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Biomedical subjects

R Nossal

Publications and source records attributed to R Nossal.

At least 19 recordsLinked to original sources
PubMed2001

Energetics of clathrin basket assembly.

A minimal thermodynamic model is used to study the in vitro equilibrium assembly of reconstituted clathrin baskets. The model contains parameters accounting for i) the combined bending and flexing rigidities of triskelion legs and hubs, ii) the intrinsic curvature of an isolated triskelion, and iii) the free energy changes associated with interactions between legs of neighboring triskelions. Analytical expressions for basket size distributions are derived, and published size distribution data (Zaremba S, Keen JH. J Cell Biol 1983;97: 1339-1347) are then used to provide estimates for net total basket assembly energies. Results suggest that energies involved in adding triskelions to partially formed clathrin lattices are small (of the order of kBT), in accord with the notion that lattice remodeling during basket formation occurs as a result of thermodynamic fluctuations. In addition, analysis of data showing the effects of assembly proteins (APs) on basket size indicates that the binding of APs increases the intrinsic curvature of an elemental triskelial subunit, the stabilizing energy of leg interactions, and the effective leg/hub rigidity. Values of effective triskelial rigidity determined in this investigation are similar to those estimated by previous analysis of shape fluctuations of isolated triskelia.

Clathrin↗
PubMed2000

Rigidity of triskelion arms and clathrin nets.

Statistical analysis is applied to a set of electron micrographic images (Kocsis, E., B. L. Trus, C. J. Steer, M. E. Bisher, and A. C. Steven. 1991. J. Struct. Biol. 107:6-14), from which quantitative measures are obtained to support the notion that the three arms of a triskelion have statistically identical properties and exhibit independent structural fluctuations. Additionally, a study of local contour fluctuations, which indicates that the elastic properties of a triskelion arm are approximately constant over the entire arm length, is used along with a small deformation statistical mechanics theory to derive an effective, average flexural rigidity for the arms. This result is used to estimate the bending energy necessary to deform a clathrin patch, and comparison is made with the deformation energy of an equivalent area of non-clathrin-coated membrane. We estimate that the rigidity of the clathrin lattice is at least comparable to that of a typical membrane. Hence, the natural curvature of a clathrin cage can stabilize, and perhaps propel, the formation of intracellular coated vesicles.

Clathrin↗
PubMed1999

A singular state of membrane lipids at cell growth temperatures.

Cells adjust their membrane lipid composition when they adapt to grow at different temperatures. The consequences of this adjustment for membrane properties and functions are not well understood. Our report shows that the temperature dependence of the diffusion of a probe molecule in multilayers formed from total lipid extracts of E. coli has an anomalous maximum at a temperature corresponding to the growth temperature of each bacterial preparation (25, 29, and 32 degrees C). This increase in the lateral diffusion coefficient, D, is characteristic of membrane lipids in a critical state, for which large fluctuations of molecular area in the plane of the bilayer are expected. Therefore, changes in lipid composition may be due to a requirement that cells maintain their membranes in a state where molecular interactions and reaction rates are readily modulated by small, local perturbations of membrane organization.

Cell Division↗
PubMed1999

Light scattering characterization of extruded lipid vesicles.

By modeling extruded unilamellar lipid vesicles as thin-walled ellipsoidal shells, mathematical analysis provides simple equations which relate the mean elongation and other morphological characteristics of a vesicle population to quantities readily obtained from combined static and dynamic light scattering measurements. For SOPC vesicles extruded through a 100 nm pore-size filter into a 72.9 mM NaCl solution, the inferred elongation ratio (vesicle long axis to short axis) is approximately 3.7 +/- 0.6. When these vesicles were dialyzed into hypertonic or hypotonic solutions, this elongation ratio varied from 1 (for spherical liposomes) in strongly hypotonic solutions to greater than 6 in increasingly hypertonic solutions, beyond which abrupt morphological transformations appear. These results are quantitatively consistent with a mechanism of vesicle formation by extrusion and with the expectation that vesicle volumes change to equalize internal and external osmolarity via water flow, subject to the constraint of constant bilayer area. Our analysis also provides simplified equations to assess the effects of vesicle elongation and polydispersity on liposome parameters that are commonly required to characterize vesicle preparations for diverse applications. The implications of this study for routine light scattering characterization of extruded vesicles are discussed.

Biophysical Phenomena↗
PubMed1998

Cell transit analysis of ligand-induced stiffening of polymorphonuclear leukocytes.

A mathematical treatment of the mechanical behavior of transiently bonded polymer networks is used to interpret measurements of the pressure-induced passage of plant cells through microporous membranes. Cell transit times are inferred to be proportional to the instantaneous shear modulus of the cell cortex, a parameters that we then relate to properties of the cortical F-actin matrix. These theoretical results are used to analyze published data on chemoattractant-induced changes of rigidity of polymorphonuclear leukocytes. We thereby rationalize previously noted, peculiar, power-law logarithmic dependences of transit time on ligand concentration. As a consequence, we are able to deduce a linear relationship between the extent of F-actin polymerization and the logarithm of the chemoattractant concentration. The latter is examined with regard to the G-protein activation that is known to occur when chemoattractants bind to receptors on the surfaces of polymorphonuclear cells.

