PubMed Health⌕ Search

Biomedical subjects

S G Elliott

Publications and source records attributed to S G Elliott.

13 recordsLinked to original sources
PubMed1997

Homodimerization of erythropoietin receptor by a bivalent monoclonal antibody triggers cell proliferation and differentiation of erythroid precursors.

Erythropoietin (EPO) stimulates proliferation and differentiation of erythroid progenitor cells. Several lines of evidence indicate that the most likely mechanism of EPO receptor (EPO-R) activation by EPO is homodimerization of the receptor on the surface of erythrocyte precursors. Therefore, we argued that it should be possible to raise EPO-R monoclonal antibodies (MoAbs) that would activate the receptor by dimerization and thus mimic EPO action. We have identified such an agonist MoAb (MoAb34) directed against the extracellular EPO binding domain of the EPO-R. This bivalent IgG antibody triggers the proliferation of EPO-dependent cell lines and induces differentiation of erythroid precursors in vitro. In contrast, the monovalent Fab fragment, which cannot dimerize the receptor, is completely inactive. The mechanism of receptor activation by homodimerization implies that at high ligand concentrations the formation of 1:1 receptor/ligand complexes is favored over 2:1 complexes, thereby turning the ligand agonist into an antagonist. Thus, EPO and MoAb34 should self-antagonize at high concentrations in both cell proliferation and differentiation assays. Our data indeed demonstrate that EPO and MoAb34 antagonize ligand-dependent cell proliferation with IC50 values of approximately 20 and 2 mumol/L, respectively. Erythroid colony formation (BFUe) is inhibited at MoAb34 concentrations above 1 mumol/L. Furthermore, we analyzed the MoAb34:EPO-R interaction using a mathematic model describing antibody-mediated receptor dimerization. The data for proliferation and differentiation activity were consistent with the receptor dimer formation on the cell surface predicted by the model.

Animals↗
PubMed1983

Coordination of growth with cell division: regulation of synthesis of RNA during the cell cycle of the fission yeast Schizosaccharomyces pombe.

In the fission yeast Schizosaccharomyces pombe the rate of RNA synthesis, as determined by pulse labeling, increases in a step-like manner in synchronous cultures prepared by centrifugal elutriation. Cultures prepared in this way show marked reductions in perturbations which can be caused by many synchrony techniques. Kinetic evidence indicates that alterations in pool metabolism are not responsible for the step pattern. The long period of increase in the rate doubling (relative to cell number increase) indicates that the period of increasing rate may be due to a growth period and not a sudden transition between a slow and a fast rate. An analysis of synchronous cultures of cells of different cell size and synchronous cultures of temperature sensitive mutants blocked in cell cycle progress indicated that neither size control, changes in DNA content nor septation are directly responsible for the steps in RNA synthesis. Instead the time of the rate change is associated with a specific point in the cell cycle, probably an event associated with nuclear division.

Ascomycota↗
PubMed1983

Regulation of the maximal rate of RNA synthesis in the fission yeast Schizosaccharomyces pombe.

Of interest to many biologists is how growth, e.g., RNA synthesis, and cell division are mutually controlled. One method of establishing the nature of the control is to determine what "factors" are limiting when cells synthesize RNA at a maximal rate. The transcription maximum (maximum rate of RNA synthesis) has been determined in cell division mutants that continue to grow but fail to divide to determine if there is a cell cycle control over RNA synthesis. There is no correlation between transcription maximum and DNA synthesis or septation which suggests that these events do not exert a direct cell cycle control over RNA synthesis in exponentially growing cells. In addition, the lack of strong correlation between the transcription maximum and cell size or gene dosage indicates that the rate of RNA synthesis is not directly regulated by either of these parameters. The possibility that the maximum rate is determined by a concentration effect of an end product which acts in the nucleus, such as a specific RNA or protein, could not be ruled out and evidence is presented in support of such a model.

Ascomycota↗
PubMed1983

Absence of step changes in activity of certain enzymes during the cell cycle of budding and fission yeasts in synchronous cultures.

Synchronous cultures prepared by selection from an elutriating rotor were used to measure activity changes during the cell cycle of the following enzymes: acid phosphatase in Schizosaccharomyces pombe and Saccharomyces cerevisiae, alpha-glucosidase in S. cerevisiae and beta-galactosidase in Kluyveromyces lactis. There was no sign of step rises in activity in acid phosphatase but there were indications in S. cerevisiae of the linear pattern with rate doublings once per cycle that had been found previously in S. pombe. There was also no sign of step rises in the other two enzymes, in contrast to earlier results using different techniques. Asynchronous control cultures showed little or no perturbations after the first hour.

