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PubMed1999

Maize transposable element Ds is differentially spliced from primary transcripts in endosperm and suspension cells.

The process by which transposable elements are spliced from the host gene transcripts remains poorly understood. We previously reported that a maize transposable element Ds (dissociation) and a copy of its host site duplication are perfectly spliced from the shrunken-2 transcript in the endosperm. Here, we have monitored splicing of the Ds element and its flanking Sh2 sequence following transient expression in maize suspension cells. The pattern of Ds splicing in suspension cells differs dramatically from that in the endosperm. In contrast to splicing in the endosperm, Ds in suspension cells was completely spliced from the transcripts using multiple donor and acceptor splice sites outside the element. In addition, noncanonical splice sites were utilized in suspension cells. Our results indicate that this difference in splicing is due to the context of Ds placement in the construct and/or to tissue specific differences in splicing.

DNA Transposable Elements↗
PubMed2001

Liposomal tacrolimus administered systemically and within the donor cell suspension improves xenograft survival in hemiparkinsonian rats.

The most widely used immunosuppressant in neural transplantation is cyclosporine- A (CsA). However, CsA has significant toxic effects when administered systemically. Tacrolimus (FK506), is a more potent immunosuppressant than CsA and can be prepared in lipid micelles (LTAC). This liposomal preparation allows for the administration of tacrolimus to the site of transplantation, possibly reducing the systemic side effects of immunosuppression. In this study we investigated the ability of LTAC to promote graft survival in hemiparkinsonian rats implanted with fetal mouse xenografts when LTAC is administered systemically to the host, when added to the donor cell suspension, or in combination. Rats with unilateral 6-hydroxydopamine lesions were transplanted with 800,000 fetal mouse ventral mesencephalic (VM) cells and were randomly divided into four groups. Group 1 was not immunosuppressed; Group 2 received daily systemic injections of LTAC; Group 3 received LTAC within the cell suspensions of mouse VM cells; and Group 4 received LTAC in the cell suspensions along with daily systemic administration of LTAC. Transplanted rats were assessed for rotational behavior 3 and 6 weeks posttransplantation. Cell survival was assessed using tyrosine hydroxylase (TH) immunohistochemistry. A significant reduction in rotational scores was observed only in the group of animals receiving the combination of LTAC-treated donor cells and systemic LTAC administration. This functional improvement correlated with a significantly greater survival of TH-immunoreactive cells in this group of animals. The other groups had poor cell survival and no significant functional improvement. We conclude that a combination of systemic immunosuppression and treatment of the cell suspension with LTAC may be a superior strategy to optimize xenograft survival. This strategy may have important implications for clinical neural transplantation.

Animals↗
PubMed1996

Optical Properties of Nonaqueous Suspensions in Electric Fields.

The optical properties of suspensions consisting of pigmented shell/polymer core composite particles dispersed in a silicone oil were studied in electric fields. In the absence of electric fields, the transmittance shows an exponential decay with sample thickness and particle concentration. The particles may be randomly dispersed in the medium. The application of electric field leads to an increase in the transmittance of suspensions. The clearing effect is very striking, especially for concentrated suspensions in a thick cell. In electric fields, the transmittance is almost independent of thickness in the range 0.15-1.1 mm and linearly decreases with increasing particle concentration. Microscopic observation shows that many discrete clusters are formed in electric fields. Based on the analysis of dipole-dipole interactions, the clusters may be composed of fully developed chains spanning the electrodes. Since the columns are aligned in the direction parallel to the optical path, the transmittance is not affected by the cell thickness. In addition, because of the increase in the cross section of the column, the transmittance shows a linear dependence on the particle concentration. The optical properties of suspensions can be explained by the column formation of particles in electric fields.

