Biomedical subjects

S J Martin

Publications and source records attributed to S J Martin.

At least 37 records · Page 2Linked to original sources

PubMed2001

The activity of 14-hydroxy clarithromycin, alone and in combination with clarithromycin, against penicillin- and erythromycin-resistant Streptococcus pneumoniae.

There are no data regarding the activity of clarithromycin's active metabolite, 14-hydroxy clarithromycin, against penicillin-intermediate, penicillin-resistant or erythromycin-resistant Streptococcus pneumoniae. Agar dilution MICs were determined for clarithromycin, 14-hydroxy clarithromycin (henceforth called 'metabolite'), azithromycin, erythromycin and clarithromycin/metabolite (2:1 and 1:1 ratio) against 24 penicillin-intermediate and 14 penicillin-resistant strains, including 13 erythromycin-resistant clinical strains and one ATCC strain of S. pneumoniae. The interaction between clarithromycin and its metabolite was determined using an agar chequerboard assay against all isolates, and time-kill tests were performed against five penicillin-intermediate (macrolide-susceptible) and five penicillin-resistant (two macrolide-resistant) strains of S. pneumoniae using all antibiotics alone at simulated peak serum concentrations, and clarithromycin/metabolite in a 2:1 ratio (physiological). MICs were as follows: clarithromycin, 0.008-->64 mg/L; metabolite, 0.015-->64 mg/L; erythromycin, 0.015-->64 mg/L; azithromycin, 0.125-->64 mg/L; clarithromycin/metabolite (1:1 and 2:1 combinations), 0.001-->64 mg/L. The MIC of the clarithromycin/metabolite combination was one or more tube dilution lower than the MIC of clarithromycin in 28 of the isolates tested. In chequerboard testing, 13 strains (seven erythromycin susceptible and six erythromycin resistant) demonstrated synergy, 18 additivity and seven indifference. In time-kill testing, bacterial eradication below detection limits occurred with clarithromycin and metabolite in seven of 10 organisms. The combination of parent and metabolite was more rapidly bactericidal than clarithromycin alone in six of the seven isolates (P = 0.026). The metabolite has potent activity against S. pneumoniae and enhances the activity of the parent compound against this organism. The metabolite's activity must be considered in evaluating clarithromycin in vitro to avoid underestimation of clarithromycin's activity against the pneumococcus.

Anti-Bacterial Agents↗

PubMed2001

A risk-benefit assessment of levofloxacin in respiratory, skin and skin structure, and urinary tract infections.

As a class, the quinolone antibacterials can no longer be assumed to be both effective and relatively free of significant adverse effects. Recent safety issues with newer generation fluoroquinolones, and concerns regarding drug-use associated bacterial resistance have made all drugs in this class subject to intense scrutiny and further study. Levofloxacin is a second generation fluoroquinolone with a post marketing history of well tolerated and successful use in a variety of clinical situations. Quinolones as a class cause a variety of adverse effects, including phototoxicity, seizures and other CNS disturbances, tendonitis and arthropathies, gastrointestinal effects, nephrotoxicity, prolonged QTc interval and torsade de pointes, hypo- or hyperglycaemia, and hypersensitivity reactions. Levofloxacin has been involved in only a few case reports of adverse events, which include QTc prolongation, seizures, glucose disturbances, and tendonitis. Levofloxacin has been shown to be effective at dosages of 250mg to 500mg once-daily in clinical trials in the management of acute maxillary sinusitis, acute bacterial exacerbations of chronic bronchitis, community-acquired pneumonia, skin and skin structure infections, and urinary tract infections. There are data suggesting that levofloxacin may promote fluoroquinolone resistance among the Streptococcus pneumoniae, and that clinical failures may result from this therapy. Other data suggest that fluoroquinolones with lower potency against Pseudomonas aeruginosa than ciprofloxacin, such as levofloxacin, may drive class-wide resistance to this pathogen. Levofloxacin is an effective drug in many clinical situations, but its cost is significantly higher than amoxicillin, erythromycin, or first and second generation cefalosporins. Because of the propensity to select for fluoroquinolone resistance in the pneumococcus and potentially other pathogens, levofloxacin should be an alternative agent rather than a drug-of-choice in routine community-acquired respiratory tract, urinary tract, and skin or skin structure infections. In areas with increasing pneumococcal beta-lactam resistance, levofloxacin may be a reasonable empiric therapy in community-acquired respiratory tract infections. Similarly, in patients with risk factors for infectious complications or poor outcome, levofloxacin may be an excellent empiric choice in severe community-acquired respiratory tract infections, urinary tract infections, complicated skin or skin structure infections, and nosocomial respiratory and urinary tract infections. Better clinical data are needed to identify the true place in therapy of the newer fluoroquinolones in common community-acquired and nosocomial infections. Until then, these agents, including levofloxacin, might best be reserved for complicated infections, infection recurrence, and infections caused by beta-lactam or macrolide-resistant pathogens.

