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Biomedical subjects

M J McLean

Publications and source records attributed to M J McLean.

At least 55 records · Page 3Linked to original sources
PubMed1990

The Marinesco-Sjögren syndrome examined by computed tomography, magnetic resonance, and 18F-2-fluoro-2-deoxy-D-glucose and positron emission tomography.

The Marinesco-Sjögren syndrome is an autosomal recessive degenerative disorder characterized by congenital cataracts, cerebellar ataxia, spasticity, mental deficiency, and skeletal abnormalities. We studied two adult siblings with Marinesco-Sjögren syndrome using anatomic and metabolic brain imaging techniques to characterize the pattern and nature of abnormalities in the brain. Computed tomographic and magnetic resonance imaging showed diffuse brain atrophy of mild to moderate degree, involving primarily the white matter of the cerebrum, cerebellum, brain stem, and cervical spinal cord. The pattern of atrophy resembled that seen in diffuse leukoencephalopathies. Measurements of local cerebral glucose metabolic rates with positron emission tomography revealed no statistically significant differences from normal control subjects in most regions, but metabolic rate was decreased in the thalamus in one patient. The findings support a diffuse white matter disorder in Marinesco-Sjögren syndrome.

Adult↗
PubMed1990

Sequence-selective, pH-dependent binding to DNA of benzophenanthridine alkaloids.

The sequence selectivity associated with binding to DNA of three alkaloids belonging to the benzophenanthridine family has been analysed by DNase I footprinting, and the results were compared with those obtained from an analysis of the behaviour of the standard intercalator, ethidium bromide. Like the ethidium, the benzophenanthridine compounds appear to bind best to regions of mixed nucleotide sequence, especially those containing alternating purines and pyrimidines, although there are some notable differences in behaviour. There is also a marked lack of binding to sequences such as (AT)n, where n greater than or equal to 3. The binding to DNA of the benzophenanthridines is specifically related to the hydrogen ion concentration of the medium, in that the DNase I footprints are considerably enhanced when the reaction is performed at a pH below 7.0. We discuss these results in terms of a greater preponderance of the intercalating species being present at lower pH.

Alkaloids↗
PubMed1989

Differences between sites of binding to DNA and strand cleavage for complexes of bleomycin with iron or cobalt.

The sequence specificity of bleomycin A5 and of its light-activated cobalt complex were compared by examining the relative cleavage of each strand of two DNA fragments by either species. Significant differences between the two metallobleomycins were observed. The iron-bleomycin (Fe-BLM) complex cleaved the DNA molecules preferentially at dinucleotides GpT and GpC, whereas the light-activated cobalt-bleomycin complex (Co-BLM) showed a preference for cutting at the dinucleotide GpA in addition to cleavage at every GpT dinucleotide. Further, new sites of preferential cleavage were noted for Co-BLM in regions of the DNA where enhanced reaction with DNAaseI can be observed in the presence of the antibiotic. No differences in the cutting behaviour of the Fe-BLM were evident upon irradiation of the reaction mixture. A reduction in the relative efficiency of cutting at GpC sequences by Co-BLM is responsible for the previously observed diminution of double-strand breaks under conditions of photoactivated cleavage. The results are discussed in terms of the likely production of highly reactive, diffusible cutting elements in the light activated reaction which cause cleavage of the DNA in regions where the antibiotic is not bound.

Autoradiography↗
PubMed1989

Echinomycin-induced hypersensitivity to osmium tetroxide of DNA fragments incapable of forming Hoogsteen base pairs.

To show conclusively that the critical structural deformation of double-helical DNA that is induced by the binding of quinoxaline antibiotics does not involve the formation of Hoogsteen base pairs, we have prepared a DNA fragment containing the nucleoside analog 7-deaza-2'-deoxyadenosine in one of the two strands. This DNA fragment was subjected to treatment with the thymidine-specific reagent osmium tetroxide and to DNase I "footprinting" in the presence or absence of micromolar concentrations of echinomycin. We report that this anti-tumor antibiotic binds to DNA containing the nucleoside analog as well as to natural DNA and that the previously reported hypersensitivity to osmium tetroxide of certain thymidine residues adjacent to echinomycin binding sites is maintained in analog-containing DNA. Since these thymidines are rendered incapable of participating in Hoogsteen base pairs by the incorporation of 7-deaza-2'-deoxyadenosine, we conclude that this unusual base-pairing scheme is not the cause of the observed hypersensitivity to osmium tetroxide and that it therefore results from a large local unwinding of the DNA in the presence of the antibiotic. Moreover, preventing the possibility of Hoogsteen base pairing does not preclude echinomycin binding.

