The conduction of protons in different stereoisomers of dioxolane-linked gramicidin A channels

Two different stereoisomers of the dioxolane-linked gramicidin A (gA) channels were individually synthesized (the SS and RR dimers;. Science. 244:813-817). The structural differences between these dimers arise from different chiralities within the dioxolane linker. The SS dimer mimics the helicity and the inter- and intramolecular hydrogen bonding of the monomer-monomer association of gA's. In contrast, there is a significant disruption of the helicity and hydrogen bonding pattern of the ion channel in the RR dimer. Single ion channels formed by the SS and RR dimers in planar lipid bilayers have different proton transport properties. The lipid environment in which the different dimers are reconstituted also has significant effects on single-channel proton conductance (g(H)). g(H) in the SS dimer is about 2-4 times as large as in the RR. In phospholipid bilayers with 1 M [H(+)](bulk), the current-voltage (I-V) relationship of the SS dimer is sublinear. Under identical experimental conditions, the I-V plot of the RR dimer is supralinear (S-shaped). In glycerylmonooleate bilayers with 1 M [H(+)](bulk), both the SS and RR dimers have a supralinear I-V plot. Consistent with results previously published (. Biophys. J. 73:2489-2502), the SS dimer is stable in lipid bilayers and has fast closures. In contrast, the open state of the RR channel has closed states that can last a few seconds, and the channel eventually inactivates into a closed state in either phospholipid or glycerylmonooleate bilayers. It is concluded that the water dynamics inside the pore as related to proton wire transfer is significantly different in the RR and SS dimers. Different physical mechanisms that could account for this hypothesis are discussed. The gating of the synthetic gA dimers seems to depend on the conformation of the dioxolane link between gA's. The experimental results provide an important framework for a detailed investigation at the atomic level of proton conduction in different and relatively simple ion channel structures.     

A hypomethylating variant of MTHFR, 677C>T, blunts the neural response to errors in patients with schizophrenia and healthy individuals

Background

Responding to errors is a critical first step in learning from mistakes, a process that is abnormal in schizophrenia. To gain insight into the neural and molecular mechanisms of error processing, we used functional MRI to examine effects of a genetic variant in methylenetetrahydrofolate reductase (MTHFR 677C>T, rs1801133) that increases risk for schizophrenia and that has been specifically associated with increased perseverative errors among patients. MTHFR is a key regulator of the intracellular one-carbon milieu, including DNA methylation, and each copy of the 677T allele reduces MTHFR activity by 35%.

Methodology/Principal Findings

Using an antisaccade paradigm, we found that the 677T allele induces a dose-dependent blunting of dorsal anterior cingulate cortex (dACC) activation in response to errors, a pattern that was identical in healthy individuals and patients with schizophrenia. Further, the normal relationship between dACC activation and error rate was disrupted among carriers of the 677T allele.

Conclusions/Significance

These findings implicate an epigenetic mechanism in the neural response to errors, and provide insight into normal cognitive variation through a schizophrenia risk gene.     

The role of catecholamines on intercellular coupling,myocardial cell synchronization and self ventricular defibrillation

