PubMed Health⌕ Search

Biomedical subjects

G Isenberg

Publications and source records attributed to G Isenberg.

At least 181 records · Page 10Linked to original sources
PubMed1981

Interaction of alpha-actinin and vinculin with actin: opposite effects on filament network formation.

The interaction of actin filaments with two actin-associated proteins, alpha-actinin and vinculin (Mr 130,000 protein), was studied in vitro with viscometry and light and electron microscopy. Vinculin, like alpha-actinin, binds to F-actin, and the two proteins were found to have different effects on the formation of filament networks: alpha-actinin crosslinks individual filaments in a manner strongly dependent on temperature and acts as a spacer, whereas vinculin forms actin bundles that display a paracrystalline substructure. In viscometric assays, alpha-actinin mimics the effect of actin gelation factors, whereas vinculin acts as a gelation inhibitor. These findings imply complementary functions of these proteins in the regulation of cellular mobility.

Actinin↗
PubMed1981

Crystalline actin sheets: their structure and polymorphism.

Crystalline sheets of Acanthamoeba actin induced by the trivalent lanthanide gadolinium exist in three different polymorphic forms, which show different striation patterns and surface topographies. We have called these different forms "rectangular" and "square" sheets, and "cylinders" and have shown that each of the three forms is constructed from common "basic" lattices associated in different ways. We have used image processing of electron micrographs to obtain a model for the actin molecule in projection to a resolution of 1.5 nm. The overall dimensions observed in these images are 5.6 x 3.3 x 4.5 nm, and the molecule itself appears distinctly bilobed with the two lobes separated by a cleft. actin monomers in the sheets are arranged with P2 symmetry and are therefore packed in a manner different from that of the molecules in actin filaments. Because approximately 35% of the surface area of the actin molecule is exposed on the surface of these sheets, the sheets should be useful to study the stoichiometric binding of actin-binding proteins to the actin molecule.

Actins↗
PubMed1980

Structure of crystalline actin sheets.

Although actin is one of the most abundant proteins found in nature, little detailed information about its molecular structure is available beyond the amino acid sequence. Electron microscopy of negatively stained filaments combined with three-dimensional image reconstruction techniques have revealed the overall size and shape of the actin monomer at 25 A resolution. Higher resolution structural data can be expected from electron microscopy of two-dimensional crystalline arrays and X-ray diffraction analysis of three-dimensional crystals, but only very preliminary results have been reported so far. The original finding by Dos Remedios and Dickens was that skeletal muscle actin forms microcrystals and tubes in the presence of the trivalent lanthanide gadolinium (Gd3+). We have modified and refined their conditions to obtain large crystalline sheets of Acanthamoeba actin and present here a model of the actin monomer in projection to 15 A resolution. We have found that, depending on the ionic strength used, these sheets occur in three different forms: 'cylinders', 'square type' sheets and 'rectangular type' sheets. These different polymorphic forms are built from the same fundamental two-dimensional crystalline actin lattice, which we call the 'basic sheet'. The present concerns the structural analysis of these basic sheets; the crystal polymorphism will be discussed in detail elsewhere (U.A. et al., in preparation). Furthermore, in addition to demonstrating that actin is an elongated globular molecule with a pronounced asymmetric shape in and perpendicular to the plane of the sheet, our results indicate that these crystalline actin sheets might be suitable for three-dimensional structure determination by low-dose electron microscopy of unstained specimens to at least 10 A resolution.

Actins↗
PubMed1980

Glycocalyx is not required for show inward calcium current in isolated rat heart myocytes.

The importance of the slow inward calcium current (Isi) in the excitation-contration coupling process of cardiac muscle is well documented. The current can be attributed mainly to a calcium translocation from the extracellular space into the cell or a subsarcolemmal compartment of it. Isi has been suggested to have its source in and to be controlled by the surface coat of the sarcolemma (glycocalyx). The glycocalyx is destroyed in myocytes dissociated from adult heart tissue with solutions containing low calcium, collagenase and hyaluronidase (Fig. 1). By comparing the Isi data obtained in isolated myocytes with those reported for trabeculae or papillary muscles, we have now obtained evidence suggesting that the glycocalyx is not important in the genesis of Isi.

Animals↗
PubMed1980

K efflux through inward rectifying K channels in voltage clamped Purkinje fibers.

