Biomedical subjects

N Dan

Publications and source records attributed to N Dan.

At least 19 recordsLinked to original sources

PubMed2006

Junctions and end-caps in self-assembled non-ionic cylindrical micelles.

Cylindrical micelles are known to exhibit two types of morphologies: branched networks and linear, worm-like (or thread-like) micelles. These structures correspond to two types of topological defects: end-caps and junction points. Although either type of defect increases the micelle energy (when compared to the cylindrical sections), they are stabilized by an increase in the translational (end-caps) or configurational (junctions) entropy. End-caps reduce the length of the cylindrical micelles, resulting in a suspension of linear, worm-like micelles. Y-junction branch points cause the formation of a network structure that may percolate and coexist thermodynamically with a "sol" of finite cylinders with end-caps. In this paper, we review current experimental and theoretical studies of non-ionic cylindrical micelles in aqueous solutions. We focus on single and multicomponent amphiphiles, and consider both small molecules and macromolecules (polymers), in order to identify the driving forces that determine the type of topological 'defect' and the resulting system morphology.

Journal Article↗

PubMed2006

The kinetics and saturation of reversible adsorption on patterned (heterogeneous) surfaces.

We analytically examine the time-dependent adsorption of analyte (solute) on a finite-sized adsorption region as a model for sensors utilizing patterned or heterogeneous surfaces. We account for both reversible adsorption (assuming first-order reaction) and saturation of the adsorption patch that may arise either from packing constraints (finite area) or because of a finite number of binding sites (ligands). Our main conclusions include the following: (1) Saturation effects, due to either finite patch size or finite number of binding sites, become significant at extremely short times. (2) Increasing the strength of binding between the analyte and the adsorption sites increases the adsorbed amount at short times, but, at long times, the mass adsorbed on a weakly binding patch is higher than that on a strongly binding one. (3) The sensitivity of detection, as defined by the adsorption of the minimal analyte mass required for signaling, over a fixed period of time, does not scale as 1/detection time. As a result, increasing the time over which adsorption occurs increases sensitivity, but not linearly. Sensitivity of detection also increases with increasing patch area and initial binding strength.

Adsorption↗

PubMed2005

Cooperativity in the adsorption of magnetic colloidal particles.

In this paper we investigate the adsorption of magnetic particles onto magnetically patterned substrates. We find that the adsorption process is cooperative, where the probability of adsorption decreases with increasing substrate occupancy (namely, density of adsorbed particles). The effect of cooperativity can be accounted for by a simple modification of the adsorption probability as manifested by the binomial distribution. The negative cooperativity found in the magnetic particle adsorption is not due to direct repulsion between particles, but to screening of the surface's magnetic field by previously adsorbed particles. Thus, the adsorption of magnetic colloids on magnetic substrates is a self-limiting process.

Journal Article↗

PubMed2004

Controlling the collapse/swelling transition in charged hydrogels.

In this paper, we investigate systematically the parameters controlling the swelling transition of charged hydrogels. Combining theoretical analysis and experimental investigations we characterize the transition pH and the width of the transition as a function of the hydrogel crosslink density (i.e. subchain molecular weight) and the solution salt concentration. We also investigate the effect of pendant polyethylene glycol (PEG) side chains on the swelling transition. Quite surprisingly, we find that (1) the degree of swelling varies non-monotonically with the solution salt concentration, first increasing and then decreasing with salinity, (2) even in the limit of high salt concentrations, where electrostatics are expected to play a minor role, we find that the degree of swelling is quite sensitive to the suspension pH, namely, to the gel degree of charging, (3) in the limit of long subchains the transition pH depends only on the chemistry of the gel charges, and the transition is quite sharp, (4) the transition pH shifts with decreasing subchain length and/or with decreasing salt concentration, and the width of the transition increases and (5) the overall degree of swelling increases significantly upon the incorporation of pendent PEG chains, while the transition pH remains unaffected and the width of the transition narrows significantly.

Absorption↗

PubMed2003

Ameliorating effect of anti-Fas ligand MAb on wasting disease in murine model of chronic colitis.