Ventricular fibrillation (VF) is one of the most life threatening events. Although in humans VF is generally sustained (SVF) requiring artificial defibrillation, in various mammals and in some cases in humans VF terminates by itself, reverting spontaneously into sinus rhythm. Since VF is one of the main causes of sudden death, one of the important clinical problems today is if and how we can transform the fatal SVF into a self limited transient one (TVF).From electrophysiological studies carried out on anaesthetized open chest animals, we have found that TVF requires a high degree of intercellular coupling and synchronization.Cardiac myocytes are electrically coupled with adjacent cells. The intercellular coupling is a focus of low electrical resistance which allows rapid transmission of electrical impulses between cells. Any decrease in intercellular coupling decreases the ability of the heart for self defibrillation. The cell-to-cell coupling decreases with age, ischemia, VF and variations in physiological conditions probably due to an increase in intercellular resistance (Ri), widening in the internexal gaps, decrease in electrotonic space constant () etc. All of these factors are known to be affected by intracellular concentration of free Ca⁺⁺ ([Ca⁺⁺]).On the basis of studies carried out on various mammals at different ages, we hypothesized that the ability of the heart to defibrillate depends on the cardiac catecholamine level [CA], during VF. This hypothesis is supported by the facts, known from the literature, that increase in [CA] decreases intracellular free Ca⁺⁺ concentration, decreases Ri and increases . By these effects, increase in [CA] enhances intercellular coupling and intercellular synchronization, and thereby, according to our hypothesis, leads to spontaneous ventricular defibrillation — TVF.During VF the sympathetic activity is enhanced but in some cases the [CA] does not reach the level needed for TVF. In order to help the heart in its effort to elevate the [CA] during VF, we proposed to treat these cases with drugs which inhibit the reuptake of [CA]. The facts that administration of [CA] reuptake inhibitors, before the induction of VF, and/or intracoronary infusion of adrenaline, during VF, transforms SVF into TVF, emphasized the validity of our hypothesis.     

Bacteria in cancer therapy: a novel experimental strategy

Resistance to conventional anticancer therapies in patients with advanced solid tumors has prompted the need of alternative cancer therapies. Moreover, the success of novel cancer therapies depends on their selectivity for cancer cells with limited toxicity to normal tissues. Several decades after Coley's work a variety of natural and genetically modified non-pathogenic bacterial species are being explored as potential antitumor agents, either to provide direct tumoricidal effects or to deliver tumoricidal molecules. Live, attenuated or genetically modified non-pathogenic bacterial species are capable of multiplying selectively in tumors and inhibiting their growth. Due to their selectivity for tumor tissues, these bacteria and their spores also serve as ideal vectors for delivering therapeutic proteins to tumors. Bacterial toxins too have emerged as promising cancer treatment strategy. The most potential and promising strategy is bacteria based gene-directed enzyme prodrug therapy. Although it has shown successful results in vivo yet further investigation about the targeting mechanisms of the bacteria are required to make it a complete therapeutic approach in cancer treatment.     

Phylogenetic evidence for horizontal transmission of group I introns in the nuclear ribosomal DNA of mushroom-forming fungi

Group I introns were discovered inserted at the same position in the nuclear small-subunit ribosomal DNA (nuc-ssu-rDNA) in several species of homobasidiomycetes (mushroom-forming fungi). Based on conserved intron sequences, a pair of intron-specific primers was designed for PCR amplification and sequencing of intron-containing rDNA repeats. Using the intron-specific primers together with flanking rDNA primers, a PCR assay was conducted to determine presence or absence of introns in 39 species of homobasidiomycetes. Introns were confined to the genera Panellus, Clavicorona, and Lentinellus. Phylogenetic analyses of nuc-ssu-rDNA and mitochondrial ssu-rDNA sequences suggest that Clavicorona and Lentinellus are closely related, but that Panellus is not closely related to these. The simplest explanation for the distribution of the introns is that they have been twice independently gained via horizontal transmission, once on the lineage leading to Panellus, and once on the lineage leading to Lentinellus and Clavicorona. BLAST searches using the introns from Panellus and Lentinellus as query sequences retrieved 16 other similar group I introns of nuc-ssu-rDNA and nuclear large-subunit rDNA (nuc-lsu-rDNA) from fungal and green algal hosts. Phylogenetic analyses of intron sequences suggest that the mushroom introns are monophyletic, and are nested within a clade that contains four other introns that insert at the same position as the mushroom introns, two from different groups of fungi and two from green algae. The distribution of host lineages and insertion sites among the introns suggests that horizontal and vertical transmission, homing, and transposition have been factors in intron evolution. As distinctive, heritable features of nuclear rDNAs in certain lineages, group I introns have promise as phylogenetic markers. Nevertheless, the possibility of horizontal transmission and homing also suggest that their use poses certain pitfalls.      

