High fidelity DNA strand-separation is the major specificity determinant in DNA methyltransferase CcrM's catalytic mechanism

Strand-separation is emerging as a novel DNA recognition mechanism but the underlying mechanisms and quantitative contribution of strand-separation to fidelity remain obscure. The bacterial DNA adenine methyltransferase, CcrM, recognizes 5'GANTC'3 sequences through a DNA strand-separation mechanism with unusually high selectivity. To explore this novel recognition mechanism, we incorporated Pyrrolo-dC into cognate and noncognate DNA to monitor the kinetics of strand-separation and used tryptophan fluorescence to follow protein conformational changes. Both signals are biphasic and global fitting showed that the faster phase of DNA strand-separation was coincident with the protein conformational transition. Non-cognate sequences did not display strand-separation and methylation was reduced > 300-fold, providing evidence that strand-separation is a major determinant of selectivity. Analysis of an R350A mutant showed that the enzyme conformational step can occur without strand-separation, so the two events are uncoupled. A stabilizing role for the methyl-donor (SAM) is proposed; the cofactor interacts with a critical loop which is inserted between the DNA strands, thereby stabilizing the strand-separated conformation. The results presented here are broadly applicable to the study of other N6-adenine methyltransferases that contain the structural features implicated in strand-separation, which are found widely dispersed across many bacterial phyla, including human and animal pathogens, and some Eukaryotes.

Phosphorylation of SAMHD1 Thr592 increases C-terminal domain dynamics, tetramer dissociation and ssDNA binding kinetics

SAM and HD domain containing deoxynucleoside triphosphate triphosphohydrolase 1 (SAMHD1) is driven into its activated tetramer form by binding of GTP activator and dNTP activators/substrates. In addition, the inactive monomeric and dimeric forms of the enzyme bind to single-stranded (ss) nucleic acids. During DNA replication SAMHD1 can be phosphorylated by CDK1 and CDK2 at its C-terminal threonine 592 (pSAMHD1), localizing the enzyme to stalled replication forks (RFs) to promote their restart. Although phosphorylation has only a small effect on the dNTPase activity and ssDNA binding affinity of SAMHD1, perturbation of the native T592 by phosphorylation decreased the thermal stability of tetrameric SAMHD1 and accelerated tetramer dissociation in the absence and presence of ssDNA (∼15-fold). In addition, we found that ssDNA binds competitively with GTP to the A1 site. A full-length SAMHD1 cryo-EM structure revealed substantial dynamics in the C-terminal domain (which contains T592), which could be modulated by phosphorylation. We propose that T592 phosphorylation increases tetramer dynamics and allows invasion of ssDNA into the A1 site and the previously characterized DNA binding surface at the dimer-dimer interface. These features are consistent with rapid and regiospecific inactivation of pSAMHD1 dNTPase at RFs or other sites of free ssDNA in cells.

Cell cycle regulated DNA methyltransferase: fluorescent tracking of a DNA strand-separation mechanism and identification of the responsible protein motif

DNA adenine methylation by Caulobacter crescentus Cell Cycle Regulated Methyltransferase (CcrM) is an important epigenetic regulator of gene expression. The recent CcrM-DNA cocrystal structure shows the CcrM dimer disrupts four of the five base pairs of the (5'-GANTC-3') recognition site. We developed a fluorescence-based assay by which Pyrrolo-dC tracks the strand separation event. Placement of Pyrrolo-dC within the DNA recognition site results in a fluorescence increase when CcrM binds. Non-cognate sequences display little to no fluorescence changes, showing that strand separation is a specificity determinant. Conserved residues in the C-terminal segment interact with the phospho-sugar backbone of the non-target strand. Replacement of these residues with alanine results in decreased methylation activity and changes in strand separation. The DNA recognition mechanism appears to occur with the Type II M.HinfI DNA methyltransferase and an ortholog of CcrM, BabI, but not with DNA methyltransferases that lack the conserved C-terminal segment. The C-terminal segment is found broadly in N4/N6-adenine DNA methyltransferases, some of which are human pathogens, across three Proteobacteria classes, three other phyla and in Thermoplasma acidophilum, an Archaea. This Pyrrolo-dC strand separation assay should be useful for the study of other enzymes which likely rely on a strand separation mechanism.

