Subcellular redistribution of trimeric G-proteins--potential mechanism of desensitization of hormone response: internalization, solubilization, down-regulation.

Agonist-induced subcellular redistribution of G-protein coupled receptors (GPCR) and of trimeric guanine-nucleotide binding regulatory proteins (G-proteins) represent mechanisms of desensitization of hormone response, which have been studied in our laboratory since 1989. This review brings a short summary of these results and also presents information about related literature data covering at least small part of research carried out in this area. We have also mentioned sodium plus potassium dependent adenosine triphosphatase (Na, K-ATPase) and 3H-ouabain binding as useful reference standard of plasma membrane purity in the brain.

Monomer dynamics in single- and double-stranded DNA coils.

In our paper (Tothova et al., Czech. J. Phys. 55, 221 (2005)), the first observation of the kinetics of individual polymer monomers using the fluorescence correlation technique (R. Shusterman et al., Phys. Rev. Lett. 92, 048303 (2004)) has been interpreted within the bead-spring theory. Optimizing the joint Rouse-Zimm model to the experimental data, the phenomenological parameters for the statistical-mechanical description of the universal behavior of double- and single-stranded DNA and the dominant types of their dynamics have been determined. Recently, these data have been corrected (R. Shusterman et al., Phys. Rev. Lett. 98, 029901 (2007)). In the present work, the fits of the theory to the new data are given. The main conclusions of our preceding paper remain unchanged but some of the polymer parameters have changed. The new data allow a significantly better agreement with the theory than the previous ones. Our calculations confirm that dsDNA follows mainly the classical Zimm-type kinetics rather than the Rouse one as it was proposed by Shusterman et al. Single-stranded DNA also behaves predominantly as the Zimm polymer. To support these conclusions, we analyze the draining effects on the monomer dynamics and the applicability of simple "universal" laws, according to which the monomer mean square displacement scales with the time as t 1/2 and t 2/3 for the Rouse and Zimm polymers, respectively.

Deformability of multilamellar vesicles.

Although a free unilamellar vesicle has zero or almost zero genuine surface tension, the multilamellar vesicle ("onion") exhibits a nonzero effective surface tension sigma(eff). The expression for sigma(eff) used in the literature is sigma(eff) approximately square root of kappaB/d(0), where B is the interaction modulus between the vesicle bilayers, d(0) the repeating distance between the bilayers in the droplet, and kappa their bending rigidity. In this paper we calculate the contributions to the effective surface tension of a lamellar droplet in the case when the layers interact with one another and when they are free. It is shown that the interaction contribution to the surface tension is small and sigma(eff) is determined mainly by kappa, the radius of the droplet R(0), and the number of the shape undulation modes l(max). A nonzero surface tension of the layers is also included in the calculation which is necessary when the vesicle membrane is stressed in the complex of other membranes.

Long-time dynamics of Rouse-Zimm polymers in dilute solutions with hydrodynamic memory.

The dynamics of flexible polymers in dilute solutions is studied taking into account the hydrodynamic memory, as a consequence of fluid inertia. As distinct from the Rouse-Zimm (RZ) theory, the Boussinesq friction force acts on the monomers (beads) instead of the Stokes force, and the motion of the solvent is governed by the nonstationary Navier-Stokes equations. The obtained generalized RZ equation is solved approximately using the preaveraging of the Oseen tensor. It is shown that the time correlation functions describing the polymer motion essentially differ from those in the RZ model. The mean-square displacement (MSD) of the polymer coil is at short times approximately t(2) (instead of approximately t). At long times the MSD contains additional (to the Einstein term) contributions, the leading of which is approximately t. The relaxation of the internal normal modes of the polymer differs from the traditional exponential decay. It is displayed in the long-time tails of their correlation functions, the longest lived being approximately t(-3/2) in the Rouse limit and t(-5/2) in the Zimm case, when the hydrodynamic interaction is strong. It is discussed that the found peculiarities, in particular, an effectively slower diffusion of the polymer coil, should be observable in dynamic scattering experiments.

Biochemistry of transmembrane signaling mediated by trimeric G proteins.

