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Biomedical subjects

E Henderson

Publications and source records attributed to E Henderson.

At least 109 records · Page 6Linked to original sources
PubMed1991

Monovalent cation induced structural transitions in telomeric DNAs: G-DNA folding intermediates.

Telomeric DNA consists of G- and C-rich strands that are always polarized such that the G-rich strand extends past the 3' end of the duplex to form a 12-16-base overhang. These overhanging strands can self-associate in vitro to form intramolecular structures that have several unusual physical properties and at least one common feature, the presence of non-Watson-Crick G.G base pairs. The term "G-DNA" was coined for this class of structures (Cech, 1988). On the basis of gel electrophoresis, imino proton NMR, and circular dichroism (CD) results, we find that changing the counterions from sodium to potassium (in 20 mM phosphate buffers) specifically induces conformational transitions in the G-rich telomeric DNA from Tetrahymena, d(T2G4)4 (TET4), which results in a change from the intramolecular species to an apparent multistranded structure, accompanied by an increase in the melting temperature of the base pairs of greater than 25 degrees, as monitored by loss of the imino proton NMR signals. NMR semiselective spin-lattice relaxation rate measurements and HPLC size-exclusion chromatography studies show that in 20 mM potassium phosphate (pH 7) buffer (KP) TET4 is approximately twice the length of the form obtained in 20 mM sodium phosphate (pH 7) buffer (NaP) and that mixtures of Na+ and K+ produce mixtures of the two forms whose populations depend on the ratio of the cations. Since K+ and NH4+ are known to stabilize a parallel-stranded quadruplex structure of poly[r(I)4], we infer that the multistranded structure is a quadruplex. Our results indicate that specific differences in ionic interactions can result in a switch in telomeric DNAs between intramolecular hairpin-like or quadruplex-containing species and intermolecular quadruplex structures, all of which involve G.G base pairing interactions. We propose a model in which duplex or hairpin forms of G-DNA are folding intermediates in the formation of either 1-, 2-, or 4-stranded quadruplex structures. In this model monovalent cations stabilize the duplex and quadruplex forms via two distinct mechanisms, counterion condensation and octahedral coordination to the carbonyl groups in stacked planar guanine "quartet" base assemblies. Substituting one of the guanosine residues in each of the repeats of the Tetrahymena sequence to give the human telomeric DNA, d(T2AG3)4, results in less effective K(+)-dependent stabilization. Thus, the ion-dependent stabilization is attenuated by altering the sequence. Upon addition of the Watson-Crick (WC) complementary strand, only the Na(+)-stabilized structure dissociates quickly to form a WC double helix.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Effect of captopril, an angiotensin-converting enzyme inhibitor, in patients with angina pectoris and heart failure.

The effects of captopril and placebo were compared in 18 patients with chronic heart failure and angina pectoris with use of a double-blind crossover trial design. Symptoms were assessed by patient treatment preference, visual analogue scores and nitroglycerin consumption. Exercise performance was assessed using two different treadmill protocols of different work intensity with simultaneous measurement of oxygen consumption and by supine bicycle exercise and simultaneous radionuclide ventriculography. Arrhythmias were assessed by 48 h ambulatory electrocardiographic monitoring. Patients generally preferred placebo to captopril, and this appeared to be due to an increase in symptoms of angina with captopril. Treadmill exercise time on a high intensity protocol was shorter with captopril than with placebo; on a low intensity protocol, angina became a more frequent limiting symptom even though overall exercise performance was not changed. The heart rate-blood pressure product was reduced, but largely because of a reduction in blood pressure rather than in heart rate. During supine bicycle exercise, no differences in symptoms, exercise performance, ejection fraction or changes in blood pressure were noted and ventricular arrhythmias were reduced. Captopril does not appear to be clinically useful in alleviating angina pectoris in patients with heart failure, and this effect may be related to a decrease in coronary perfusion pressure. Nonetheless, desirable metabolic effects, a reduction in arrhythmias and potential effects on survival require further study of captopril in patients with both angina and heart failure.

Aged↗
PubMed1991

Submaximal, but not maximal, exercise testing detects differences in the effects of beta-blockers during treadmill exercise: a study of celiprolol and atenolol. II.