Actins↗
PubMed1996

Dynamics of temperature dependent optical properties of tissue: dependence on thermally induced alteration.

Thermal damage in heated bovine myocardial tissue is assessed from measured changes in total reflection and transmission of light. Mathematical expressions, based on random walk analysis of light propagation within tissue slabs, are used to relate the diffuse reflection and transmittance to the absorption coefficient, mu a, and effective scattering coefficient, mu's, for samples of myocardial tissue which were subjected to rapid step changes in temperature. Time-dependent changes in mu's, indicate two processes, one with a fast and temperature-dependent rate the other with a slow and apparently temperature-independent rate. For final temperatures above 56.8 degrees C and for the first 500 s after the temperature change, the optical parameters are well fit by exponential forms that exhibit temperature-dependent time constants as predicted by Arrhenius reaction rate theory of thermal damage. The scattering changes are associated with an apparent activation energy, delta E, of 162 kJ/mole and a frequency constant, A, of 3 x 10(23) s-1. This method provides a means for estimating optical coefficients which are needed to assess laser tissue dosimetry.

Absorption↗
PubMed1996

Point spread functions of photons in time-resolved transillumination experiments using simple scaling arguments.

Simple scaling arguments are used to determine spatial resolution achievable in time-resolved transillumination experiments involving highly diffuse media. These arguments allow us to obtain relationships linking target resolution at different planes inside an optically turbid slab to the gating times of the imaging system. We show that this approach yields the same results as those obtained previously from an approximate and rather complicated analytical derivation. In addition, we are now able to assess the effects of scattering anisotropy on spatial resolution attainable when gating times are so short that a constant scaling of photon transport scattering length is not appropriate. These results should enable one to devise more accurate and simpler image reconstruction algorithms.

Algorithms↗
PubMed1994

Small angle neutron scattering studies of structural characteristics of agarose gels.

The 30 m small angle neutron scattering facility at the National Institutes of Standards and Technology has been used to examine neutron scattering from agarose gels formed in D2O. Differential scattering cross sections have been acquired over a continuous range of Q between 0.005 and 0.3 A-1. Subtle changes in gel structure are observed when pre-gelation agarose concentration is varied. Similarly, except when the gelling solution is rapidly cooled to a low temperature, the rate at which the gels are formed does not seem to have much effect. Clearer evidence of structural rearrangement is observed when the solvent quality is changed by the addition of dimethyl sulfoxide, or when the temperature of the gel is elevated above 70 degrees C. These data are consistent with a description of a randomly structured polymer network containing discrete self-similar, hydrogen-bonded, junctions normally of minimal thickness approximately 35-40 A.

Carbohydrate Conformation↗
PubMed1994

Resolution limits for optical transillumination of abnormalities deeply embedded in tissues.

Random walk theory is used to calculate the line spread function (LSF) of photons as they cross the midplane of a slab of finite thickness. The relationship between the LSF and the photon transit time in transillumination time-resolved experiments is investigated. It is found that the LSF is approximately Gaussian distributed, with a standard deviation, sigma, which can be used as a criterion of the spatial resolution of the imaging system. Results are substantiated by comparison with actual data in the literature. Any given resolution can be improved by reducing the excess transit time delta t, but heterogeneity of the scattering medium and low levels of detected light enormously complicate the achievement of subcentimeter spatial resolution. The latter point is discussed by using optical parameters of breast tissues for visible and near-infrared radiation (NIR) light.

Breast Diseases↗
PubMed1993

Rheological effects of the presence of hyaluronic acid in the extracellular media of differentiated 3T3-L1 preadipocyte cultures.

The viscoelastic properties of culture medium obtained from confluent 3T3-L1 preadipocytes, after differentiation with isobutyl-methylxanthine and dexamethasone, were studied with a rotational Couette viscometer. In close association with adipocyte differentiation, the culture medium showed gel-like properties, in concert with an increase in viscosity. This behavior vanishes after digestion by Streptomyces hyaluronidase or chondroitinase ABC, but not after application of collagenase, pronase, trypsin, DNase, or neuraminidase, or by treatment with EDTA or mercaptoethanol, indicating that the primary substance responsible for this behavior is hyaluronic acid. The material revealed a non-Newtonian behavior with an irreversible disruption of the network by shear force at high speeds. The viscosity of the medium, containing about 1 microgram/ml of hyaluronic acid, was calculated to be similar to that of a solution containing 1.7 mg high molecular weight hyaluronic acid per milliliter of stock culture medium. The comparison of rheological properties between the culture medium and solutions of hyaluronic acid indicated the possibility of a highly organized network in the culture medium that is more complicated than a simple interaction between homologous hyaluronic acid molecules. The non-Newtonian behavior depends on the hyaluronic acid concentration in the medium as well as on the length of exposure of the 3T3-L1 cells to the isobutyl-methylxanthine/dexamethasone mixture. The results point toward the possibility of interaction between hyaluronic acid and binding proteins.