Aminoglycosides↗
PubMed1988

Multiple DNA secondary structures in perfect inverted repeat inserts in plasmids. Right-handed B-DNA, cruciforms, and left-handed Z-DNA.

The capabilities of five recombinant plasmids, containing relatively long (approximately 60-100 base pairs) perfect inverted repeat (IR) inserts, to support supercoil stabilized non-B-DNA structures were studied in vitro. The IRs were also alternating purine-pyrimidine sequences, thus, each could form either left-handed Z-DNA or cruciforms. Single-strand specific endonucleases, restriction endonucleases and methylases, and OsO4 modifications were used to characterize the DNA structures. Two-dimensional gel electrophoretic studies indicated that three of the five IRs formed both cruciforms and Z-DNA. (C-G) containing inserts preferred to form Z-DNA, whereas (T-G) sequences favored cruciforms. In vivo supercoil relaxation experiments demonstrated the existence of cruciforms in Escherichia coli. The physiological significance of these structures is discussed.

Base Sequence↗
PubMed1988

The role of DNA sequence in the formation of Z-DNA versus cruciforms in plasmids.

The capacities of four synthetic sequences containing runs of perfectly alternating purine-pyrimidine base pairs (bp) to adopt left-handed structures were evaluated in a homologous family of recombinant plasmids. All the sequences had the same G+C content (50%) and consisted of simple tetranucleotide repeat units but differed in the relative orientations of these units. For some of the sequences, several alternate secondary structures were theoretically possible; a variety of probes (S1 nuclease, bromoacetaldehyde, OsO4, T7 gene 3 endonuclease, supercoil-induced gel relaxation studies) under a wide range of reaction conditions was used to determine which structures were adopted as a function of superhelical stress. The precise positions at the bp level of reactions with these chemical and enzymatic probes were determined. We conclude that for short (20-24 bp) sequences containing runs of alternating (T-G) and (C-A), the cruciform state is preferred over the similarly allowable left-handed form provided that symmetry constraints allow. However, these sequences can be induced to form a left-handed helix under appropriate conditions. This is the first demonstration of plasmid inserts which will adopt more than one unusual DNA structure in response to negative superhelical stress. The structural properties of a molecule containing a Z-Z junction were studied, and we conclude that the disruption caused by this feature extends over only a few bp although it requires a high energetic penalty.

Base Sequence↗
PubMed1988

Characteristics of Z-DNA helices formed by imperfect (purine-pyrimidine) sequences in plasmids.

The capacities of three synthetic sequences to adopt left-handed helices were evaluated in recombinant plasmids. The sequences consisted of very short runs of (CG)n (n = 2-4) interspersed with runs of alternating A.T base pairs and/or with regions of non-alternating base pairs. The plasmids were studied by two-dimensional gel electrophoresis to determine the natures of the conformational transitions and their free energies of formation. These results coupled with analyses with chemical (diethyl pyrocarbonate, osmium tetroxide, and bromoacetaldehyde) and enzymatic (S1 nuclease, T7 gene 3 product, and MHhaI) probes indicated that the entire sequence was adopting a left-handed helix in each case. In one of these sequences, Z-DNA formation necessitated the retention of the anti conformation of one of the guanines in a region of non-alternation. In a sequence which contains out-of-phase regions of alternation, our results indicate the formation of a separate left-handed helix in the central (CG)2 region, thus forming two Z-Z junctions. In summary, we conclude that only very short regions of alternating CG are necessary to effect the B to Z transition and that this conformational change can be transmitted through non-alternating regions. A set of empirical rules governing the characteristics of the B to Z transition and the types of left-handed helices in supercoiled plasmids was derived from studies on a systematic series of 17 plasmids.

Acetaldehyde↗
PubMed1988

Diethyl pyrocarbonate can detect a modified DNA structure induced by the binding of quinoxaline antibiotics.

The reactivity of the 160 bp tyrT DNA fragment towards diethyl pyrocarbonate (DEPC) has been investigated in the presence of bis-intercalating quinoxaline antibiotics and the synthetic depsipeptide TANDEM. At moderate concentrations of each ligand, specific purine residues (mainly adenosines) exhibit enhanced reactivity towards the probe, and several sites of enhancement appear to be related to the sequence selectivity of drug binding. Further experiments were performed with echinomycin at pH 5.5 and 4.6 to facilitate the protonation of cytosine required for formation of Hoogsteen GC base pairs. No significant increase in reactivity was observed under these conditions. Additionally, no protection of deoxyguanosine residues from methylation by dimethyl sulphate was observed in the presence of echinomycin. We conclude that the structural anomaly giving rise to drug-dependent enhanced DEPC reaction is not simply the formation of Hoogsteen base pairs adjacent to antibiotic binding sites. Nor is it due to a general unwinding of the double helix, since we show that conditions which are supposed to unwind the helix lead to a uniform increase in purine reactivity, regardless of the surrounding nucleotide sequence.