Animals↗

PubMed2000

Executioner caspase-3, -6, and -7 perform distinct, non-redundant roles during the demolition phase of apoptosis.

Apoptosis is orchestrated by a family of cysteine proteases known as the caspases. Fourteen mammalian caspases have been identified, three of which (caspase-3, -6, and -7) are thought to coordinate the execution phase of apoptosis by cleaving multiple structural and repair proteins. However, the relative contributions that the "executioner" caspases make to the demolition of the cell remains speculative. Here we have used cell-free extracts immuno-depleted of either caspase-3, -6, or -7 to examine the caspase requirements for apoptosis-associated proteolysis of 14 caspase substrates as well as nuclear condensation, chromatin margination, and DNA fragmentation. We show that caspase-3 is the primary executioner caspase in this system, necessary for cytochrome c/dATP-inducible cleavage of fodrin, gelsolin, U1 small nuclear ribonucleoprotein, DNA fragmentation factor 45 (DFF45)/inhibitor of caspase-activated DNase (ICAD), receptor-interacting protein (RIP), X-linked inhibitor of apoptosis protein (X-IAP), signal transducer and activator of transcription-1 (STAT1), topoisomerase I, vimentin, Rb, and lamin B but not for cleavage of poly(ADP-ribose) polymerase (PARP) or lamin A. In addition, caspase-3 was also essential for apoptosis-associated chromatin margination, DNA fragmentation, and nuclear collapse in this system. Surprisingly, although caspase-6 and -7 are considered to be important downstream effector caspases, depletion of either caspase had minimal impact on any of the parameters investigated, calling into question their precise role during the execution phase of apoptosis.

Adenosine Triphosphate↗

PubMed2000

Failure of Bcl-2 to block cytochrome c redistribution during TRAIL-induced apoptosis.

Tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family of cytokines that promotes apoptosis and NF-kappaB activation. Here we show that recombinant hu-TRAIL initiates the activation of multiple caspases, the loss of mitochondrial transmembrane potential, the cleavage of BID and the redistribution of mitochondrial cytochrome c. However, whereas Bcl-2 efficiently blocked UV radiation-induced cytochrome c release and consequent apoptosis of CEM cells, it failed to do either in the context of TRAIL treatment. Thus, TRAIL engages a death pathway that is at least partially routed via the mitochondria, but in contrast with other stimuli that engage this pathway, TRAIL-induced cytochrome c release is not regulated by Bcl-2.

Animals↗

PubMed2000

Equivalent-circuit model for the thickness-shear mode resonator with a viscoelastic film near film resonance.

We derive a lumped-element, equivalent-circuit model for the thickness-shear mode (TSM) resonator with a viscoelastic film. This modified Butterworth-Van Dyke model includes in the motional branch a series LCR resonator, representing the quartz resonance, and a parallel LCR resonator, representing the film resonance. This model is valid in the vicinity of film resonance, which occurs when the acoustic phase shift across the film is an odd multiple of pi/2 rad. For low-loss films, this model accurately predicts the frequency changes and damping that arise at resonance and is a reasonable approximation away from resonance. Elements of the parallel LCR resonator are explicitly related to film properties and can be interpreted in terms of elastic energy storage and viscous power dissipation. The model leads to a simple graphical interpretation of the coupling between the quartz and film resonances and facilitates understanding of the resulting responses. These responses are compared with predictions from the transmission-line and Sauerbrey models.

Elasticity↗

PubMed2000

Conformational changes during proteolytic processing of a picornavirus capsid proteins.