Fas/Fas ligand (FasL) interaction has been implicated in the pathogenesis of various diseases. To clarify the involvement of Fas/FasL in the pathogenesis of intestinal inflammation, we investigated the preventive and therapeutic effects of neutralizing anti-FasL monoclonal antibody (MAb) on the development of chronic colitis induced by adaptive transfer of CD4+CD45RBhigh T cells to SCID mice. Administration of anti-FasL MAb from 1 day after T cell transfer (prevention study) resulted in a significant improvement of clinical manifestations such as wasting and diarrhea. However, histological examination showed that mucosal inflammation in the colon, such as infiltration of T cells and macrophages, was not improved by the anti-FasL MAb treatment. In vitro studies showed that anti-FasL MAb did not inhibit IFN-gamma production by anti-CD3/CD28-stimulated lamina propria CD4+ T cells but suppressed TNF-alpha and IL-1beta production by lamina propria mononuclear cells. Therapeutic administration of anti-FasL MAb from 3 wk after T cell transfer also improved ongoing wasting disease but not intestinal inflammation. These results suggest that the Fas/FasL interaction plays a critical role in regulating systemic wasting disease but not local intestinal inflammation.

Adoptive Transfer↗

PubMed2001

Determinants of infants' understanding of supporting relations: amount of contact versus position of the center of gravity.

This study investigated the developmental age at which infants recognize about supporting relations between objects and what information they use to judge whether a supported object will fall down or not. Four kinds of events were used. All events involved support in relation of two boxes, which differed in the amount of contact between objects and the amount of discrepancy between the supported object's position and its most balanced position. 115 infants (3 to 13 mo.) saw 4 events which differed on these two variables. Infants 10 months and older looked longer at the event in which the center of a supported box was just outside of the edge of a supporting box, that is, a support relation in which it was difficult to anticipate whether the box would fall down or not. Analysis suggested that infants' attention is not determined by only one simple stimulus variable but by more complicated variables (such as uncertainty of prediction).

Child Development↗

PubMed2000

Determination of enantiomers in a synthetic argininal peptide using capillary zone electrophoresis and high-performance liquid chromatography.

SCH 201781 is a synthetic argininal peptide containing two chiral centers and an aromatic sulfonamide group. It can exist as four reversible forms, the aldehyde, the hydrate, and two diastereomeric aminals. Capillary zone electrophoresis (CZE) and reversed-phase high-performance liquid chromatographic (HPLC) methods were developed to separate and quantitate the enantiomers in SCH 201781. Comparable results were obtained using both methods. The CZE method uses direct injection, while the HPLC method requires a precolumn derivatization and is more time consuming. The CZE method provides superior sensitivity to the HPLC method. Both methods were shown to be precise and reproducible.

Arginine↗

PubMed2000

Vector unpacking as a potential barrier for receptor-mediated polyplex gene delivery.

Ligand-conjugated polymer (polyplex) gene delivery vectors have strong potential as targeted, in vivo gene transfer vehicles; however, they are currently limited by low delivery efficiency. A number of barriers to polyplex-mediated delivery have been previously identified, including receptor binding, internalization, endosomal escape, and nuclear localization. However, based on understanding of viral gene delivery systems, yet another potential barrier may exist; a limited ability to unpackage the plasmid DNA cargo following localization to the nucleus. We have developed a model system that employs a cationic polymer linked to epidermal growth factor (EGF) as a ligand to target delivery of plasmid DNA encoding the green fluorescent protein to mouse fibroblasts bearing the EGF receptor. Using fluorescence microscopy to simultaneously trace both the plasmid and polymer during gene delivery in combination with an in vitro transcription assay, we provide evidence that plasmid unpackaging can indeed be a limiting step for gene expression for sufficiently large polymer constructs. Short-term expression is significantly enhanced by using short polycations that dissociate from DNA more rapidly both in vitro and in vivo. Finally, we describe a thermodynamic model that supports these data by showing that shorter polycations can have a higher probability of dissociating from DNA. This work demonstrates that vector unpackaging should be added to the list of barriers to receptor-mediated polyplex gene delivery, thus providing an additional design principle for targeted synthetic delivery vehicles.

Animals↗

PubMed2000

Synthesis of hierarchical materials.

A major goal of material science is to produce hierarchical materials that are ordered on all length scales, from the molecular (1-100 A) via the nano (10-100 nm) to the meso (1-100 microm). In these materials, the larger-scale properties can be controlled by choosing molecular characteristics. Methods developed to produce three-dimensional, bulk-like hierarchical structures include biomimetic methods, which use polypeptides as building blocks, and amphiphile and colloidal templating, which use amphiphilic or colloidal mesophases as templates for inorganic mesoporous materials. Designing finite mesostructures with a given geometry still remains a challenge.