Journal Article↗
PubMed1997

Aqueous Suspensions of Fumed Silica and Adsorption of Proteins

Thermally stimulated depolarization spectra of frozen aqueous suspensions of fumed silica show a dependence of relaxation processes on the concentration of silica (CSiO2) due to a change in the particle-particle interaction (their electrical double layers) and the concentration of bulk water (ice) with increasing CSiO2. A significant fraction of agglomerates of primary particle aggregates are not decomposed in aqueous suspensions of fumed silica. The thickness of an interfacial water layer perturbed by the silica surface or protein molecules was estimated from the dependence of 1H NMR signal intensity of unfrozen water on temperature below 273 K, separating the signals of water molecules weakly and strongly bound to the surface. Protein molecules adsorbed on fumed silica aggregates form hydrogen bonds with identical withSiOH or identical withSiO- groups by amino groups, e.g., H+NR from the zwitterion fragments. Aqueous suspensions of mechanochemically activated fumed silica do not lose protein adsorption ability during a long period in which the particles remain as microscaled agglomerates. According to AM1-SM1 calculations, complexes such as identical withSiO(H)···H+NRCOO- are stabilized in aqueous suspensions of silica and the protein desorption rate decreases with increasing molecular weight of proteins.

Journal Article↗
PubMed1997

Stability of Dilute Colloidal Silica Suspensions in the Vicinity of the Binodal Curve of the System 2-Butoxyethanol/Water

In the temperature-composition diagram of the system 2-butoxyethanol (abbreviated C4E1)/water, SiO2 [mass fraction of SiO2, y(SiO2) approximately 0.01; volume fraction phi(SiO2) approximately 10(-3)] there exists a flocculation temperature-composition curve running below the binodal curve for x > xc (x, mole fraction of C4E1; xc = 0.0598). The concentration of the colloidal particles is low enough to consider them an "impurity." In the region of the phase diagram bound by the binodal and the flocculation curve the suspension of the colloidal SiO2 particles is unstable and the particles flocculate reversibly. The difference between the temperature of phase separation and the flocculation temperature increases with increasing values of x - xc up to a maximum value of x (xmax). For x > xmax the suspensions are unstable at all temperatures studied. For x < xc the suspension of the SiO2 particles is stable up to the temperature of phase separation of the C4E1/water mixture. The flocculation curve is assumed to reflect the influence of local concentration fluctuations with long range correlations on the stability of the SiO2 suspensions. For x > xc these concentration fluctuations are water rich and interact with the hydrophilic surface of the colloidal SiO2 particles by forming an adsorption layer. This layer is assumed to modify the interparticle potential energy-distance curve and to trigger flocculation. For x < xc the concentration fluctuations are C4E1 rich and no adsorption layer is formed at the hydrophilic surface of the colloidal particles. Copyright 1997Academic Press

Journal Article↗
PubMed1998

Effect of Adsorption Affinity of Polymers on the Viscosity Behavior of Suspensions.

The viscosity behavior of suspensions flocculated by bridging was examined as a function of adsorption affinity of polymers for the particle surface. The polymer chains having strong affinity form fragile bridges between particles as a result of irreversible adsorption. Because the flocs are broken down on the application of shear fields, the flow is shear-thinning. The decrease in the adsorption affinity by the addition of small amounts of surfactant causes the reversible adsorption. The suspensions flocculated by reversible bridging are Newtonian at low shear rates because the polymer bridges are forming and breaking in a quiescent state. When the coil size is comparable to the particle diameter, the suspensions show shear-thickening flow in high shear fields. The shear thickening is induced by the elastic effects of extended bridges. The flow profiles of suspensions flocculated by polymer bridging are controlled by changing the adsorption affinity of polymers. Copyright 1998 Academic Press.

Journal Article↗
PubMed1999

Colloid Vibration Potential in a Concentrated Suspension of Spherical Colloidal Particles.