We have used synthetic peptide antibodies to probe conformational changes that occur during the cleavage cascade which generates the capsid proteins of a picornavirus. The initial translation product of 97 kDa, the precursor of all four structural proteins, is cleaved to form a 63 kDa fragment which, we show, has significantly different folding characteristics to both its larger parent and its products. We demonstrate that proteolytic cleavages as distant as 520 residues from epitopes confer sufficiently large conformational changes as to render them unrecognisable. To our knowledge, this is the first demonstration of this phenomenon in the picornavirus system.

Animals↗

PubMed2000

Cleavage of BID during cytotoxic drug and UV radiation-induced apoptosis occurs downstream of the point of Bcl-2 action and is catalysed by caspase-3: a potential feedback loop for amplification of apoptosis-associated mitochondrial cytochrome c release.

BID, a pro-apoptotic Bcl-2 family member, promotes cytochrome c release during apoptosis initiated by CD95L or TNF. Activation of caspase-8 in the latter pathways results in cleavage of BID, translocation of activated BID to mitochondria, followed by redistribution of cytochrome c to the cytosol. However, it is unclear whether BID participates in cytochrome c release in other (non-death receptor) cell death pathways. Here, we show that BID is cleaved in response to multiple death-inducing stimuli (staurosporine, UV radiation, cycloheximide, etoposide). However BID cleavage in these contexts was blocked by Bcl-2, suggesting that proteolysis of BID occurred distal to cytochrome c release. Furthermore, addition of cytochrome c to Jurkat post-nuclear extracts triggered breakdown of BID at Asp-59 which was catalysed by caspase-3 rather than caspase-8. We provide evidence that caspase-3 catalysed cleavage of BID represents a feedback loop for the amplification of mitochondrial cytochrome c release during cytotoxic drug and UV radiation-induced apoptosis.

Animals↗

PubMed2000

The viral nucleocapsid protein of transmissible gastroenteritis coronavirus (TGEV) is cleaved by caspase-6 and -7 during TGEV-induced apoptosis.

The transmissible gastroenteritis coronavirus (TGEV), like many other viruses, exerts much of its cytopathic effect through the induction of apoptosis of its host cell. Apoptosis is coordinated by a family of cysteine proteases, called caspases, that are activated during apoptosis and participate in dismantling the cell by cleaving key structural and regulatory proteins. We have explored the caspase activation events that are initiated upon infection of the human rectal tumor cell line HRT18 with TGEV. We show that TGEV infection results in the activation of caspase-3, -6, -7, -8, and -9 and cleavage of the caspase substrates eIF4GI, gelsolin, and alpha-fodrin. Surprisingly, the TGEV nucleoprotein (N) underwent proteolysis in parallel with the activation of caspases within the host cell. Cleavage of the N protein was inhibited by cell-permeative caspase inhibitors, suggesting that this viral structural protein is a target for host cell caspases. We show that the TGEV nucleoprotein is a substrate for both caspase-6 and -7, and using site-directed mutagenesis, we have mapped the cleavage site to VVPD(359) downward arrow. These data demonstrate that viral proteins can be targeted for destruction by the host cell death machinery.

Amino Acid Chloromethyl Ketones↗

PubMed2000

Synaptic plasticity and memory: an evaluation of the hypothesis.

Changing the strength of connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. In its most general form, the synaptic plasticity and memory hypothesis states that "activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the information storage underlying the type of memory mediated by the brain area in which that plasticity is observed." We outline a set of criteria by which this hypothesis can be judged and describe a range of experimental strategies used to investigate it. We review both classical and newly discovered properties of synaptic plasticity and stress the importance of the neural architecture and synaptic learning rules of the network in which it is embedded. The greater part of the article focuses on types of memory mediated by the hippocampus, amygdala, and cortex. We conclude that a wealth of data supports the notion that synaptic plasticity is necessary for learning and memory, but that little data currently supports the notion of sufficiency.

Amygdala↗

PubMed1999

Characteristics of acoustic plate modes on rotated Y-cuts of quartz utilized for biosensing applications.

Acoustic plate modes (APM) on various quartz substrates have been investigated in order to determine their usefulness for liquid-sensing applications. The modes have been characterized in terms of their mass sensitivity, mode separation, temperature sensitivity, and reproducibility of the experimental results. Promising characteristics are found for rotated Y-cuts of quartz with the direction of acoustic mode propagation being perpendicular to the X-axis of the quartz crystal. Experiments on the detection of immunochemical reactions are performed using different quartz APM sensors, and the results are compared to similar experiments utilizing APM devices on ZX-LiNbO3.