Celiprolol is a new-generation beta-blocker with ancillary properties that include partial beta 2-agonism and direct vasodilating activity. The effects of atenolol and celiprolol on maximal exercise capacity and on both respiratory variables and subjective indices of breathlessness and fatigue during submaximal exercise were compared in a placebo-controlled crossover study of 12 trained volunteers. Both atenolol and celiprolol equally and significantly reduced exercise capacity and maximal oxygen consumption. During constant submaximal exercise at 70% maximal oxygen uptake, however, differences emerged between the two beta-blockers. Atenolol was associated with a significantly higher minute ventilation than placebo. In contrast, values for minute ventilation and respiratory exchange ratio with celiprolol were similar to values with placebo. During the early stages of exercise, treatment with atenolol was also associated with higher scores for the subjective indices of breathlessness and fatigue. Thus submaximal exercise, which may be physiologically more relevant to the everyday activities of patients, may demonstrate potentially useful differences between drugs that are not seen during maximal exercise testing.

Adrenergic beta-Antagonists↗
PubMed1991

Pancreatic beta-cell-type-specific transcription of the insulin gene is mediated by basic helix-loop-helix DNA-binding proteins.

The pancreatic beta-cell-specific expression of the insulin gene is mediated, at least in part, by the interaction of unique trans-acting beta-cell factors with a cis-acting DNA element found within the insulin enhancer (5'-GC CATCTG-3'; referred to as the insulin control element [ICE]) present in the rat insulin II gene between positions -100 and -91. This sequence element contains the consensus binding site for a group of DNA-binding transcription factors called basic helix-loop-helix proteins (B-HLH). As a consequence of the similarity of the ICE with the DNA sequence motif associated with the cis-acting elements of the B-HLH class of binding proteins (CANNTG), the ability of this class of proteins to regulate cell-type-specific expression of the insulin gene was addressed. Cotransfection experiments indicated that overexpression of Id, a negative regulator of B-HLH protein function, inhibits ICE-mediated activity. Antibody to the E12/E47 B-HLH proteins attenuated the formation, in vitro, of a previously described (J. Whelan, S. R. Cordle, E. Henderson, P. A. Weil, and R. Stein, Mol. Cell. Biol. 10:1564-1572, 1990) beta-cell-specific activator factor(s)-ICE DNA complex. Both of these B-HLH proteins (E12 and E47) bound efficiently and specifically to the ICE sequences. The role of B-HLH proteins in mediating pancreatic beta-cell-specific transcription of the insulin gene is discussed.

Animals↗
PubMed1991

Insulin gene expression in nonexpressing cells appears to be regulated by multiple distinct negative-acting control elements.

Selective transcription of the insulin gene in pancreatic beta cells is regulated by its enhancer, located between nucleotides -340 and -91 relative to the transcription start site. Transcription from the enhancer is controlled by both positive- and negative-acting cellular factors. Cell-type-specific expression is mediated principally by a single cis-acting enhancer element located between -100 and -91 in the rat insulin II gene (referred to as the insulin control element [ICE]), which is acted upon by both of these cellular activities. Analysis of the effect of 5' deletions within the insulin enhancer has identified a region between nucleotides -217 and -197 that is also a site of negative control. Deletion of these sequences from the 5' end of the enhancer leads to transcription of the enhancer in non-insulin-producing cells, even though the ICE is intact. Derepression of this ICE-mediated effect was shown to be due to the binding of a ubiquitously distributed cellular factor to a sequence element which resides just upstream of the ICE (i.e., between nucleotides -110 and -100). We discuss the possible relationship of these results to cell-type-specific regulation of the insulin gene.

Animals↗
PubMed1990

Identification of a pancreatic beta-cell insulin gene transcription factor that binds to and appears to activate cell-type-specific expression: its possible relationship to other cellular factors that bind to a common insulin gene sequence.

The insulin gene is expressed almost exclusively in pancreatic beta-cells. Previous work in our laboratory has shown that pancreatic beta-cell-specific expression of the rat insulin II gene is controlled by a number of positive and negative cis-acting DNA elements within the enhancer. We have shown that one element within the enhancer, located between nucleotides -100 and -91 (GCCATCTGCT; referred to as the insulin control element [ICE]) relative to the transcription start site, is controlled by both positive- and negative-acting cellular transcription factors. The positive-acting factor appears to be uniquely active in beta-cells. To identify the nucleotides within the ICE that mediate positive cell-type-specific regulation, point mutations within this element were generated and assayed for their effects on expression. Base pairs -97, -94, -93, and -92 were found to be crucial for the activator function of this region, while mutations at base pairs -100, -96, and -91 had little or no effect on activity. The gel mobility shift assay was used to determine whether specific cellular factors associated directly with the ICE. Several specific protein-DNA complexes were detected in extracts prepared from insulin-producing and non-insulin-producing cells, including a complex unique to beta-cell extracts. The ability of unlabeled wild-type and point mutant versions of the ICE to compete for binding to these cellular factors demonstrated that the beta-cell-specific complex appears to contain the insulin gene activator protein(s). Interestingly, the adenovirus type 2 major late promoter upstream element (USE; GCCACGTGAC) also competed in the gel mobility shift assay for binding of cellular proteins to the ICE. These results suggested that the cellular factor that binds to the USE (i.e., USF) also interacts with the ICE. This was directly demonstrated by showing that ICE and USE sequences completed for the USF required for adenovirus type 2 major late promoter transcription in vitro and by showing that reticulocyte lysate-translated human USF products bound to the ICE. However, the USE sequences were unable to stimulate beta-cell-type-specific activity in vivo. We discuss the possible relationship of these observations to positive and negative control mediated by the ICE.