Many extracellular signals are at the cell surface received by specific receptors, which upon activation transduce information to the appropriate cellular effector molecules via trimeric G proteins. The G protein-mediated cascades ultimately lead to the highly refined regulation of systems such as sensory perception, cell growth, and hormonal regulation. Transmembrane signaling may be seriously deranged in various pathophysiological conditions. Over the last two decades the major experimental effort of our group has been devoted to better understanding the molecular mechanisms underlying transmembrane signaling regulated by G proteins and to the closely related process of desensitization of hormone response. This review provides general information about the basic principles of G protein-regulated transmembrane signaling as well as about our contribution to the current progress in the field.

Kerr constant of vesicle-like droplets.

The Kerr effect on vesicle-likedroplets (vesicles, cells or emulsion droplets) is described. Wegive a derivation of the Kerr constant for a dielectric fluiddroplet immersed in another fluid, assuming that the droplet in aweak electric field becomes a prolate ellipsoid. The Kerr constantis evaluated also for a droplet covered by a membrane of nonzerothickness. Comparing the theory with the experiment on dropletmicroemulsions from the literature, the bending rigidity constantof the surface layer is estimated.

Nitric oxide synthase inhibition and glutamate binding in quinolinate-lesioned rat hippocampus.

The effect of lesions induced by bilateral intracerebroventricular (i.c.v.) injection of quinolinate (250 nmol of QUIN/ventricle), a selective N-methyl-D-aspartate (NMDA) receptor agonist, on [3H]glutamate ([3H]Glu) binding to the main types of both ionotropic and metabotropic glutamate receptors (iGluR and mGluR) was investigated in synaptic membrane preparations from the hippocampi of 50-day-old rats. The membranes from QUIN injured brains revealed significantly lowered binding in iGluR (by 31%) as well as in mGluR (by 22%) as compared to the controls. Using selected glutamate receptor agonists as displacers of [3H]Glu binding we found that both the NMDA-subtype of iGluR and group I of mGluR are involved in this decrease of binding. Suppression of nitric oxide (NO) production by N(G)-nitro-L-arginine (50 nmol of NARG/ventricle) or the increase of NO generation by 3-morpholinylsydnoneimine (5 nmol of SIN-1/ventricle) failed to alter [3H]Glu or [3H]CPP (3-((D)-2-carboxypiperazin-4-yl)-[1,2-(3)H]-propyl-1-phosphonic acid; NMDA-antagonist) binding declines caused by QUIN-lesions. Thus, our findings indicate that both the NMDA-subtype of iGluR and group I of mGluR are susceptible to the QUIN-induced neurodegeneration in the rat hippocampus. However, the inhibition of NO synthesis did not reveal any protective action in the QUIN-evoked, NMDA-receptor mediated decrease of [3H]Glu binding. Therefore, the additional mechanisms of QUIN action, different from direct NMDA receptor activation/NO production (e.g. lipid peroxidation induced by QUIN-Fe-complexes) cannot be excluded.

Quinolinic acid induces oxidative stress in rat brain synaptosomes.

The oxidative action of quinolinic acid (QUIN), and the protective effects of glutathione (GSH), and 2-amino-5-phosphonovaleric acid (APV), were tested in rat brain synaptosomes, Reactive oxygen species (ROS) formation was quantified after the exposure of synaptosomes to increasing concentrations of QUIN (25-500 microM). The potency of QUIN to induce lipid peroxidation (LP) was tested as a regional index of thiobarbituric acid-reactive substances (TBARS) production, and the antioxidant actions of both GSH (50 microM) and APV (250 microM) on QUIN-induced LP were evaluated in synaptosomes prepared from different brain regions. QUIN induced concentration-dependent increases in ROS formation and TBARS in all regions analyzed, but increased production of fluorescent peroxidized lipids only in the striatum and the hippocampus, whereas both GSH and APV decreased this index. These results suggest that the excitotoxic action of QUIN involves regional selectivity in the oxidative status of brain synaptosomes, and may be prevented by substances exhibiting antagonism at the NMDA receptor.