Animals↗

PubMed1999

Regulation of apoptotic protease activating factor-1 oligomerization and apoptosis by the WD-40 repeat region.

Apoptotic protease activating factor-1 (Apaf-1) has been identified as a proximal activator of caspase-9 in cell death pathways that trigger mitochondrial damage and cytochrome c release. The mechanism of Apaf-1 action is unclear but has been proposed to involve the clustering of caspase-9 molecules, thereby facilitating autoprocessing of adjacent zymogens. Here we show that Apaf-1 can dimerize via the CED-4 homologous and linker domains of the molecule providing a means by which Apaf-1 can promote the clustering of caspase-9 and facilitate its activation. Apaf-1 dimerization was repressed by the C-terminal half of the molecule, which contains multiple WD-40 repeats, but this repression was overcome in the presence of cytochrome c and dATP. Removal of the WD-40 repeat region resulted in a constitutively active Apaf-1 that exhibited greater cytotoxicity in transient transfection assays when compared with full-length Apaf-1. These data suggest a mechanism for Apaf-1 function and reveal an important regulatory role for the WD-40 repeat region.

Apoptosis↗

PubMed1999

Ordering the cytochrome c-initiated caspase cascade: hierarchical activation of caspases-2, -3, -6, -7, -8, and -10 in a caspase-9-dependent manner.

Exit of cytochrome c from mitochondria into the cytosol has been implicated as an important step in apoptosis. In the cytosol, cytochrome c binds to the CED-4 homologue, Apaf-1, thereby triggering Apaf-1-mediated activation of caspase-9. Caspase-9 is thought to propagate the death signal by triggering other caspase activation events, the details of which remain obscure. Here, we report that six additional caspases (caspases-2, -3, -6, -7, -8, and -10) are processed in cell-free extracts in response to cytochrome c, and that three others (caspases-1, -4, and -5) failed to be activated under the same conditions. In vitro association assays confirmed that caspase-9 selectively bound to Apaf-1, whereas caspases-1, -2, -3, -6, -7, -8, and -10 did not. Depletion of caspase-9 from cell extracts abrogated cytochrome c-inducible activation of caspases-2, -3, -6, -7, -8, and -10, suggesting that caspase-9 is required for all of these downstream caspase activation events. Immunodepletion of caspases-3, -6, and -7 from cell extracts enabled us to order the sequence of caspase activation events downstream of caspase-9 and reveal the presence of a branched caspase cascade. Caspase-3 is required for the activation of four other caspases (-2, -6, -8, and -10) in this pathway and also participates in a feedback amplification loop involving caspase-9.

Animals↗

PubMed1999

Reversible neural inactivation reveals hippocampal participation in several memory processes.

Studies of patients and animals with brain lesions have implicated the hippocampal formation in spatial, declarative/relational and episodic types of memory. These and other types of memory consist of a series of interdependent but potentially dissociable memory processes-encoding, storage, consolidation and retrieval. To identify whether hippocampal activity contributes to these processes independently, we used a novel method of inactivating synaptic transmission using a water-soluble antagonist of AMPA/kainate glutamate receptors. Once calibrated using electrophysiological and two-deoxyglucose techniques in vivo, drug or vehicle was infused chronically or acutely into the dorsal hippocampus of rats at appropriate times during or after training in a water maze. Our findings indicate that hippocampal neural activity is necessary for both encoding and retrieval of spatial memory and for either trace consolidation or long-term storage.

Animals↗

PubMed1999

Serial killers: ordering caspase activation events in apoptosis.

Caspases participate in the molecular control of apoptosis in several guises; as triggers of the death machinery, as regulatory elements within it, and ultimately as a subset of the effector elements of the machinery itself. The mammalian caspase family is steadily growing and currently contains 14 members. At present, it is unclear whether all of these proteases participate in apoptosis. Thus, current research in this area is focused upon establishing the repertoire and order of caspase activation events that occur during the signalling and demolition phases of cell death. Evidence is accumulating to suggest that proximal caspase activation events are typically initiated by molecules that promote caspase aggregation. As expected, distal caspase activation events are likely to be controlled by caspases activated earlier in the cascade. However, recent data has cast doubt upon the functional demarcation of caspases into signalling (upstream) and effector (downstream) roles based upon their prodomain lengths. In particular, caspase-3 may perform an important role in propagating the caspase cascade, in addition to its role as an effector caspase within the death programme. Here, we discuss the apoptosis-associated caspase cascade and the hierarchy of caspase activation events within it.