Behavioral and fMRI evidence of the differing cognitive load of domain-specific assessments

Standards-referenced educational reform has increased the prevalence of standardized testing; however, whether these tests accurately measure students' competencies has been questioned. This may be due to domain-specific assessments placing a differing domain-general cognitive load on test-takers. To investigate this possibility, functional magnetic resonance imaging (fMRI) was used to identify and quantify the neural correlates of performance on current, international standardized methods of spelling assessment. Out-of-scanner testing was used to further examine differences in assessment results. Results provide converging evidence that: (a) the spelling assessments differed in the cognitive load placed on test-takers; (b) performance decreased with increasing cognitive load of the assessment; and (c) brain regions associated with working memory were more highly activated during performance of assessments that were higher in cognitive load. These findings suggest that assessment design should optimize the cognitive load placed on test-takers, to ensure students' results are an accurate reflection of their true levels of competency.

Driving, dementia and Australian physicians: primum non nocere?

Older Australians are increasingly reliant on automobiles as their sole form of transport. As our population is ageing and the prevalence of dementia is increasing, it is anticipated that the number of drivers with dementia will rise over time. Much of the literature relating to driving and dementia focuses on safety rather than mobility. The objective of this paper is to highlight several topical ethical issues that pertain to Australian drivers with dementia. It is recommended that future research, policy and practice should centre on the crucial mobility and transport needs of our senior citizens.

Involvement of abscisic acid-dependent and -independent pathways in the upregulation of antioxidant enzyme activity during NaCl stress in cotton callus tissue

The role of abscisic acid (ABA) in the signal transduction pathway associated with NaCl-induced up-regulation of antioxidant enzyme activity was examined in a NaCl-tolerant cotton callus cell line treated with NaCl, ABA, paraquat, or H2O2 in the presence and absence or fluridone, an inhibitor of terpene, and therefore, ABA synthesis. Treatment with NaCl resulted in a rapid increase (within 30 minutes) in the ABA levels of the callus tissue, and the NaCl, ABA, and paraquat treatments induced rapid increases in the activities of superoxide dismutase, catalase, peroxidase, and glutathione reductase. Pre-treatment with fluridone significantly suppressed the NaCl-induced increases, but only slightly delayed the increases in tissue subjected to exogenous ABA treatment. This implies that ABA is involved in the signal transduction pathway associated with the NaCl-induced up-regulation of these antioxidant enzymes. Pre-treatment with fluridone had no effect on the paraquat-induced increases, suggesting that these enzymes can also be up-regulated by a pathway other than the one mediated by ABA. Both the NaCl and paraquat treatments produced significant increases in the superoxide levels within the callus, but the increase resulting from the paraquat treatment was significantly higher than the increase resulting from the NaCl treatment. These data suggest that NaCl stress results in the production of reactive oxygen intermediates (ROI) which signals the induction of an ABA-dependent signaling pathway. The production of very high levels of ROI, such as those that occur with paraquat treatment or perhaps during periods of prolonged or extreme stress, may induce an ABA-independent signaling pathway.

Effect of gender on anti-inflammatory and analgesic actions of two kappa-opioids

The higher incidence of inflammatory and painful disorders in women and recent reports that have emphasized the importance of gender in nociceptive sensitivity and responsiveness to analgesics prompted us to investigate gender as a factor in the variability in response to opioids. We studied the anti-inflammatory and antinociceptive effects of two kappa-opioid agonists in adjuvant-induced arthritis, one that acts both peripherally and centrally (PNU50488H; 20 mg/kg/day), the other which is peripherally selective (asimadoline; 5 mg/kg/day). Both drugs had equally powerful anti-inflammatory effects in both male and female rats (reducing measures by 60-80%). In contrast, there were gender-based heterogeneities in their analgesic actions, contingent on the method of stimulation (mechanical or thermal); males were insensitive to the analgesic effects of asimadoline with thermal but not mechanical nociceptive stimuli. We also sought evidence for gender influences on the joint content of Substance P (SP), a peptide suggested to have a role in producing inflammation and found that levels were higher in the untreated arthritic females, although there were no gender differences in disease sensitivity or nociception in arthritic animals receiving no drugs. Paradoxically, both drugs elevated SP concentrations in the joints, perhaps as a consequence of an action of kappa-opioids to suppress SP release from peripheral nerves, but the gender differences remained. Further experiments are required to determine exact mechanisms responsible for the gender distinction in analgesic response to kappa-opioids that may involve differential activation of primary afferents.