Acute, nongenomic effect of thyroid hormones in preventing calcium overload in newborn rat cardiocytes

In this study, we examined the acute effects of thyroid hormones (TH) T(3) and T(4), leading to improvement of myocardial function through activation of Ca(2+) extrusion mechanisms and, consequently, prevention of intracellular calcium overload. Extracellular calcium elevation from 1.8 to 3.8 mM caused immediate increase in intracellular calcium level ([Ca(2+)](i)) in newborn cardiomyocyte cultures. Administration of 10 or 100 nM T(3) or T(4) rapidly (within 10 sec) decreased [Ca(2+)](i) to its control level. Similar results were obtained when [Ca(2+)](i) was elevated by decreasing extracellular Na(+) concentration, causing backward influx of Ca(2+) through Na(+)/Ca(2+) exchanger, or by administration of caffeine, releasing Ca(2+) from the sarcoplasmic reticulum (SR). Under these conditions, T(3) or T(4) decreased [Ca(2+)](i). T(3) and T(4) also exhibited protective effects during ischemia. T(3) or T(4) presence during hypoxia for 120 min in culture medium restricted the increase of [Ca(2+)](i) and prevented the pathological effects of its overload. An inhibitor of SR Ca(2+)-ATPase (SERCA2a), thapsigargin, increases [Ca(2+)](i) and in its presence neither T(3) nor T(4) had any effect on the [Ca(2+)](i) level. The reduction of [Ca(2+)](i) level by T(3) and T(4) was also blocked in the presence of H-89 (a PKA inhibitor), and by calmodulin inhibitors. The effect of TH on the reduction of [Ca(2+)](i) was prevented by propranolol, indicating that the hormones exert their effect through interaction with adrenergic receptors. These results support our hypothesis that TH prevent calcium overload in newborn rat cardiomyocytes, most likely by a direct, acute, and nongenomic effect on Ca(2+) transport into the SR.     

Modulation of intracellular Ca(2+) concentration by tedisamil,a class III antiarrhythmic agent,in isolated heart preparation

New class III antiarrhythmic/defibrillating compound tedisamil was shown to facilitate termination of atrial and ventricular fibrillation in experimental as well as clinical conditions. However, class III-related inhibition of K(+) current associated with prolongation of repolarization can not solely explain its defibrillating ability. Following recent findings it was hypothesized that defibrillating effect of tedisamil is likely due to its sympathomimetic feature linked with modulation of intracellular calcium. Results of this study obtained in isolated heart preparation showed that elevated intracellular Ca(2+) free concentration was decreased by administration of tedisamil in concentration that did not induce Q-T interval prolongation. Due to species differences the effective concentration was in rat 10(-7) M, while in guinea pig 10(-5) M. On the contrary, further dramatic increase of elevated Ca(2+) was detected upon administration of tedisamil in concentration that markedly prolonged Q-T interval (10(-5) M in rat). It is concluded that defibrillating ability of tedisamil is most likely associated with attenuation of abnormal and harmful intracellular Ca(2+) elevation (that is highly arrhythmogenic) than with prolongation of APD or Q-T interval.     

Repeated evolution of an acetate-crossfeeding polymorphism in long-term populations of Escherichia coli

Six out of 12 independent replicate populations of Escherichia coli maintained in long-term glucose-limited continuous culture for up to approximately 1,750 generations evolve polymorphisms maintained by acetate crossfeeding. In all cases, the acetate-crossfeeding phenotype is associated with semiconstitutive overexpression of acetyl CoA synthetase, which allows for the enhanced uptake of low levels of exogenous acetate. Mutations in the 5' regulatory region of the acetyl CoA synthetase locus are responsible for all the acetate crossfeeding phenotypes found. These changes were either transposable-element insertions or a single T-->A nucleotide substitution at position -93 relative to the acs gene translation start site.