A relation between the dynamic electrophoretic mobility of spherical colloidal particles in a concentrated suspension and the colloid vibration potential (CVP) generated in the suspension by a sound wave is obtained from the analogy with the corresponding Onsager relation between electrophoretic mobility and sedimentation potential in concentrated suspensions previously derived on the basis of Kuwabara's cell model. The obtained expression for CVP is applicable to the case where the particle zeta potential is low, the particle relative permittivity is very small, and the overlapping of the electrical double layers of adjacent particles is negligible. It is found that CVP shows much stronger dependence on the particle volume fraction &phi; than predicted from the &phi; dependence of the dynamic electrophoretic mobility. It is also suggested that the same relation holds between the electrokinetic sonic amplitude of a concentrated suspension of spherical colloidal particles and the dynamic electrophoretic mobility. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Surface Characterization of NH4PAA-Stabilized Zirconia Suspensions.

Ammonium polyacrylate (NH4PAA) stabilized yttria-doped (3 mol%) tetragonal zirconia (Y-TZP) aqueous suspensions were prepared. Surface chemistry of the suspension was investigated by a variety of techniques. Isotherm adsorption of dispersants on ZrO2 particles demonstrated an optimum coverage of 0.25 mg/g at pH 10. The isoelectric point (IEP) of ZrO2 suspensions was shifted to low pH regions with the presence of NH4PAA. Scanning auger microscopy (SAM) and FTIR and UV spectrophotometry were employed to determine the surface properties of ZrO2 suspensions, which confirmed that the surface Zr-O bonds were affected by the adsorbed polymers. Atomic force microscopy (AFM) analysis indicated that the interactions between particles were evidently influenced by polymer dispersants. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Effect of pH on Rheological Properties of Purified Sodium Bentonite Suspensions.

The effect of pH on the rheological behavior of three purified sodium bentonite (HP, GP, and WP) suspensions is studied. Each clay is studied as a function of pH, at a chosen constant concentration. These concentrations correspond to the lower concentrations at which the yield stress can be detected. They are 12.5, 8.5, and 5.5% w/w, respectively, for the three clay suspensions. The natural pH of these samples measured by a pH meter or by a colorimetric test paper are usually neutral or weakly basic. To obtain a large range of pH values, HCl or NaOH solutions are used in the preparation of the suspensions. From the natural pH and when pH values become more basic, the yield stress measured, using the vane method, decreases. It reaches a minimum before it increases sharply, at very high basic medium. When the pH decreases, the yield stress increases and reaches a maximum for pH in weakly acidic media before decreasing again, in a very highly acidic medium where the structure of the clay is probably attacked. The pH where the maximum appears depends on the nature of the clay. At the same time, the conductivity and the electrophoretic mobility are measured, at the different pH values, on the supernatants obtained after centrifugation of the previous suspensions. The isoelectric points of the edges of the three studied purified sodium bentonites, which are of a great importance for the understanding of the interactions between the clay particles, have been determined. In summary, a correlation between rheological and electrokinetic results, is shown. And moreover, different types of interactions between clay particles are deduced, depending on the pH value of the medium. At acidic medium, there is a dominance of the attractive forces between particles. At high basic medium, interparticle interactions lead to a card-house-like structure based on edge to edge, edge to face, and face to face repulsion, instead of attraction. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Correlation between Generated Shear Stress and Generated Permittivity for the Electrorheological Response of Colloidal Silica Suspensions.

Electrorheological response was experimentally studied by the use of silicone oil suspensions containing submicrometer-sized and supermicrometer-sized silica particles with different amounts of adsorbed water. The simultaneous measurements of dielectric permittivity and shear stress of the suspensions were carried out after the application of alternating current voltage under steady shear in ranges of shear rate (150-1400 s(-1)), electric field strength (0-4 kV/mm), its frequency (30-1000 Hz), and particle volume fraction (0.1-0.3). For the particles with small amounts of adsorbed water, steady shear stress was attained within several minutes after the application of electric field. The steady-state data for both the particles at each electric field strength showed that the shear stress generated by the application of electric field, Deltatau, varied correlatively with the generated permittivity, Deltaepsilon(r) (= epsilon(r) - epsilon(r,oil)), where epsilon(r) and epsilon(r,oil) are the permittivities of the suspension and the silicon oil, respectively. Under a wide variety of experimental conditions, the steady-state data for both the particle sizes could be correlated with a simple relationship, Deltatau ~ (Deltaepsilon(r)E)(2), where E is electric field strength. For the particles with large amounts of adsorbed water, steady state was not attained, and the evolutions of shear stress and permittivity of the suspensions were measured after the application of electric field. Remarkably, the transient values of Deltatau varied with (Deltaepsilon(r)E)(2) and fell along the same correlation line as the steady-state data. Copyright 1999 Academic Press.