Animals↗

PubMed1999

Surfactant-mediated two-dimensional crystallization of colloidal crystals

Colloidal particles can form unexpected two-dimensional ordered colloidal crystals when they interact with surfactants of the opposite charge. Coulomb interactions lead to self-limited adsorption of the particles on the surface of vesicles formed by the surfactants. The adsorbed particles form ordered but fluid rafts on the vesicle surfaces, and these ultimately form robust two-dimensional crystals. This use of attractive Coulomb interaction between colloidal particles and surfactant structures offers a potential new route to self-assembly of ordered colloidal structures.

Journal Article↗

PubMed1999

Electrostatic repulsion of positively charged vesicles and negatively charged objects

A positively charged, mixed bilayer vesicle in the presence of negatively charged surfaces (for example, colloidal particles) can spontaneously partition into an adhesion zone of definite area and another zone that repels additional negative objects. Although the membrane itself has nonnegative charge in the repulsive zone, negative counterions on the interior of the vesicle spontaneously aggregate there and present a net negative charge to the exterior. Beyond the fundamental result that oppositely charged objects can repel, this mechanism helps to explain recent experiments on surfactant vesicles.

Journal Article↗

PubMed1998

The structure of DNA complexes with cationic liposomes-cylindrical or flat bilayers?

DNA complexes with cationic lipids promise to be versatile and effective synthetic transfection agents. Recent experiments identified both flat lamellar structures, where DNA strands are sandwiched between lipid bilayers, and cylindrical ones where the DNA is coated by a curved bilayer. Using a simple model we compare the stability of the two structures, and find that flat-bilayer aggregates are always more stable than the cylindrical ones. The different experimental observations are explained within the framework of the model predictions.

Cations↗

PubMed1998

Effect of lipid characteristics on the structure of transmembrane proteins.

The activity of embedded proteins is known to vary with lipid characteristics. Indeed, it has been shown that some cell-membrane proteins cannot function unless certain non-bilayer-forming lipids (i.e., nonzero spontaneous curvature) are present. In this paper we show that membranes exert a line tension on transmembrane proteins. The line tension, on the order of 1-100 kT/protein, varies with the lipid properties and the protein configuration. Thus, membranes composed of different lipids favor different protein conformations. Model predictions are in excellent agreement with the data of Keller et al. (Biophys. J. 1993, 65:23-27) regarding the conductance of alamethicin channels.

Alamethicin↗

PubMed1998

Conserved sequences in enzymes of the UDP-GlcNAc/MurNAc family are essential in hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase.

The UDP-GlcNAc/MurNAc family of eukaryotic and prokaryotic enzymes use UDP-GlcNAc or UDP-MurNAc-pentapeptide as donors, dolichol-P or polyprenol-P as acceptors, and generate sugar-P-P-polyisoprenols. A series of six conserved sequences, designated A through F and ranging from 5 to 13 amino acid residues, has been identified in this family. To determine whether these conserved sequences are required for enzyme function, various mutations were examined in hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT). Scramble mutations of sequences B-F, generated by scrambling the residues within each sequence, demonstrated that each is important in GPT. While E and F scrambles appeared to prevent stable expression of GPT, scrambling of B-D resulted in GPT mutants that could be stably expressed and bound tunicamycin, but lacked enzymatic activity. Further, the C and D scramble mutants had an unexpected sorting defect. Replacement of sequences B-F with prokaryotic counterparts from either the B.subtilis mraY or E.coli rfe genes also affected GPT by preventing expression of the mutant protein (B, F) or inhibiting its enzymatic activity (C-E). For the C-E replacements, no acquisition of acceptor activity for polyprenol-P, the fully unsaturated natural bacterial acceptor, was detected. These studies show that the conserved sequences of the UDP-GlcNAc/MurNAc family are important, and that the eukaryotic and prokaryotic counterparts are not freely interchangeable. Since several mutants were efficiently expressed and bound tunicamycin, yet lacked enzymatic activity, the data are consistent with these sequences having a direct role in product formation.

Amino Acid Sequence↗

PubMed1997

Oligomerization of hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase, an enzyme with multiple transmembrane spans.