Lack of a role for substance P in the control of dural arterial flow

The role of the neuropeptide substance P (SP) in the control of dural arterial blood flow was examined in barbiturate-anaesthetised rats. The parietal skull was trephinised and the blood flow in branches of the medial meningeal artery was monitored with a laser Doppler flowmeter. Electrical stimulation of the dura mater encephali at a parasagittal site with pulses of 0.5 ms (10-20 V, 5-10 Hz, 30 s) caused a transient increase in dural blood flow which was reproducible in size with repetitive stimulation. Neither the basal flow nor the stimulus-evoked flow was significantly changed by topical administration of SP, the SP analog septide, or the NK1 antagonist RP 67580. It is concluded that SP released from dural nerve fibres upon local stimulation does not play an important role in the regulation of dural arterial flow.

The case. Bad care, good care, and spiritual preservation

A cancer patient learning to walk again finds that his relationships with his two physical therapists affect his recovery. One woman gives hope as well as instruction. He advises healers to meet their patients as equals, "to take down as many barriers as they can, give away as many distancing privileges as possible."

Correlating CBC profile and infectious outcome. A study of febrile infants evaluated for sepsis

The cases of 1,009 febrile infants who were evaluated for sepsis as outpatients during a seven-year period were reviewed to correlate their complete blood count (CBC) profiles with the infectious outcomes. Eighty-one infants had serious bacterial infections (SBIs); the remainder (928) were culture-negative. The infants with SBIs had a significantly greater mean total white blood cell (WBC) count and absolute band count (ABC) than did those who were culture-negative, whereas the difference in mean percent of polymorphonuclear leukocytes was not significant. The sensitivity of the ABC was significantly superior to that of total WBC count in predicting the outcome of SBI. The diagnostic data provided by the ABC can aid physicians in determining the predictive value of CBC profiles for infectious outcome in febrile infants receiving outpatient sepsis evaluation.

Effects on recovery of soleus and extensor digitorum longus muscles of prolonged wheel running during a period of repeated nerve damage

The right sciatic nerve in NMRI mice was frozen under anaesthesia 13 times at three-week intervals for a total period of 8.5 months. During this period, but not afterwards, one sub-group of these mice had access to running wheels in which the animals ran several kilometres per night, thereby actively or passively training reinnervated or denervated leg muscles, as well as the intact contralateral muscles. A number of distinct effects persisted for as long as 14-18 weeks after the termination of this "endurance training". In reinnervated soleus muscle, tetanic force was significantly higher (37%) in the trained muscles as was muscle weight (36%); in general, negative effects of the nerve damage persisted. In the reinnervated extensor digitorum longus, tetanic force and muscle weight were significantly smaller in the trained animals (by 11 and 16%, respectively) which are considered typical effects of endurance training. The resistance of the soleus neuromuscular junction to block by both curare and Mg2+ was depressed on the damaged side but this property was not influenced by the training; in extensor digitorum longus the pattern was similar. It is concluded that training during the period of repeated cycles of denervation-reinnervation produced significant effects which impressively outlasted the training period. The possible nature of these effects is discussed.

Nimodipine reduces morbidity, mortality after disruption of cerebral blood flow

Nimodipine is a 1,4 dihydropyridine with Ca++ channel blocking properties. It is more lipophilic than most calcium channel blockers, and thus has a greater volume of distribution within the brain. Many actions of nimodipine have been proposed from work done in animal models following induction of complete or partial cerebral ischemia for varying lengths of time. However, these experimental results have not been sufficiently documented in patient populations, and there exists conflicting data as to nimodipine's mechanism of action in the cerebral ischemic patient. Regardless of the exact mechanisms of action, studies suggest that nimodipine may significantly reduce morbidity and mortality in patients following disruption of cerebral blood flow.