Animals↗
PubMed1990

Non-invasive determination of cardiac output by Doppler echocardiography and electrical bioimpedance.

Cardiac output measured by thermodilution in 25 patients within 24 hours of acute myocardial infarction was compared with cardiac output measured by Doppler echocardiography (24 patients) and electrical bioimpedance (25 patients). The mean (range) cardiac outputs measured by Doppler (4.03 (2.2-6.0) 1/min) and electrical bioimpedance (3.79 (1.1-6.2) 1/min) were similar to the mean thermodilution value (3.95 (2.1-6.2) 1/min). Both non-invasive techniques agreed closely with thermodilution in most patients. None the less, three results with each method disagreed with thermodilution by more than 1 1/min. Both non-invasive techniques were reproducible and accurate in most patients with acute myocardial infarction. Doppler echocardiography was time consuming and technically demanding. Electrical bioimpedance was simple to use and had the additional advantage of allowing continuous monitoring of the cardiac output.

Aged↗
PubMed1990

Novel exercise protocol suitable for use on a treadmill or a bicycle ergometer.

Many exercise protocols are in use in clinical cardiology, but no single test is applicable to the wide range of patients' exercise capacity. A new protocol was devised that starts at a low workload and increases by 15% of the previous workload every minute. This is the first protocol to be based on exponential rather than linear increments in workload. The new protocol (standardised exponential exercise protocol, STEEP) is suitable for use on either a treadmill or a bicycle ergometer. This protocol was compared with standard protocols in 30 healthy male volunteers, each of whom performed four exercise tests: the STEEP treadmill and bicycle protocols, a modified Bruce treadmill protocol, and a 20 W/min bicycle protocol. During the two STEEP tests the subjects' oxygen consumption rose gradually and exponentially and there was close agreement between the bicycle and the treadmill protocols. A higher proportion of subjects completed the treadmill than the bicycle protocol. Submaximal heart rates were slightly higher during the bicycle test. The STEEP protocol took less time than the modified Bruce treadmill protocol, which tended to produce plateaux in oxygen consumption during the early stages. The 20 W/min bicycle protocol does not take account of subjects' body weight and consequently produced large intersubject variability in oxygen consumption. The STEEP protocol can be used on either a treadmill or a bicycle ergometer and it should be suitable for a wide range of patients.

Adult↗
PubMed1989

Recognition and elongation of telomeres by telomerase.

Telomeres stabilize chromosomal ends and allow their complete replication in vivo. In diverse eukaryotes, the essential telomeric DNA sequence consists of variable numbers of tandem repeats of simple, G + C rich sequences, with a strong strand bias of G residues on the strand oriented 5' to 3' toward the chromosomal terminus. This strand forms a protruding 3' over-hang at the chromosomal terminus in three different eukaryotes analyzed. Analysis of yeast and protozoan telomeres showed that telomeres are dynamic structures in vivo, being acted on by shortening and lengthening activities. We previously identified and partially purified an enzymatic activity, telomere terminal transferase, or telomerase, from the ciliate Tetrahymena. Telomerase is a ribonucleoprotein enzyme with essential RNA and protein components. This activity adds repeats of the Tetrahymena telomeric sequence, TTGGGG, onto the 3' end of a single-stranded DNA primer consisting of a few repeats of the G-rich strand of known telomeric, and telomere-like, sequences. The shortest oligonucleotide active as a primer was the decamer G4T2G4. Structural analysis of synthetic DNA oligonucleotides that are active as primers showed that they all formed discrete intramolecular foldback structures at temperatures below 40 degrees C. Addition of TTGGGG repeats occurs one nucleotide at a time by de novo synthesis, which is not templated by the DNA primer. Up to 8000 nucleotides of G4T2 repeats were added to the primer in vitro. We discuss the implications of this finding for regulation of telomerase in vivo and a model for telomere elongation by telomerase.

Animals↗