Hamster UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT), which initiates N-linked glycosylation by catalyzing the synthesis of GlcNAc-P-P-dolichol, has multiple transmembrane spans and a catalytic site that probably exists on the cytosolic face of the endoplasmic reticulum membrane (Dan, N., Middleton, R. M., and Lehrman, M. A. (1996) J. Biol. Chem. 271, 30717-30725). In this report, we demonstrate that GPT forms functional oligomers, probably dimers. Oligomers were detected by chemical cross-linking of GPT and by a dominant-negative effect caused by co-expression of enzymatically inactive (but properly folded) GPT mutants. The GPT mutants had no effect on two other dolichol-P-dependent endoplasmic reticulum enzymes. Mixing experiments indicated that mature GPT was competent for oligomerization. Oligomerization appeared to be favored in detergent extracts compared with intact microsomes. Detergent treatments were found to prevent, rather than promote, nonspecific aggregation of GPT. These results demonstrate that GPT subunits can physically interact and influence each other. The implications of oligomerization for enzyme function are discussed. From these results, we conclude that GPT is one of a very small number of multitransmembrane span enzymes that can form multimers.

Animals↗

PubMed1997

Multilamellar structures of DNA complexes with cationic liposomes.

Studies of DNA complexes with cationic liposomes are prompted by the search for nonviral DNA carriers for gene therapy. Recent experiments have identified a stable multilamellar phase in which ordered smectic layers of DNA alternate with cationic bilayers. In this paper we identify the forces governing DNA adsorption on cationic lamellae, including a membrane-induced attraction between the adsorbed DNA. Calculating the DNA interhelical spacing as a function of system composition, the model successfully explains recent surprising observations.

Adsorption↗

PubMed1996

Hamster UDP-N-acetylglucosamine:dolichol-P N-acetylglucosamine-1-P transferase has multiple transmembrane spans and a critical cytosolic loop.

UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT) is an endoplasmic reticulum (ER) enzyme responsible for synthesis of GlcNAc-P-P-dolichol, the committed step of dolichol-P-P-oligosaccharide synthesis. The sequence of hamster GPT predicted multiple transmembrane segments (Zhu, X., and Lehrman, M. A. (1990) J. Biol. Chem. 265, 14250-14255). GPT has also been predicted to act on the cytosolic face of the ER membrane, based on topological studies of its substrates and products. In this report we test these predictions by: (i) immunofluorescence microscopy with antibodies specific for native GPT sequences or epitope tags inserted into GPT, after selective permeabilization of the plasma membrane with digitonin; (ii) insertion of Factor Xa cleavage sites; (iii) in vitro translation of GPT; and (iv) site-directed mutagenesis. The loops between the 1st and 2nd and between the 9th and 10th predicted transmembrane spans of GPT were found to be cytosolic. In contrast, the loop between the 6th and 7th transmembrane spans, as well as the carboxyl terminus, were lumenal. Thus, hamster GPT must cross the ER membrane at least three times, consistent with previous computer-assisted predictions. There was no apparent N-glycosylation or signal sequence cleavage detected by in vitro translation. The cytosolic loop between the 9th and 10th transmembrane spans is the largest hydrophilic segment in GPT and, as judged by site-directed mutagenesis, has a number of conserved residues essential for activity. Hence, these results directly support the hypothesis that dolichol-P-P-oligosaccharide assembly is initiated in the cytosol and that a downstream intermediate must translocate to the lumenal face of the ER membrane.

Amino Acid Sequence↗

PubMed1996

Interaction between inclusions embedded in membranes.

We calculate the membrane-induced interaction between inclusions, in terms of the membrane stretching and bending moduli and the spontaneous curvature. We find that the membrane-induced interaction between inclusions varies nonmonotonically as a function of the inclusion spacing. The location of the energy minimum depends on the spontaneous curvature and the membrane perturbation decay length, where the latter is set by the membrane moduli. The membrane perturbation energy increases with the inclusion radius. The Ornstein-Zernike theory, with the Percus-Yevick closure, is used to calculate the radial distribution function of inclusions. We find that when the spontaneous curvature is zero, the interaction between inclusions due to the membrane deformation is qualitatively similar to the hard-core interaction. However, in the case of finite spontaneous curvature, the effective interaction is dramatically modified.

Kinetics↗