The role of upstream sequences in selecting the reading frame on tmRNA

BACKGROUND

tmRNA acts first as a tRNA and then as an mRNA to rescue stalled ribosomes in eubacteria. Two unanswered questions about tmRNA function remain: how does tmRNA, lacking an anticodon, bypass the decoding machinery and enter the ribosome? Secondly, how does the ribosome choose the proper codon to resume translation on tmRNA? According to the -1 triplet hypothesis, the answer to both questions lies in the unique properties of the three nucleotides upstream of the first tmRNA codon. These nucleotides assume an A-form conformation that mimics the codon-anticodon interaction, leading to recognition by the decoding center and choice of the reading frame. The -1 triplet hypothesis is important because it is the most credible model in which direct binding and recognition by the ribosome sets the reading frame on tmRNA.

RESULTS

Conformational analysis predicts that 18 triplets cannot form the correct structure to function as the -1 triplet of tmRNA. We tested the tmRNA activity of all possible -1 triplet mutants using a genetic assay in Escherichia coli. While many mutants displayed reduced activity, our findings do not match the predictions of this model. Additional mutagenesis identified sequences further upstream that are required for tmRNA function. An immunoblot assay for translation of the tmRNA tag revealed that certain mutations in U85, A86, and the -1 triplet sequence result in improper selection of the first codon and translation in the wrong frame (-1 or +1) in vivo.

CONCLUSION

Our findings disprove the -1 triplet hypothesis. The -1 triplet is not required for accommodation of tmRNA into the ribosome, although it plays a minor role in frame selection. Our results strongly disfavor direct ribosomal recognition of the upstream sequence, instead supporting a model in which the binding of a separate ligand to A86 is primarily responsible for frame selection.

Ultrasound enhanced delivery of molecular imaging and therapeutic agents in Alzheimer's disease mouse models

Alzheimer's disease is a neurodegenerative disorder typified by the accumulation of a small protein, beta-amyloid, which aggregates and is the primary component of amyloid plaques. Many new therapeutic and diagnostic agents for reducing amyloid plaques have limited efficacy in vivo because of poor transport across the blood-brain barrier. Here we demonstrate that low-intensity focused ultrasound with a microbubble contrast agent may be used to transiently disrupt the blood-brain barrier, allowing non-invasive, localized delivery of imaging fluorophores and immunotherapeutics directly to amyloid plaques. We administered intravenous Trypan blue, an amyloid staining red fluorophore, and anti-amyloid antibodies, concurrently with focused ultrasound therapy in plaque-bearing, transgenic mouse models of Alzheimer's disease with amyloid pathology. MRI guidance permitted selective treatment and monitoring of plaque-heavy anatomical regions, such as the hippocampus. Treated brain regions exhibited 16.5±5.4-fold increase in Trypan blue fluorescence and 2.7±1.2-fold increase in anti-amyloid antibodies that localized to amyloid plaques. Ultrasound-enhanced delivery was consistently reproduced in two different transgenic strains (APPswe:PSEN1dE9, PDAPP), across a large age range (9–26 months), with and without MR guidance, and with little or no tissue damage. Ultrasound-mediated, transient blood-brain barrier disruption allows the delivery of both therapeutic and molecular imaging agents in Alzheimer's mouse models, which should aid pre-clinical drug screening and imaging probe development. Furthermore, this technique may be used to deliver a wide variety of small and large molecules to the brain for imaging and therapy in other neurodegenerative diseases.

Genetic analysis of the structure and function of transfer messenger RNA pseudoknot 1

