentials↗
PubMed1996

The alpha 2-adrenoreceptor agonist clonidine suppresses seizures, whereas the alpha 2-adrenoreceptor antagonist idazoxan promotes seizures in amygdala-kindled kittens: a comparison of amygdala and pontine microinfusion effects.

PURPOSE: We sought to determine whether local, in vivo microinfusion of an alpha 2-adrenoreceptor agonist and antagonist into either the amygdala or the pons (locus ceruleus, LC) would have contrasting effects on evoked amygdala-kindled seizure susceptibility. METHODS: The study population consisted of 6 amygdala-kindled kittens, each undergoing the same protocol, in which the amygdala microinfusion paradigm preceded the pontine microinfusion series. Microinfusions (1 microliter) of the alpha 2-agonist clonidine (CLON) and the alpha 2-antagonist idazoxan (IDA) were made over 1 min through cannulas adjacent to stimulating electrodes in the kindled amygdala or through cannulas adjacent to recording electrodes in the ipsilateral LC. Order of administered drugs (CLON vs. IDA) and dosages (n = 3 each) was partly counterbalanced. Focal and convulsive seizure thresholds were evaluated 10-12 min postinfusion and compared to thresholds obtained during two interspersed control conditions (vehicle control = 1 microliter microinfusion of sterile saline; sham control = needle insertion only). RESULTS: CLON significantly increased focal and generalized seizure thresholds, whereas IDA significantly reduced seizure thresholds when compared to controls. Magnitude of effects was dose dependent and more potent after pontine than amygdala microinfusion. CONCLUSIONS: Our results confirm and extent findings of previous researchers who used unlocalized in vivo manipulations to show that norepinephrine (NE) is a highly antiepileptic agent in the amygdala kindling preparation. With further investigation, the results may ultimately lead to development of microinfusion techniques as an alternative treatment option for limbic epilepsy.

Adrenergic alpha-Agonists↗
PubMed1996

Relation between rise times and amplitudes of GABAergic postsynaptic currents.

1. We recorded rise times and amplitudes of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) in melanotropes of Xenopus laevis. Average rise times did not vary with amplitude, but the rise times of larger IPSCs were less variable. A simple linear one-step Markov model for channel opening following the binding of a transmitter molecule can quantitatively account for the average rise time and its coefficient of variation as a function of amplitude. Our results indicate that the observed variations in the rise times are not due to variations in transmitter concentrations, but result from stochastic variations in the opening of the receptor channels.

Animals↗
PubMed1995

Paroxysmal microarousals in amygdala-kindled kittens: could they be subclinical seizures?

Amygdala-kindled kittens exhibit frequent epileptiform EEG transients, often in conjunction with phasic arousal events of sleep [k-complexes, pontogeniculo-occipital (PGO) waves, and/or sleep spindles]. In this study, paroxysmal microarousals occurred throughout the sleep-wake cycle after kindling, but were most frequent during seizure-prone states of slow-wave sleep (SWS) and the transition into rapid-eye-movement sleep (REM). Their incidence correlated with interictal sleep fragmentation as well as onset of spontaneous convulsions. Results could reflect transsynaptic kindling effects on brainstem and forebrain arousal mechanisms with which amygdala is reciprocally connected. Increased discharge rates of neural generators for normal EEG and behavioral arousal could disrupt sleep at some times and recruit epileptic neurons in the kindled focus to precipitate seizures at others. Alternatively, epileptiform EEG paroxysms were accompanied by subtle behavioral stereotypes (a head nod, limb elevation, eye twitch, lip smack, or a combination of these). Behavioral correlates were elements of partial kindled seizures, suggesting that paroxysmal microarousals may be subclinical seizures. Whether or not the microarousals are true seizures, our findings may link ictal onset and interictal sleep disorders to a subclinical paroxysmal arousal disorder and suggest a common epileptic mechanism.

Amygdala↗
PubMed1994

The alpha 2-agonist clonidine suppresses seizures, whereas the alpha 2-antagonist idazoxan promotes seizures--a microinfusion study in amygdala-kindled kittens.

This is the first report showing that local, in vivo microinfusion of alpha 2-adrenoreceptor agonists and antagonists have contrasting effects on amygdala-kindled seizure susceptibility. Microinfusions (1 microliter) of the alpha 2-agonist clonidine (CLON) and of the alpha 2-antagonist idazoxan (IDA) were made over 1 min through cannulae adjacent to stimulating electrodes in five amygdala-kindled kittens. Order of administered drugs (CLON vs. IDA) and dosages (n = 3 each) was partly counterbalanced. Focal and convulsive seizure thresholds were evaluated 10-12 min post-infusion and compared to thresholds obtained during two, interspersed control conditions (vehicle control: 1 microliter microinfusion of sterile saline; sham control: needle insertion only). CLON significantly elevated focal and generalized seizure thresholds, whereas IDA significantly reduced seizure thresholds when compared to controls. Magnitude of effects was dose-dependent. Results confirm and extend previous findings which employed unlocalized, in vivo manipulations to show that norepinephrine is a potent antiepileptic agent in the amygdala kindling preparation.

Adrenergic alpha-2 Receptor Agonists↗
PubMed1994

Variance analysis of gamma-aminobutyric acid (GABA)-ergic inhibitory postsynaptic currents from melanotropes of Xenopus laevis.

We have studied the variance in the decay of large spontaneous gamma-aminobutyric acid (GABA)-ergic inhibitory postsynaptic currents (IPSCs) in melanotropes of Xenopus laevis to obtain information about the number of GABAA receptor channels that bind GABA during the IPSCs. The average decay of the IPSCs is well described by the sum of two exponential functions. This suggests that a three-state Markov model is sufficient to describe the decay phase, with one of the three states being an absorbing state, entered when GABA dissociates from the GABAA receptor. We have compared the variance in the decay of large spontaneous IPSCs with the variance calculated for two different three-state models: a model with one open state, one closed state, and one absorbing state (I), and a model with two open states and one absorbing state (II). The data were better described by the more efficient model II. This suggests that the efficacy of GABA at synaptic GABAA receptor channels is high and that only a small number of channels are involved in generating the GABA-ergic IPSCs.

Analysis of Variance↗