Acid Phosphatase↗
PubMed1982

Synthesis of specific identified, phosphorylated, heat shock, and heat stroke proteins through the cell cycle of Saccharomyces cerevisiae.

The methods of centrifugal elutriation, two-dimensional gel electrophoresis, and dual isotopic labeling were applied to the study and identification of a number of purified yeast proteins. The location of polypeptide spots corresponding to specific proteins was determined on two-dimensional gels. A dual-label method was used to determine the rates of synthesis through the cell cycle of the identified proteins as well as to confirm the results of previous studies from our laboratory on unidentified proteins. The identified proteins, and the more generally defined phosphorylated, heat shock, and heat stroke proteins were found to follow the general pattern of exponential increase in rate of synthesis through the cell cycle. In addition, colorimetric enzyme activity assays were used to examine the catabolic enzyme alpha-glucosidase (EC 3.2.1.20). Both the activity and synthesis of alpha-glucosidase were found to be nonperiodic with respect to the cell cycle. These data contrast with earlier reports of periodicity, which employed induction and selection synchrony to study enzyme expression through the yeast cell cycle.

Cell Cycle↗
PubMed1981

Replication of double-stranded RNA of the virus-like particles in Saccharomyces cerevisiae.

The mode of replication of the L double-stranded RNA (dsRNA) present in virus-like particles in Saccharomyces cerevisiae was examined by density transfer experiments. After transfer to light medium, significant amounts of fully heavy dsRNA persisted over a number of cell doublings. In addition, very little material of hybrid density was ever formed, and the accumulation of fully light material began as early as 0.5 doubling after transfer to light medium. Our results are compatible with a conservative mode of replication or with a semiconservative mode of replication carried out by a small portion of the total dsRNA population. In additional experiments the synthesis of dsRNA relative to the cell cycle was studied. This was done by determining the ratio of short-term to long-term radioactive label in size-separated cell fractions of a prelabeled exponential culture. The ratio of short-term to long-term label remained constant for all fractions, implying that dsRNA is synthesized throughout the cell cycle, increasing through the cell cycle at an exponential rate.

Cell Cycle↗
PubMed1979

Regulation of RNA synthesis in yeast. III. Synthesis during the cell cycle.

Centrifugal elutriation was used to separate cells in different stages of the cell cycle from a culture of Saccharomyces cerevisiae in balanced exponential growth. The rate of DNA and RNA synthesis was determined using a pulse--long-term label technique that is capable of distinguishing between exponential, linear, and periodic variations in the rate of synthesis through the cell cycle. It was found that while the rate of DNA synthesis varies periodically through the cell cycle, the rate of synthesis of mRNA, rRNA, and tRNA increases exponentially through the cell cycle. The implications of these findings for the control of RNA synthesis are discussed.

Cell Cycle↗
PubMed1979

Synthesis and modification of proteins during the cell cycle of the yeast Saccharomyces cerevisiae.

We have used a novel technique to study the synthesis, modification and degradation of proteins during the cell cycle in Saccharomyces cerevisiae. Logarithmically growing cells were pulse-labeled twice, with the pulses separated in time by more than one generation. Subsequently, the cells were fractionated as to their position in the cell cycle by centrifugal elutriation, and for different proteins the ratio of radioactive material from the two pulses was then determined. Periodic degradation, synthesis, or modification would produce periodic variations in the ratio of counts. Two-dimensional gel electrophoresis was used to examine 110 different proteins at different times of the cell cycle. All but two proteins had a constant ratio of counts through the cell cycle. This indicates that the rate of synthesis of individual proteins increases exponentially during the cell cycle and that periodic degradation or modification of proteins is not a general feature of the cell cycle in S. cerevisiae.

Cell Cycle↗
PubMed1978

Rate of macromolecular synthesis through the cell cycle of the yeast Saccharomyces cerevisiae.

Centrifugal elutriation was used to separate cells of Saccharomyces cerevisiae in balanced exponential growth according to position in the cell cycle. Macromolecular synthesis was examined. DNA synthesis was found to be periodic, but RNA and protein synthesis showed an exponential increase in rate. Two-dimensional electrophoresis was used to determine the rate of synthesis of individual proteins, with 111 of the more abundant cellular proteins selected for analysis from among the more than 1000 proteins that migrate in the system. All the examined proteins showed an exponentially increasing rate of synthesis.

Cell Cycle↗