Interpretation of static and dynamic neutron and light scattering from microemulsion droplets: effects of shape fluctuations

The theory of static and dynamic scattering of neutrons and light on microemulsion droplets is developed. The droplets are modeled by double-layered fluid spheres immersed in another fluid. The surface layer of arbitrary thickness thermally fluctuates in the shape. The scattering functions are consistently calculated up to the second order of the fluctuations. The bulk fluids and the layer are characterized by different scattering length densities (or dielectric constants). Involving the Helfrich's concept of interfacial elasticity, the theory is applied for the description of small-angle neutron scattering (SANS), neutron spin echo (NSE), and dynamic light scattering (DLS) experiments on dilute microemulsions. From the fits to the experimental data the bending elasticity and the Gaussian modulus are extracted. Due to the corrected account for the fluctuations, their values differ markedly from those obtained in the original works. The theory well describes the SANS experiments. In the case of DLS, we had to assume the shell of the solvent molecules to be built of several layers. Previous theories were in a sharp disagreement with the NSE experiments. A better agreement with these experiments is obtained if the dissipation in the surface layer is included into the consideration. From the experiments, the viscosity of the layer is estimated for a concrete microemulsion system.

Characterization of low-salt and high-salt conformation of poly(dI-dC) by hydrogen-deuterium exchange kinetics: a classical Raman spectroscopy study.

Poly(dI-dC) in H2O and D2O solution can undergo different equilibrium geometries which strongly depend on the salt nature and concentration. These structures were studied by classical Raman spectroscopy in order to monitor a hydrogen-deuterium exchange kinetics in 8-CH group in inosine. Spectral and isotopic exchange rate changes depending on NaCl concentration were observed and interpreted on the basis of previously obtained results from resonance and classical Raman spectroscopy studies of poly(dI-dC) and hydrogen-deuterium exchange measurements of different conformations of nucleic acids. It is shown that: i) the Raman spectrum of low-salt poly(dI-dC) corresponds to the right-handed polymer with characteristic bands for B conformation, but the value of the retardation factor of isotopic exchange suggests that this form is not a pure canonical B form and that it contains some portion of the A form, ii) the Raman spectrum of the high-salt poly(dI-dC) corresponds to the right-handed polymer with characteristic bands for both the A and B conformations, iii) the retardation factor of hydrogen deuterium exchange for the high-salt form of poly(dI-dC) is essentially higher than in the low-salt form which indicates a dominant presence of the A form in the high-salt conformation of poly(dI-dC). This leads to the conclusion that the high-salt conformation of poly(dI-dC) is a mixture of A and B forms with the predominant A form.

Thermal excitations of hydrated macromolecules: the effects of hydration on the Mössbauer absorption and scattering spectra.

The spectra of Rayleigh scattering of Mössbauer radiation (RSMR) and Mössbauer absorption by globular macromolecules are calculated. The dependence of the spectra parameters on hydration is modeled with the account for thermal low-frequency vibrations of the particles constituting the globule. Deformational motions of the macromolecule fragments leading to deviations from its equilibrium spherical shape are considered introducing collective dynamical variables governed by Langevin equations with random sources of external forces. The macromolecule is modeled by a double-layered sphere: a rigid (elastic) core is surrounded by a porous hydration shell filled with fluid. The dynamical properties of the bound water inside the shell are described by the Debye-Brinkman equations. The degree of hydration is introduced by means of a combination of the mass coefficients of the porous shell with fluid and the mass coefficients in the limiting cases when the flow inside the shell is "frozen" and in the case of free flow. The hydration-dependent Lamb-Mössbauer factor and the elastic fraction of the RSMR are calculated and compared with experimental data from the literature.

Specific [3H]glutamate binding in the cerebral cortex and hippocampus of rats during development: effect of homocysteine-induced seizures.

Specific [3H]glutamate binding to synaptic membranes from the cerebral cortex and hippocampus of 7-, 12- and 18-day-old rats was examined, both in control animals and during seizures induced by homocysteine. In the cerebral cortex a transient peak of glutamate binding was observed in 7-day-old group, whereas in the hippocampus it occurred in 12-day-old animals. Total specific [3H]glutamate binding was not influenced by preceding seizure activity in either of the age groups and both the studied regions. NMDA- and QA-sensitive glutamate bindings represent the highest portion of the total binding. Moreover, NMDA-sensitive binding in the cerebral cortex of 7-day-old rats is significantly higher as compared to the two more mature groups. The proportion of individual receptor subtypes on total binding in each age group was not influenced by preceding seizure activity. However, NMDA-sensitive binding in the hippocampus of 12-day-old rats, sacrificed during homocysteine-induced seizures, was significantly increased as compared to corresponding controls. In contrast to the effect of NMDA, AMPA, kainate and quisqualate which displaced to a different extent [3H]glutamate binding, homocysteine had no effect when added to membrane preparations. Similarly, [3H]CPP and [3H]AMPA bindings were not affected in the presence of homocysteine. It thus seems unlikely that homocysteine is an effective agonist for conventional ionotropic glutamate receptors. Its potential activity at some of the modulatory sites at the NMDA receptor channel complex or at metabotropic receptors has to be clarified in further experiments.