Journal Article↗
PubMed2001

Dispersion Stability and Electrorheological Properties of Polyaniline Particle Suspensions Stabilized by Poly(vinyl methyl ether).

In the present article, a novel method of stabilization of a semi conductive polyaniline particle nonaqueous suspension of electrorheological (ER) materials was introduced. Using as the steric stabilizer poly(vinyl methyl ether) (PVME), a dispersion polymerization of aniline was performed and stable aqueous dispersions of the polyaniline were obtained. However, a stable colloidal suspension of polyaniline in silicone oil medium can be obtained only with a low concentration of PVME (0.75%), although the average size of the individual, redispersed particles in oil generally decreased with increasing concentration of PVME. At higher concentrations of PVME, formation of highly packed particle aggregates was investigated, as seen in the scanning electron microscopy images. The polyaniline suspension stabilized by 0.75% PVME content exhibited the best dispersing state and, therefore, showed the highest zero-field viscosity. Such a well-stabilized polyaniline suspension also showed the maximum ER property, namely the largest normalized yield stress in an electric field. Copyright 2001 Academic Press.

Journal Article↗
PubMed2001

Characteristics of Alkyltrimethoxysilane-Treated Fumed Silicas and Rheological Behavior of Fumed Silica Suspensions in an Epoxy Resin.

Fumed silica was surface treated by hexyl (C6), octyl (C8), decyl (C10), hexadecyl (C16), and octadecyl(C18)trimethoxysilanes, and the products were evaluated from various aspects. The amounts of alkyl group introduced on silica surface were all within the range from 22 to 25 mmol/100 g. Such fumed silicas were suspended in an epoxy resin, bisphenol type A, and their rheological properties were measured. The rheological behavior of the silica suspensions was investigated as to steady-state viscosity and dynamic moduli in order to examine the effect of alkyl chain length on interactions of hydrophobic groups on the silica surface. The stronger shear thinning behavior was observed in the surface-modified fumed silica with longer alkyl groups. However, the suspensions of the nontreated fumed silica and the fumed silicas treated by C6 silane showed Newtonian behavior even at the silica concentration of 7 wt%. The dynamic moduli G' of the silica suspensions increased with increasing alkyl chain length. The interaction of hydrophobic groups on the silica surface is mainly attributed to the rheological behavior of fumed silica suspensions in the epoxy resin. Copyright 2001 Academic Press.

Journal Article↗
PubMed2002

Analysis of the dielectric permittivity of suspensions by means of the logarithmic derivative of its real part.

Measurement of the dielectric permittivity of colloidal suspensions in the kilohertz frequency range (the so-called low-frequency dielectric dispersion) is a promising tool for the electrokinetic characterization of colloids. However, this technique is less used than would be desirable because of the difficulties associated with the measurements, the most important of which is the electrode polarization (EP). Recently (M. Wübbenhorst and J. Van Turnhout, Dielectrics Newsl. November (2000)) a method was proposed that appears capable of separating the unwanted electrode effects from the double-layer relaxation that we are interested in. The method, based on the logarithmic derivative of raw epsilon'(omega) data (epsilon'(omega) is the real part of the permittivity of the suspension for a frequency omega of the applied AC field), is first checked against the well-known theory of the AC permittivity of colloidal suspensions developed by DeLacey and White (E. H. B. DeLacey and L. R. White, J. Chem. Soc. Faraday Trans. 277, 2007 (1981)). We show that the derivative epsilon''(D)(omega)=-(pi/2)(partial differential epsilon'/partial differential ln omega) gives an excellent representation of the true imaginary part of the permittivity, epsilon''(omega). The technique is then applied to experimental data of the dielectric constant of polystyrene and ethylcellulose suspensions. We found that epsilon''(D) displays two separated behaviors when plotted against log omega in the frequency range 100 Hz-1 MHz: a monotonous decrease (associated with EP) followed by an absorption peak (associated with the double-layer relaxation, or alpha-relaxation). Interestingly, they are separated enough to make it possible to easily find the characteristic frequency of the alpha-relaxation. Fitting a relaxation function to epsilon''(D)(omega) after eliminating the part due to EP, we could calculate the real part epsilon'(omega) and compare it to the DeLacey and White (DW) theoretical predictions. A significantly better agreement between DW calculations and experimental epsilon'(omega) data is obtained when the logarithmic derivative method is used, as compared to the classical electrode-separation techniques.