Anti-Bacterial Agents↗
PubMed1988

Chemical probes reveal no evidence of Hoogsteen base pairing in complexes formed between echinomycin and DNA in solution.

Five different DNA fragments have been treated with a range of conformationally sensitive reagents in an effort to probe structural changes in DNA associated with binding of the bis-intercalating antibiotic echinomycin. For each probe, the intensity and pattern of its reactivity with DNA have been analyzed in order to elucidate the effect of antibiotic binding on the accessibility of a specific site or sites to chemical attack. It was found that in one of the DNA fragments, pTyr2 DNA, several purine residues exhibit enhanced reactivity to diethyl pyrocarbonate (DEPC) in the absence of bound antibiotic, and that this strongly sequence specific reaction is enhanced in the presence of quite low echinomycin concentrations. The echinomycin-dependent reactivities towards DEPC of three homologous DNA fragments, chosen for their subtly different antibiotic binding characteristics, were also investigated. It was found that small changes in base sequence generate striking changes in susceptibility to modification by DEPC. The abolition of one antibiotic binding site leads to the creation of a new, intense DEPC-reactive site. In the presence of moderate concentrations of echinomycin, specific thymidine residues exhibit enhanced reactivity towards osmium tetroxide. No differences in the reactivities of the DNA fragments towards bromoacetaldehyde, S1 nuclease, dimethyl sulphate or potassium tetrachloropalladinate were observed in the presence of the antibiotic. DEPC reactions were performed on tubercidin (7-deaza-adenosine) to determine the DEPC reactive positions in situation where N-7 is inaccessible. Tubercidin was found to be generally resistant to attack by DEPC followed by treatment with base. We conclude that the bulk of structural changes induced by the binding of echinomycin to DNA do not involve Hoogsteen base pairing, but rather are due to sequence-specific unwinding of the helix in a manner which is strongly dependent on the nature of surrounding nucleotide sequences.

Acetaldehyde↗
PubMed1988

Benzodiazepines, but not beta carbolines, limit high frequency repetitive firing of action potentials of spinal cord neurons in cell culture.

Effects of benzodiazepines (BDZs) and beta carbolines (beta CCs) on sustained repetitive firing at high frequency (SRF) of action potentials of mouse spinal cord neurons in cell culture were examined using intracellular recording techniques. In control medium neurons responded to depolarizing current pulses with SRF. Limitation of SRF was produced by the anticonvulsant BDZs (diazepam, clonazepam, nitrazepam and lorazepam) at low to mid nanomolar concentrations, by a convulsant BDZ which does not bind to high affinity BDZ receptors (Ro 5-4864) at high nanomolar concentrations and by a BDZ receptor weak partial agonist (Ro 15-1788) at micromolar concentrations. The limitation of SRF was accompanied by use- and voltage-dependent reduction of maximal rate of rise (Vmax) of sodium-dependent action potentials. Partial agonist and inverse agonist beta CCs did not limit SRF at concentrations up to 200 nM. The limitation of SRF by diazepam was not prevented by inverse or partial agonists at the BDZ receptor, including Ro 15-1788 and the beta CCs. These findings suggest that limitation of SRF was produced by binding of BDZs, but not beta CCs, to voltage-dependent sodium channels and not to high affinity central BDZ receptors, and that BDZs limit SRF by slowing recovery of sodium channels from inactivation. We propose that the limitation of SRF may contribute to the efficacy of BDZs against generalized tonic-clonic seizures and status epilepticus.

Action Potentials↗
PubMed1987

Reaction conditions affect the specificity of bromoacetaldehyde as a probe for DNA cruciforms and B-Z junctions.

The reaction of bromoacetaldehyde (BAA) was investigated further with recombinant plasmids containing tracts of (CG)16, in pRW756, or (CA)32, in pRW777, which adopt left-handed Z-structures under the influence of negative supercoiling. The cruciform structures adopted by the inverted repeat sequences near the replication origins of the pBR322 vectors served as internal controls for the number of unpaired bases. The extent of reaction with the B-Z junctions and the cruciforms was dependent on the reaction and analysis conditions, the method of preparation of BAA, ionic conditions, and the amount of negative supercoiling. In contrast to the previous results of Kang and Wells, B-Z junctions in addition to cruciforms do react with BAA. However, more forcing conditions are required to detect this reaction since B-Z junctions appear to be less reactive than the single stranded loops of cruciforms. The site of reaction with DNA was readily mapped with high precision at the nucleotide level. Also, a simple method is described for determining the concentration of BAA as well as its intrinsic reactivity in a given ionic medium.