Internal DNA modes below 25 cm-1: a resonance Raman spectroscopy observation.

The first resonance Raman scattering observation of the low-frequency (LF) region (below 40 up to 12 cm-1) of DNA motions is presented. Since the concentration of the studied DNA solution was very low (1 mg/ml), the spectra features reflect internal vibrations of the macromolecule. The decomposition of the spectra into Lorentzians clearly indicate three intrahelical DNA modes: the corresponding peaks are located at the frequencies 16, 19, and 23 (+/- 1) cm-1. This result is in agreement with our quasi-continuity model of the LF B-form DNA dynamics (V. Lisy, P. Miskovsky and P. Schreiber, J. Biomol. Struct. Dyn. 13, 707 (1996)). The fit of the experimental frequencies to the theory, using the Genetic Algorithms approach, allowed us to make some conclusions about the model force constants which could be found by independent conformational energy calculations. Possible positions of five lowest-frequency DNA peaks, predicted by the model, are discussed.

Biochemical characteristics of gamma-glutamyl transpeptidase in capillaries from entorhinohippocampal complex of quinolinate-lesioned rat brain.

Quinolinic acid (QUIN) is an endogenous excitotoxic agonist of the N-methyl-D-aspartate (NMDA) type of glutamate receptor, which causes slowly progressing degeneration of vulnerable neurons in some brain regions. Using changes in the activity of membrane-bound gamma-glutamyl transpeptidase (GGT) as a marker of cell damage, we found a significant decrease of this enzyme activity, which was preferentially located in the ipsilateral hippocampal formation and entorhinal cortex, 4 d after the unilateral intracerebroventricular (icv) injection of 0.5 mumol QUIN. The dose of QUIN divided into two half-doses injected bilaterally led to a symmetrical decline of GGT activity in hippocampal areas. The lesion was characterized by a suppression of GGT activity in hippocampal and entorhinal capillaries, corresponding to 60 and 81% of their initial value, respectively, but no significant changes were ascertained in synaptosomal membranes. The changes in the activity of capillary GGT were associated with the decrease of apparent maximal velocity Vmaxapp, whereas apparent Michaelis constant K(m)app (0.69-0.79 mM) remained unaffected. In the nonlesioned brain, concanavalin A (Con A) affinity chromatography revealed five glycoforms of synaptosomal GGT in contrast to only one found in hippocampal and entorhinal capillaries. The results document that neither the saccharide moiety of GGT nor the value of enzyme K(m)app is significantly affected by the QUIN-induced lesion of the rat brain. However, the suppression of GGT activity, which is accompanied by a decrease in the value of Vmaxapp in brain microvessels, may suggest dysfunction of the blood-brain barrier (BBB) in the QUIN-injured rat brain.

Analysis of kinetic properties of gamma-glutamyl transpeptidase from rat kidney.

The initial rate kinetics of rat kidney gamma-glutamyl transpeptidase were measured using L-gamma-glutamyl-p-nitroanilide and glycyl-glycine as the donor and the acceptor substrate, respectively. Experimental data were fitted with the initial rate equation, and the obtained results indicated that: (1) Michaelis constants for transpeptidation (Kb), autotranspeptidation (Ka), and hydrolysis (Kh) are 8.56 mmol/l, 2.02 mmol/l and 0.005 mmol/l, respectively. (2) The maximum rate of transpeptidation (Vb) exceeds that of hydrolysis (Vh) and autotranspeptidation (Va) 160 times and 5 times, respectively. (3) A comparison of the ratios of maximal rate: Michaelis constant of individual reactions shows that hydrolysis is approximately 10 times more efficient than the remaining two reactions. (4) Under routine conditions used for gamma-glutamyl transpeptidase estimation, transpeptidation is the prevalent reaction.

