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Biomedical subjects

Stephen P Bell

Publications and source records attributed to Stephen P Bell.

7 recordsLinked to original sources
PubMed2003

Endothelin receptor blockade has an oxygen-saving effect in Dahl salt-sensitive rats with heart failure.

The effects of endothelin (ET) receptor blockade on energy utilization in heart failure (HF) are unknown. We administered ET type A (ETA), ET type B (ETB), and ETA/ETB antagonists to isolated hearts from Dahl salt-sensitive (DS) rats with HF and controls. Contractile efficiency was assessed as slope-1 of myocardial O consumption (VO2)-pressure-volume area relation. In HF, ETA and ETA/ETB but not ETB blockade decreased the contractility index (Emax)(-15 +/- 3% and -17 +/- 2%, P < 0.05), excitation-contraction (E-C) coupling VO2 (-39 +/- 4% and -37 +/- 5%, P < 0.01), and efficiency (-15 +/- 4% and -17 +/- 2%, P < 0.05). Despite decreased efficiency, ETA and ETA/ETB blockade decreased total VO2 (-24 +/- 3% and -22 +/- 2%, P < 0.05). Na+/H+ exchanger inhibition decreased Emax and E-C coupling VO2 similar to ETA and ETA/ETB blockade, but did not alter efficiency. In HF, endogenous ET-1 maintains contractility at expense of increased VO2 through ETA receptor activation, likely mediated by Na+/H+ exchange.

Amiloride↗
PubMed2002

Cell-cycle control of the establishment of mating-type silencing in S. cerevisiae.

Transcriptional silencing in Saccharomyces cerevisiae involves the assembly of a heterochromatic domain that is heritable from generation to generation. To maintain the silenced state, the propagation of silent heterochromatin must be coordinated with the events of chromosome duplication and segregation. Here we present an in vivo analysis of the cell-cycle events required for the establishment of the silenced state at the HMR silent mating-type locus. We show that Sir protein recruitment to and spreading from the HMRE silencer is poor during S phase, but is robust during G2 and subsequent phases. Despite abundant Sir protein association in cells arrested in G2/M phase, silencing is not fully established by this stage of the cell cycle. Rather, robust silencing is not observed until telophase. Interestingly, the elimination of the cohesin subunit Scc1/Mcd1p allows the full establishment of silencing to occur in G2/M phase. Furthermore, expression of a noncleavable allele of Scc1/Mcd1p inhibits the establishment of silencing. Our findings reveal both S- and M-phase requirements for the establishment of silencing and implicate the loss of sister-chromatid cohesion as a critical event in this process.

Cell Cycle↗
PubMed2002

Visualization of replication initiation and elongation in Drosophila.

Chorion gene amplification in the ovaries of Drosophila melanogaster is a powerful system for the study of metazoan DNA replication in vivo. Using a combination of high-resolution confocal and deconvolution microscopy and quantitative realtime PCR, we found that initiation and elongation occur during separate developmental stages, thus permitting analysis of these two phases of replication in vivo. Bromodeoxyuridine, origin recognition complex, and the elongation factors minichromosome maintenance proteins (MCM)2-7 and proliferating cell nuclear antigen were precisely localized, and the DNA copy number along the third chromosome chorion amplicon was quantified during multiple developmental stages. These studies revealed that initiation takes place during stages 10B and 11 of egg chamber development, whereas only elongation of existing replication forks occurs during egg chamber stages 12 and 13. The ability to distinguish initiation from elongation makes this an outstanding model to decipher the roles of various replication factors during metazoan DNA replication. We utilized this system to demonstrate that the pre-replication complex component, double-parked protein/cell division cycle 10-dependent transcript 1, is not only necessary for proper MCM2-7 localization, but, unexpectedly, is present during elongation.

Animals↗
PubMed2002

Changes in titin isoform expression in pacing-induced cardiac failure give rise to increased passive muscle stiffness.

BACKGROUND: Titin contains a molecular spring segment that underlies passive myocardial stiffness. Myocardium coexpresses titin isoforms with molecular spring length variants and, consequently, distinct stiffness characteristics: the stiff N2B isoform (short spring) and more compliant N2BA isoform (long spring). We tested whether changes in titin isoform expression occur in the diastolic dysfunction that accompanies heart failure. METHODS AND RESULTS: We used the tachycardia-induced dilated cardiomyopathy canine model (4-week pacing) and found that control myocardium coexpresses the N2B and N2BA isoforms at similar levels, whereas in dilated cardiomyopathy the expression ratio had shifted, without affecting the amount of total titin, toward more prominent N2B expression. This shift was accompanied by elevated titin-based passive muscle stiffness. Pacing also resulted in significant upregulation of obscurin, an approximately 800-kDa elastic protein with several signaling domains. CONCLUSIONS: Coexpression of titin isoforms with distinct mechanical properties allows modulation of passive stiffness via adjustment of the isoform expression ratio. The canine pacing-induced heart failure model uses this mechanism to increase myocardial stiffness. Thus, changes in titin isoform expression may play a role in diastolic dysfunction in heart failure.

Animals↗
PubMed2001

The B2 element of the Saccharomyces cerevisiae ARS1 origin of replication requires specific sequences to facilitate pre-RC formation.

The minimal requirements for a eukaryotic origin of replication are an initiator binding site and a region of helically unstable DNA [DNA unwinding element (DUE)]. Budding yeast origins consist of modular elements, and one of these elements, B2, has been proposed to act as a DUE. To test this hypothesis, we screened for sequences that function at the B2 element of ARS1. We found that the B2 element required A-rich sequences, but that the function of these identified sequences did not correlate with helical instability. Instead, the sequences that substituted fully for B2 function showed similarity to the ARS consensus sequence (ACS). The ACS is the binding site for the initiator origin recognition complex (ORC), but the selected sequences are not strong ORC binding sites in vitro. Nonfunctional B2 sequences show a corresponding loss in Mcm2-7p origin association. The function of these mutant sequences is rescued by Cdc6p overexpression. We propose that the B2 element requires specific sequences to bind a component of the pre-RC.

Binding Sites↗
PubMed2001

DNA replication in eukaryotic cells.

The maintenance of the eukaryotic genome requires precisely coordinated replication of the entire genome each time a cell divides. To achieve this coordination, eukaryotic cells use an ordered series of steps to form several key protein assemblies at origins of replication. Recent studies have identified many of the protein components of these complexes and the time during the cell cycle they assemble at the origin. Interestingly, despite distinct differences in origin structure, the identity and order of assembly of eukaryotic replication factors is highly conserved across all species. This review describes our current understanding of these events and how they are coordinated with cell cycle progression. We focus on bringing together the results from different organisms to provide a coherent model of the events of initiation. We emphasize recent progress in determining the function of the different replication factors once they have been assembled at the origin.

Adenosine Triphosphate↗