tmRNA rescues stalled ribosomes in eubacteria by forcing the ribosome to abandon its mRNA template and resume translation with tmRNA itself as a template. Pseudoknot 1 (pk1), immediately upstream of this coding region in tmRNA, is a structural element that is considered essential for tmRNA function based on the analysis of pk1 mutants in vitro. pk1 binds near the ribosomal decoding site and may make base-specific contacts with tmRNA ligands. To study pk1 structure and function in vivo, we have developed a genetic selection that ties the life of Escherichia coli cells to tmRNA activity. Mutation of pk1 at 20% per base and selection for tmRNA activity yielded sequences that retain the same pseudoknot fold. In contrast, selection of active mutants from 10(6) completely random sequences identified hairpin structures that functionally replace pk1. Rational design of a hairpin with increased stability using an unrelated sequence yielded a tmRNA mutant with nearly wild-type activity. We conclude that the role of pk1 in tmRNA function is purely structural and that it can be replaced with a variety of hairpin structures. Our results demonstrate that in the study of functional RNAs, the inactivity of a mutant designed to destroy a given structure should not be interpreted as proof that the structure is necessary for RNA function. Such mutations may only destabilize a global fold that could be formed equally well by an entirely different, stable structure.

Effects of isoflurane on coronary arteries and coronary arterioles in the intact dog

To determine the site of isoflurane-associated coronary vasodilatation, the authors measured epicardial coronary artery diameter and examined the effects of isoflurane on coronary arteriolar tone. Angiograms of the left coronary system were obtained in seven fentanyl-pentobarbital anesthetized dogs and quantitated with a computerized analysis system. Cross-sectional areas of the proximal, mid, and distal left anterior descending and proximal circumflex coronary arteries were obtained at three arterial pressures, and then the measurement repeated following administration of 0.75%, 1.5%, and 2.25% end-tidal isoflurane. At the same time coronary blood flow was measured using a 123Xe washout technique. Isoflurane was found to have no effect on epicardial coronary artery dimensions. No dilatation was observed throughout the range of isoflurane concentrations and coronary perfusion pressures investigated. However, despite the absence of epicardial coronary effects, coronary arterioles were dilated by both 1.5% and 2.25% isoflurane. Coronary blood flow corresponding to a myocardial oxygen consumption of 7.5 ml oxygen X 100 gm-1 X min-1 was calculated as 99 +/- 17 ml X 100 gm-1 X min-1 (mean +/- SD) during control conditions, and it increased to 150 +/- 26 ml X 100 gm-1 X min-1 at 1.5% isoflurane (P less than .004) and to 197 +/- 35 ml X 100 gm-1 X min-1 at 2.25% isoflurane (P less than .001). Although higher concentrations of isoflurane dilated intramyocardial arterioles, isoflurane had no effect on epicardial coronary arteries.

Effects of thromboxane analog U46619 on endothelial damaged canine coronary arteries in vivo

We studied the effects of the thromboxane analog, U46619, infused into the left anterior descending (LAD) artery of intact dogs before and after producing endothelial denudation of the mid portion of the LAD. Proximal artery cross-sectional area (CSA) decreased by 47% with 0.1 microgram/min infusion of U46619 with intact and denuded endothelium, while resting CSA reduced spontaneously following denudation. Coronary resistance vessels demonstrated a marked constrictor response to U46619 with a rise in resistance and a fall in flow and myocardial O2 consumption. U46619 produces significant narrowing of proximal epicardial coronary arteries as well as resistance coronary vessels. This effect could cause ischemia in patients with moderate coronary atherosclerosis.

Proximal coronary vasomotor reactivity after exercise training in dogs

The effects of exercise on large coronary vasoreactivity were determined in eight dogs trained by treadmill running for 8 weeks. Six nontrained dogs comprised the control group. The trained group showed a significant reduction in heart rate during graded submaximal exercise testing when compared with the controls, and resting plasma levels of norepinephrine (nontrained group, 331 +/- 99 pg/ml; trained group, 142 +/- 30 pg/ml; p less than .05) and epinephrine (nontrained, 424 +/- 105; trained, 258 +/- 45 pg/ml; p less than .05) were reduced significantly in the trained group. Epicardial coronary responses to intracoronary infusion of serotonin and phenylephrine were evaluated by quantitative coronary angiography, and myocardial blood flow was measured with 15 microns radioactive microspheres. Left ventricular/body weight ratio was similar in the trained (4.81 +/- 0.24 g/kg) and nontrained groups (4.79 +/- 0.17), and no differences were noted in resting myocardial oxygen consumption or coronary arteriovenous oxygen difference. The constriction of the proximal left anterior descending artery (LAD) in response to serotonin infusion was not different in the two groups, but the LAD and circumflex artery constrictor responses to phenylephrine were attenuated in the trained when compared with the nontrained dogs. The data indicate that endurance exercise diminishes the large epicardial coronary vasoconstrictor response to alpha-adrenergic stimulation, but not to serotonin. The blunted constrictor response in the trained animals suggests that exercise may be useful in reducing epicardial coronary vasoconstriction, which is thought to be important in some patients with coronary artery disease.