Animals↗

PubMed1998

Time-dependent reversal of dentate LTP by 5 Hz stimulation.

Field potential recordings were made from the dentate gyrus of urethane-anaesthetized rats in order to investigate the ability of 5 Hz stimulation to reverse long-term potentiation (LTP) induced by a high frequency tetanus. A 10 min train of 5 Hz was found to reverse LTP in a time-dependent fashion: as the interval between tetanus and 5 Hz was increased, LTP became progressively less susceptible to reversal. If 10 min or 30 min intervened between tetanization and 5 Hz stimulation, LTP was unaffected. These results indicate that dentate LTP in vivo exhibits a similar limited time window of vulnerability to reversal by low frequency stimulation to that previously reported in area CA1 in vitro.

Analysis of Variance↗

PubMed1998

Inhibition of TNF-induced apoptosis by NF-kappa B.

Tumour necrosis factor (TNF) is an inflammatory cytokine possessing a unique property: it can induce cells to undergo apoptosis. The sensitivity of different cell types to TNF-induced apoptosis can vary dramatically, but most cells become very sensitive upon simultaneous treatment with inhibitors of protein synthesis. It has been suggested therefore that a gene, or set of genes, is induced upon TNF receptor activation that downregulates the apoptosis signal. Recent results have shown that NF-kappa B, a transcription factor activated upon TNF signalling, is at least partly responsible for this effect. These findings have broadened the role of NF-kappa B from that of a regulator of immune and inflammatory responses to include an involvement in the regulation of apoptosis.

Animals↗

PubMed1998

Anti-apoptotic oncogenes prevent caspase-dependent and independent commitment for cell death.

Apoptosis is a morphologically defined type of cell death associated with the activation of certain proteases belonging to the ICE/CED-3 family, known as caspases. Resistance to apoptosis has been implicated as one of the mechanisms that participates in oncogenesis. We found that the broad-spectrum peptide inhibitor of the caspases, zVAD-fmk, interferes in a dose-dependent way with all the morphological and biochemical changes associated with apoptosis induced by anti-CD95 mAb, staurosporine, VP-16 and Act-D. However, with the exception of anti-CD95-triggered apoptosis, the insulted cells lost their clonogenic potential, even when pre-treated with a high dose of zVAD-fmk. Under these circumstances, the dying cells displayed no signs of apoptosis, including activation of caspases, externalization of phosphatidylserine, nuclear condensation, or DNA fragmentation. Instead, this cell death was characterized by cytoplasmic and nuclear vacuolization followed by the loss of plasma membrane integrity. Thus, preventing the onset of apoptosis by blocking caspase activity did not rescue cells from dying in response to drugs such as staurosporine, VP-16 and Act-D. In comparison, ectopic expression of anti-apoptotic oncogenes such as bcl-2 and bcr-abl not only inhibited apoptosis but also preserved the clonogenic potential of the cells. Therefore, oncogenesis is promoted not by simply interfering with caspase-mediated apoptosis, but by preventing an upstream event which we define as the commitment point for cell death.

Amino Acid Chloromethyl Ketones↗

PubMed1998

Bactericidal activity and postantibiotic effect of clarithromycin and 14-hydroxyclarithromycin, alone and in combination, against Legionella pneumophila.

Time-kill curves using simulated peak and trough serum concentrations, and postantibiotic effect (PAE) were determined for clarithromycin and 14-hydroxyclarithromycin alone and in combination against four strains of Legionella pneumophila. Both compounds were bactericidal at both peak and trough concentrations. The combination at trough concentrations demonstrated lower killing activity than the parent drug in three of the four strains. PAE ranges were 7.28-17.3 h for clarithromycin, 6.78-14.77 h for the metabolite, and 5.15-13.23 h for the combination.

Anti-Bacterial Agents↗