On a simple model of low-frequency vibrations in DNA macromolecules.

A simple quasi-continuity model of low-frequency dynamics of DNA macromolecules is presented. The model is based on the phenomenological theory by Volkov and Kosevich (J. Biomol. Struct. Dyn. 8, 1069 (1991)). We propose improvements to their theory by correcting the model energy and corresponding equations of motion, recalculating the model DNA parameters, and estimating the effects of hydration and counterion binding on the masses of DNA subunits. Comparing the calculated low-frequency vibration spectrum and the results of classical Raman scattering experiments from the literature, the values for the model force constants are determined. These values differ significantly from the estimations made by Volkov and Kosevich. A good quantitative agreement with experiment can be obtained both for the case when the "25 cm-1 mode" is an external one, and when it is considered as an intrahelical mode. Problems connected with the qualitative assignments of the calculated and experimentally observed modes are discussed.

Relationship between kinetic properties of gamma-glutamyl transpeptidase and the structure of its saccharide moiety.

Gamma-glutamyl transpeptidase (EC 2.3.2.2; GGT) is a plasma-membrane bound glycoenzyme, the saccharide moiety of which is rather heterogeneous and organ specific. It has been stated that GGT catalyses three types of reactions, i.e., hydrolysis, transpeptidation and autotranspeptidation. The initial velocity equation, involving all these reactions, is shown in the present report. Mathematical analysis of the equation resulting in a definition of the constant of half saturation (Khs). The value of Khs was used for characterization of kinetics of GGT from rat organs differing in the structure of GGT oligosaccharide chains. No significant organ differences were found, when the Khs values of GGT from the brain, kidney and pancreas equalled 0.61 mM, 0.68 mM and 0.68, respectively. On the contrary, when two different glycoforms of GGT from the pancreas were compared, distinct values of Khs were obtained (1.43 mM and 0.67 mM, respectively). It is therefore being suggested that the saccharide chains of GGT are involved in its kinetic properties. However, this effect is masked when the enzyme, non-fractionated into glycoforms, is analysed, even though the saccharide moiety is specific for the organ studied.

Conformational transitions of poly(dI-dC) in aqueous solution as studied by classical Raman spectroscopy.

Poly(dI-dC) in aqueous solution can undergo different equilibrium geometries which strongly depend on the salt nature and the concentration. These structures were studied by classical Raman spectroscopy (RS). Spectral changes depending on NaCl concentration and on the presence of Ni2+ ions were observed and interpreted on the basis of previously obtained results from resonance RS studies of poly(dI-dC) and classical RS studies for other alternating purine-pyrimidine polydeoxyribonucleotides, i.e. poly(dG-dC), poly(dA-dT) and poly(dA-dC)(dG-dT), which also showed B to Z conformational transitions upon varying the salt concentrations. It is shown that: i) The low-salt structure (0.1 mol/l NaCl) is in the pure canonical B conformation. ii) The high-salt (5 mol/l NaCl) Raman spectrum is similar to that obtained for the low-salt concentration. Thus the high-salt structure corresponds to the right-handed polymer with characteristic bands for both the B (predominant) and A conformations with some weak Z conformation markers which indicate a tendency for B to Z conformational transition of the polymer. iii) The addition of 9.10(-3) mol/l NiCl2 to the high-salt solution induces Z-conformation of the polymer.

Relationship between gamma-glutamyl transpeptidase activity and sialic acid content in some organs and brain regions of the developing rat.

The activity of gamma-glutamyl transpeptidase (GGT) and the sialic acid (SA) content were found to be specific in five brain regions of 7- and 50-day-old rats. While a low GGT activity was accompanied by high sialylation in the whole brain a high GGT activity and low sialylation were observed in kidney and pancreas. Similar findings were obtained for some brain regions investigated. However, the developmental differences were not characterized by such relationship. Concanavalin A affinity chromatography of GGT from brain of 7- and 50-day-old rats revealed no remarkable changes in GGT sialylation although the enzyme activity is almost 3 times higher in brain membranes of the older group. It can be concluded that in spite of an apparent relationship between GGT activity and membrane sialylation, the amount of SA linked to the GGT molecule is not related to differences in enzyme activity.