Increased conjugated dopamine in plasma after exercise training

To evaluate the effect of endurance exercise on plasma catecholamines, we exercise trained eight dogs (group T) by treadmill running for 8 weeks. Six sedentary dogs constituted a nontrained control group (group NT). Heart rate response to a graded submaximal stress test was reduced in group T dogs (p less than 0.05), but mean resting aortic blood pressure (NT, 84 +/- 5 mm Hg; T, 82 +/- 4 mm Hg) and heart rate (NT, 87 +/- 1 bpm; T, 84 +/- 2 bpm) were unchanged by exercise, and no cardiac hypertrophy occurred after exercise. Plasma norepinephrine and epinephrine levels were reduced in group T at rest and during a fixed exercise workload. Plasma conjugated dopamine showed a marked increase in group T dogs (NT, 1398 +/- 130 pg/ml; T, 11346 +/- 1291 pg/ml; p less than 0.01) at rest, and no change in conjugated dopamine occurred in either group after short-term exercise stress. No intergroup differences were noted in resting coronary flow or coronary arteriovenous oxygen, or in myocardial oxygen consumption. The data verify previous findings of lower plasma levels of norepinephrine and epinephrine after training, and indicate that a marked rise in conjugated dopamine occurs after training. These findings suggest that norepinephrine and epinephrine metabolism is shifted toward conjugated dopamine by exercise training, thereby reducing active catecholamines in plasma, but retaining a large pool of usable metabolite.

Effects of serotonin and histamine on proximal and distal coronary vasculature in dogs: comparison with alpha-adrenergic stimulation

The effect of several vasoactive agents on epicardial conductance arteries and distal resistance arteries was studied in intact dogs using a special catheter system to infuse vasoactive mediators directly into the left anterior descending coronary artery of intact dogs. Serotonin produced significant epicardial vasoconstriction (42% cross-sectional area reduction, p less than 0.01), whereas histamine had no effect on proximal coronary arteries. Phenylephrine, an alpha-adrenergic agonist, produced an 11% reduction in cross-sectional area. Distal coronary vascular resistance (pressure/flow) changes were small for serotonin and phenylephrine, whereas histamine significantly dilated the peripheral vascular bed and caused flow measured by xenon-133 washout to increase from 30.4 +/- 4.0 to 72.4 +/- 12.6 ml/min . 100 g (p less than 0.05). These results show that vasoactive mediators can have different actions on coronary resistance and conductance vessels. Serotonin is a potent vasoconstrictor of epicardial coronary arteries but does not produce significant constriction of coronary resistance vessels.

Failure of interventions to maintain mitochondrial function in ischemic myocardium

The purpose of this study was to investigate whether pyruvate (2.5 mmol/kg), the combination of glucose (3.9 mmol/kg) and insulin (0.13 unit/kg) with potassium (9.27 meq/kg), or sodium dichloroacetate (120 mg/kg) infused for 15 minutes before and 20 minutes after ligation of the left anterior descending coronary arterv in anesthetized dogs restricted the depression in mitochondrial respiratory function induced by ischemia. Myocardial blood flow after ligation in the three groups was o.2 ml/min per gram or less in ischemic subendocardium and was similar to that in saline-infused controls that had also undergone coronary artery ligation. The depressions induced in mitochondrial respiratory control index and state 3 respiration by ischemia in the subendocardium and subepicardium of the three treatment groups, when compared with corresponding nonischemic tissue, were not significantly improved from the control values. It was concluded that these three interventions fail to preserve mitochondrial respiration in ischemic myocardium.