The 42K efflux was measured in voltage clamped sheep Purkinje fibers. The voltage dependence of the K efflux can be described as N-shaped, showing a negative slope region. At potentials negative to -30mV, the K efflux is largely due to K flowing through a channel which rectifies in the inward direction and which is blocked by external application of 20mM Cs+. At potentials positive to -30mV an outward rectifier dominates the shape of the K efflux-voltage relationship. This component is insensitive to short external application of Cs+. Both components were also found when Na+ was replaced by tetramethylammonium. When the steady-state current-voltage relationship is compared with the K efflux one can conclude that the outward rectifying K flux largely determines the shape of this curve at positive membrane potentials, while the negative slope region of the K efflux correlates with the negative slope of the steady-state current-voltage relation. The K efflux is only slightly enhanced by stimulation of the preparation, corroborating the finding of inward-going rectification of the K channel. A clamp program repetitively activating the positive dynamic current e.g. by alternating the membrane potential between -70 and +10mV, increases the K efflux by about 50% as compared to the efflux measured in steady-state at this positive membrane potential. 4-Aminopyridine suppresses both this extra K efflux and the positive dynamic current. It is concluded that K ions contribute to the positive dynamic current.

Animals↗
PubMed1980

Definition of a cell clone with astroglial characteristics derived from a chemically induced rabbit brain glioma.

Three-month-old rabbits were started on a fortnightly schedule of intravenous injections of N-methyl-N-nitrosourea. All but two of the central nervous system tumors induced in this manner were propagated in culture as permanent cell lines. On the 76RB-G-414-H line established from a grade 2 astrocytoma of this series of neoplasms, a cloning procedure was carried out using a laser microbeam. The clonal line originated in this way has been maintained in long-term culture and given the 76/RB-G-414-H-C designation. The cells of the clone display invariably a bipolar or multipolar configuration with long processes. Intermediate filaments are common and even abundant in some cells. Positivity for S-100 and GFA proteins is a regular finding in these cells. In addition, dibutyryl cyclic adenosine monophosphate treatment reduces cell division and stimulates cell process formation of these cells. Thus, it appears that we succeeded in establishing in vitro and maintaining in long-term culture a clone of tumor cells with astrocytic characteristics.

Animals↗
PubMed1979

Risk and advantages of using strongly beveled microelectrodes for electrophysiological studies in cardiac Purkinje fibers.

Conventional microelectrodes (tips with a diameter of 0.5 micrometer, a resistance of 8 MOhm, and a tip potential of -4 mV) were mechanically beveled over a length of 1--2 micrometer (resistance 2.5 MOhm, tip potential between 0 and -1 mV). Properties thought to be relevant for intracellular techniques were studied on the cardiac Purkinje fiber. The comparison with conventional microelectrodes suggests that beveled microelectrodes have an advantage as stated: 1. Intracellular impalement is favoured by the proper shape and the greater mechanical stability; the beveled tip penetrates the connective tissue smoothly without breaking or plugging. 2. Current injection (constant current mode) can be done without blocking or polarizing the tip. The 2 microelectrode voltage clamp technique (Deck et al., 1964) is improved by having lower noise and better stability, by a faster response time, and a greater range of clamp potentials (up to +80 mV). 3. The spontaneous release of the electrolytes filling the microelectrode has a 3-fold greater rate (0.015 pMol/s); this enlargement does not change the electrophysiological properties of the fiber. 4. Intracellular pressure injection requires pressures of 0.3--1.5 bar only to inject the solutes with rates between 1 and 100 pMol/s.

Animals↗
PubMed1978

Correlation between actin polymerization and surface receptor segregation in neuroblastoma cells treated with concanavalin A.

In response to concanavalin A (Con A), neuroblastoma cells undergo marked morphological changes which involve the retraction of neurites and the induction of broad and extensive lamellar regions around the cell periphery. From the use of FITC-Con A it was shown that the membrane formed on the induced lamellar regions lacked receptors to Con A from the onset of lamella formation. These receptors were confined to the cell body; they initially showed a uniform distribution and were subsequently collected into patches and finally into aggregates or caps. When the aggregates occurred on the cell periphery their position coincided with areas free of lamellae. Investigations of the lamellar regions in Triton-extracted cell monolayers showed them to consist of a meshwork of actin filaments containing radiating thin filament bundles or microspikes. With increasing time in the presence of Con A there was a progressive increase in the number of radiating microspikes. Previous studies have shown the actin in these lamellar regions to be singly polarized with respect to the cell body. From the segregation of Con A receptors away from areas of actin polymerization in the lamellae it is concluded that actin is involved in some indirect way in surface receptor movement.

Actins↗