Journal Article↗
PubMed2002

Polymer depletion-induced instability of silica suspensions in the presence of flexible and semiflexible polymers.

Polymer depletion-induced instability of silica suspensions in dilute toluene solutions of flexible polystyrene (PS) or semiflexible poly(hexyl isocyanate) (PHIC) was investigated by direct observation and oscillatory moduli measurements as functions of silica volume fraction and polymer molecular weight. Addition of the respective polymers to the gelled silica suspensions induces a gel settling. Below the silica volume fraction phi=4 vol%, PS chains compress the volume of the sediment silica phase, whereas PHIC chains expand it and play a role in swelling agents. Thus, PHIC chains lead to the formation of aggregates in the silica suspensions that are larger and less compact than those formed by the PS chains. On the other hand, above phi=4 vol% where the silica suspension occurs gelation, the effect of nonadsorbing polymers on changes in the volume of the silica phase is opposite. Moreover, polymer depletion interaction results in a mechanically stronger gelled phase, leading to the storage modulus G' values larger than those without polymer, but the effects of polymer concentration and polymer molecular weight on the G' values are not clear.

Journal Article↗
PubMed1996

Enhanced recovery of a secreted mammalian protein from suspension culture of genetically modified tobacco cells.

Increasing the level of recovery of mammalian proteins secreted by a genetically modified Nicotiana tabacum was explored in suspension culture. As a model protein system, a mouse monoclonal antibody heavy chain gamma (MAb HC) with an antigen specificity for p-azophenylarsonate was used. Consistent with findings for other plant cell suspension culture systems expressing proteins with mammalian leader sequences, the synthesized mouse MAb HC was secreted through the plasma membrane. In addition, the majority of the MAb HC was also secreted through the cell wall into the growth medium. However, efficient recovery of the protein was only possible when the protein stabilizing agent, polyvinylpyrrolidone (PVP) was present in the plant cell growth medium. The presence of PVP increased the recovered concentration of secreted protein 35-fold from 0.010 to 0.36 micrograms protein/ml culture medium. Biological activity of the approximately 50-kDa MAb HC polypeptide was demonstrated by arsonate affinity matrix binding as determined by Western blot analysis. In addition to antigen binding activity, the secreted protein also exhibited reactivity to protein G, a protein which specifically binds mouse IgG. These findings are important because they demonstrate that culture conditions can significantly influence the concentration of a biologically active foreign protein secreted from plant cells into the media of suspension cultures. The ability to increase the efficiency of mammalian protein production in plant suspension culture systems should provide significant advantage over protein production in intact transgenic plants which require cultivation, harvesting, and expensive extraction procedures to obtain nonsecreted foreign proteins.

Animals↗
PubMed2001

Comparison of in vitro surface properties of clove oil-phospholipid suspensions with those of ALEC, Exosurf and Survanta.