1-Methyl-3-isobutylxanthine↗
PubMed1993

Topological mechanisms involved in the formation of clathrin-coated vesicles.

By examining the basic characteristics of clathrin lattices, we discover that simple topological rules impose strict constraints on clathrin lattice transformations. These constraints require that internal bond rearrangements take place in conjunction with the addition or removal of pairs of clathrin triskelions within the interior of existing clathrin lattice patches. Similar constraints also are relevant to coated-vesicle shape changes and their budding-off from pit lattices. Via specific illustrations, successive vesicles with hexagonal-barrel and other coats are shown to grow out from the interior of a initially flat clathrin-coated pit so long as free triskelions are available from cytoplasm. Concomitantly, we present mathematical derivations of several simple and useful topological equations. These equations govern the numbers of nonhexagonal clathrin lattice facets and their variations during internal shape transformations and justify the proposed mechanisms of triskelion pair insertion and removal.

Animals↗
PubMed1990

Small angle neutron scattering studies of HbA in concentrated solutions.

Differential cross-sections for neutrons scattered by normal human hemoglobin have been determined over the range of concentrations from 2 to approximately 35 weight percent. Data are compared with structure factors calculated from models of monodisperse hard spheres interacting through a screened Coulomb potential. Good agreement is noted when the volume fraction eta is adjusted during multivariate fitting of data, but the fitted value of eta is always lower than expected from the known Hb concentration of the samples. Calculations of cross-sections for polydisperse scatterers suggest that the samples may contain oligomers of the fundamental tetrameric Hb molecule.

Carboxyhemoglobin↗
PubMed1988

SANS studies of interacting hemoglobin in intact erythrocytes.

Small angle neutron scattering (SANS) was used to investigate interaction forces between hemoglobin (Hb) molecules contained within human red cells. The scattering separately attributable to cell membranes and intracellular Hb was identified. A series of D2O-H2O contrast variation measurements were made in order to establish conditions for which scattering from the cell membrane is minimized (approximately 15% D2O). Measurements then were performed to examine changes in intermolecular Hb interactions occurring when the cells are contracted or swollen by varying the ionic strength of the suspension buffer. The scattering cross-sections were fitted to structure factors computed by a mean spherical approximation, and molecular parameters thereby extracted. Oxygenation studies on normal cells were performed, and results contrasted with those of similar studies of erythrocytes obtained from sickle cell disease patients.

Anemia, Sickle Cell↗
PubMed1988

On the elasticity of cytoskeletal networks.

Models relating to the gelation and elasticity of complex cytoskeletal networks are formulated and investigated. Kinetic equations for reversible elongation of nucleated actin filaments are analyzed when the filaments are acted upon by capping proteins and cross-linking factors. Analytical expressions are obtained that relate the low frequency elastic shear modulus of a network, G, to chain growth kinetics, the number of nucleation sites, monomer concentration, and the amount of capping and cross-linking protein. Elasticity curves that relate G to such factors as the association constant for cross-linking are derived and then used to determine solation-gelation phase contours.

Actins↗
PubMed1988

[Diagnosis and therapy of hyponatremic syndromes].

Hyponatremia is the most common abnormality in electrolyte and water metabolism. In adult patients it is related to high morbidity and mortality. The degree of CNS-damage depends 1st on the absolute serum sodium concentration (NaS) and 2nd on the rapidity with which NaS is lowered. The most frequent etiology of hyponatremia in pediatric patients is dilutional hyponatremia (SIADH, infusion-therapy). Nephrotic syndromes and congestive heart insufficiency associated with cardiac low output are further causes. Being aware of the different pathophysiological mechanism prevention of hyponatremia is easily achieved by monitoring serum electrolytes, water balance and compensating renal factors in critically ill patients. Hyponatremias accompanied by neurological symptoms should be corrected by rapid infusion of hypertonic saline (514 mmol/l). NaS concentration should increase at a rate of 2 mmol/1 hour. Symptoms of central pontine myelinolysis in hyponatremia were not yet described in pediatric patients.

Child↗
PubMed1987

Model for photon migration in turbid biological media.

Various characteristics of photon diffusion in turbid biological media are examined. Applications include the interpretation of data acquired with laser Doppler blood-flow monitors and the design of protocols for therapeutic excitation of tissue chromophores. Incident radiation is assumed to be applied at an interface between a turbid tissue and a transparent medium, and the reemission of photons from that interface is analyzed. Making use of a discrete lattice model, we derive an expression for the joint probability gamma(n, rho)d2 rho that a photon will be emitted in the infinitesimal area d2 rho centered at surface point rho = (x, y), having made n collisions with the tissue. Mathematical expressions are obtained for the intensity distribution of diffuse surface emission, the probability of photon absorption in the interior as a function of depth, and the mean path length of detected photons as a function of the distance between the site of the incident radiation and the location of the detector. We show that the depth dependence of the distribution of photon absorption events can be inferred from measured parameters of the surface emission profile. Results of relevant computer simulations are presented, and illustrative experimental data are shown to be in accord with the theory.

Computer Simulation↗