Low temperature worsens mammalian oxygen toxicity

Oxygen toxicity was assessed in mice exposed to 5 ATA of oxygen. Central nervous system toxicity was measured as the latent period before convulsions, and lung damage estimated by wet and dry weight measurements. Our results confirmed previous findings that hyperbaric oxygen induces hypothermia in animals, and this effect is profound in mice exposed to 5 ATA of oxygen at ambient temperatures of 15 degrees C and 5 degrees C. However, even marked hypothermia had very little effect on the latent times to convulsions in mice. Unexpectedly, the combination of hypothermia and hyperbaric oxygen produced much more severe lung damage than either treatment alone, with a 2.7-fold increase in weight in the 5 degrees C group (average rectal temperature of 16.1 degrees C). These results indicate that hyperoxic-induced hypothermia cannot be considered a protective mechanism against oxygen toxicity and indeed hypothermia can markedly potentiate hyperbaric oxygen toxicity.

Endogenous opioids are not involved in the pathology induced by hyperbaric oxygen treatment

In mice, oxygen at hyperbaric pressures (515 kPa; 5 ATA) induces convulsions and lung damage (edema and hemorrhage). Morphine treatment (15 mg X kg-1, i.p.) significantly protects against the development of this pathology. The protection is abolished by naloxone (1 mg X kg-1, i.p.). Electric footshock, which induces diverse opioid effects, affords no protection against hyperbaric oxygen damage. Possible mechanisms of the morphine action are discussed.

Swim stress reduces chronic pain in mice through an opioid mechanism

Chronic nociception has been studied in male mice by means of the formalin test in which forelimb motor behaviour is scored after subcutaneous formalin injection. The rating remained above 2.0 for 30 min after the injection (scale range 0-3). The magnitude of the nociception has been compared with that reported in other animal types. Mice are more sensitive than rats, cats and monkeys. The stress of a swim of 3 min has been found to reduce nociception by up to 25%. This analgesia is wholly opioid in nature, being abolished by a moderate dose of naloxone (1 mg/kg).

Opioid-independent hyperalgesia induced in mice by pentobarbitone at low dosage

The abdominal constriction (writhing) test was used to investigate the effect of pentobarbitone on nociception in mice. At dosages of 10, 20 and 30 mg/kg, pentobarbitone produced marked increases in nociceptive scores. The extent of the enhancement of nociception was uninfluenced by naloxone or swim-stress. The swim-stress induced analgesia but, in contrast to previous results using other tests for nociception, the analgesia was wholly non-opioid with the abdominal constriction test.

Patterns of the use of benzodiazepines in Australia

In six suburban areas of Sydney, chosen to provide a socioeconomic cross-section of the city, complete records of dispensing of benzodiazepine were collected over a four-week period. These drugs constituted 3.7% of all dispensing, female patients outnumbered males by 2.3:1, and 98% of patients were over 20 years of age. The predominance of females, and higher age groups was found in all the areas studied, but no socioeconomic correlation was detected in the use of the drugs. Analysis of national dispensing figures confirmed the higher consumption in higher age groups, and revealed no heterogeneity in per capita prescribing rates amongst the States.

Inhibition within the trigeminal nucleus induced by afferent inputs and its influence on stimulus coding by mechanosensitive neurones

1. In decerebrate, unanaesthetized cats two thirds of slowly adapting mechanosensitive neurones sampled in the trigeminal nucleus oralis exhibited inhibition in response to conditioning mechanical stimulation applied beyond their excitatory receptive fields. The influence of this inhibition was examined over the response range of these neurones using controlled, reproducible natural stimulation procedures.2. The extent of the inhibition was graded according to the intensity of the conditioning stimulus. It was evoked most strongly by vibratory skin indentation which very effectively excites rapidly adapting afferent fibres. Tonic conditioning inputs associated with steady skin indentation were less effective.3. The slope of stimulus-response relationships constructed from responses to inputs from the excitatory receptive field was reduced in 42% of trigeminal nuclear cells in the presence of afferent-induced inhibition. In the remainder the slope was unchanged.4. There was no evidence, in the neurones subject to inhibition, of an expansion of their dynamic range defined as the range of stimulus intensities over which a neurone exhibited a graded responsiveness.5. The variability in responses of an individual neurone at a given stimulus intensity was unchanged by this inhibition.6. Analysis of the stimulus-response data using information theory statistics revealed that neurones which underwent a reduction in the slope of their stimulus-response relationship in the presence of inhibition displayed a reduced capacity for defining the intensity of skin indentation. This capacity was not modified in those neurones where the slope was unchanged by the peripherally evoked inhibition.