Dipalmitoyl phosphatidylcholine, the main component of lung surfactant is ineffective as a replacement surfactant due to its poor adsorption. We studied clove oil as a possible additive for improving the surface activity of protein-free phospholipid suspensions. We added low doses of clove oil, to phospholipid suspensions and studied the surface properties by in vitro analysis using a pulsating bubble surfactometer and a Wilhelmy balance. Survanta, ALEC and Exosurf were used as controls for comparison. The test surfactants, which were phospholipid-oil suspensions at 1% concentration, in buffer containing either 2 or 5 mM calcium, were pulsated at 40 cpm in a pulsating bubble surfactometer. The phospholipids studied were dipalmitoyl phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), binary mixtures of PC:PE (2:3) and PC:PG (2:3). The addition of clove oil (CO) to each of the above phospholipids was in the ratio of nine parts of phospholipid to one part of oil. The presence of CO caused a significant improvement in the adsorption and minimum surface tension of all the phospholipid suspensions studied. The mixtures PC with CO, both in the presence of 2 and 5 mM calcium, and PCPE with CO at 2 mM calcium concentration had surface properties significantly better than those of ALEC and Exosurf and equivalent to those of Survanta. The addition of clove oil helps improve the surface properties of phospholipids.

Adsorption↗
PubMed1987

The effect of denbufylline on the viscosity of rat whole blood and on the deformability (filterability) of rat blood cell suspensions.

The novel alkylxanthine, denbufylline [1,3-di-n-butyl-7-(2-oxopropyl)-xanthine] has been examined, in vitro, for effects on the viscosity of rat whole blood and on the filterability of rat blood cell suspensions. For comparison, pentoxifylline was also examined for rheological activity. Denbufylline reduced the viscosity of whole blood at all shear rates utilised, up to 128.5 s-1. The effect was, however, more pronounced at a low (0.7 s-1) than at a high (94.5 s-1) shear rate indicating that the compound reduces blood cell aggregation and increases blood cell deformability. Denbufylline also increased the filterability of blood cell suspensions. This is further evidence that the compound increases the deformability of blood cells. Denbufylline elevated the filterability of both pure erythrocyte and mixed erythrocyte/leucocyte suspensions, the effect being greatest with the latter. This suggests that denbufylline may influence the deformability of both red and white blood cells. However, the effect on white blood cells, under the experimental conditions employed, is apparently more marked. Pentoxifylline also reduced the viscosity of rat whole blood and increased the filterability of rat blood cell suspensions. However, denbufylline was 10-100-fold more potent in these tests than pentoxifylline.

Animals↗
PubMed1992

High frequency, heat treatment-induced inactivation of the phosphinothricin resistance gene in transgenic single cell suspension cultures of Medicago sativa.

One descendant of the Medicago sativa Ra-3 transformant T304 was analysed with respect to the somatic stability of the synthetic phosphinothricin-N-acetyltransferase (pat) gene which was used as a selective marker and was under the control of the 5'/3' expression signals of the cauliflower mosaic virus (CaMV) gene VI. In order to quantify gene instability, we developed a system for culturing and regenerating individual cells. Single cell suspension cultures derived from T304 and the ancestral non-transgenic M. sativa cultivar Ra-3, were established. The cells were regenerated into monoclonal calli. In transgenic calli, the phosphinothricin (Pt)-resistance phenotype was retained after more than 2 months of non-selective growth. In contrast, up to 12% of the suspension culture cells grown under nonselective conditions and at constant temperature (25 degrees C) lost the herbicide-resistance phenotype within 150 days. Surprisingly, a heat treatment (37 degrees C), lasting for 10 days, during the culture period resulted in an almost complete (95%) loss of the Pt resistance of the suspension culture cells. However, the frequency of cell division was identical in cultures grown under normal and heat treatment conditions. A biochemical test revealed that no phosphinothricin-N-acetyltransferase activity was present in heat treated, Pt-sensitive cells. The resistance level of the Pt-sensitive transgenic cells was equivalent to that of the wild-type cells. A PCR analysis confirmed the presence of the pat gene in heat treated, Pt-sensitive cells. From these results it is concluded that the Pt resistance gene was heat-inactivated at a high frequency in the M. sativa suspension cultures.

Acetyltransferases↗