Acetaldehyde↗
PubMed1987

The segment inversion site of herpes simplex virus type 1 adopts a novel DNA structure.

The 12-base pair (bp) tandem direct repeat sequences (DR2) at the joint region (a sequence) of herpes simplex virus type 1 (strain F) adopt a new type of DNA conformation under the influence of negative supercoiling. The novel conformation is dependent on the number of the DR2 repeats; the 19 mer (228 bp total) and the 14 mer (168 bp) readily form the alternate structure whereas pentamer, trimer, and dimer repeats show somewhat different properties. S1 and P1 nuclease studies reveal that the new conformation has a major structural aberration at its center and conformational periodicities which are not identical on the complementary strands. Also, the effect of salt and pH, the location of reaction with bromo- and chloroacetaldehyde, the type of sequence (direct repeat) involved, and the nature and extent of supercoil-induced relaxations demonstrate that this structure differs from previously recognized conformations including left-handed Z helices, cruciforms, bent DNA, and slipped structures. We propose the existence of a novel conformation, anisomorphic DNA, with different structures on the complementary strands which elicit a structural aberration at the physical center of the tandem sequences. Since the oligopurine X oligopyrimidine sequence may be inherently inflexible, this supercoil-induced structural change and the physical stress on these inserts in recombinant plasmids tend to deform (crack) the DR2 sequences at their centers. Possible roles for anisomorphic DNA in the functions of this segment of intense biological activity are proposed.

Acetaldehyde↗
PubMed1987

Mechanisms of anticonvulsant drug action.

The actions of clinically used anticonvulsant drugs on sustained high frequency repetitive firing of action potentials and on responses to gamma aminobutyric acid (GABA) have been determined using mouse neurons in cell culture, and a classification of anticonvulsant drug actions has been developed based on these cellular actions. Actions of the anticonvulsant drugs were accepted as clinically relevant only if they occurred at concentrations achieved in cerebrospinal fluid or in plasma unbound to plasma proteins. Based on their cellular mechanisms of actions, drugs have been divided into 3 categories: (1) Phenytoin, carbamazepine and valproic acid limited sustained high frequency repetitive firing but did not alter GABA responses. (2) Phenobarbital and the benzodiazepines, clonazepam, diazepam and nitrazepam, augmented postsynaptic GABA responses. These drugs limited sustained high frequency repetitive firing only at concentrations above the therapeutic range in ambulatory patients, but equal to concentrations achieved in the acute treatment of status epilepticus. (3) Ethosuximide failed to reduce sustained high frequency repetitive firing or enhance GABA responses even at supertherapeutic concentrations. Limitation of sustained high frequency repetitive firing by anticonvulsant drugs correlated well with efficacy against generalized tonic-clonic seizures in man and against maximal electroshock seizures in experimental animals. Enhancement of postsynaptic GABA responses correlated with efficacy against generalized absence seizures in man and against pentylenetetrazol seizures in animals. Ethosuximide, however, did not alter GABA responses or sustained high frequency repetitive firing suggesting that its action against generalized absence seizures in man and pentylenetetrazol seizures in experimental animals occurs by an additional, as yet unknown, mechanism.

Action Potentials↗
PubMed1986

Synthesis and antiviral activity of (E)-5-(2-bromovinyl)uracil and (E)-5-(2-bromovinyl)uridine.

(E)-5-(2-Bromovinyl)uracil (BVU) and (E)-5-(2-bromovinyl)uridine (BVRU) were synthesized starting from 5-formyluracil via (E)-5-(2-carboxyvinyl)uracil or starting from 5-iodouridine via (E)-5-(2-carbomethoxyvinyl)uridine and (E)-5-(2-carboxyvinyl)uridine, respectively. Depending on the choice of the cell system, BVU and BVRU exhibited a marked activity against herpes simplex virus type 1 (HSV-1) in vitro. Although BVU and BVRU were less potent than the reference compound (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), their antiviral activity spectrum was remarkably similar to that of BVDU. The latter findings suggest that BVU and BVRU are metabolically converted to BVDU or a phosphorylated product thereof. In vivo, BVU protected mice against a lethal disseminated HSV-1 infection.

Animals↗
PubMed1986

Consecutive A X T pairs can adopt a left-handed DNA structure.

The capacity of six sequences with different numbers and orientations of A.T pairs flanked by alternating C.G pairs to adopt left-handed structures was evaluated in recombinant plasmids. A series of synthetic oligodeoxynucleotides were cloned into the BamHI site of pRW790, a small plasmid (approximately 2 kilobases) prepared especially for conformational studies of this type. Supercoil relaxation studies by two-dimensional gel electrophoresis on topoisomers of each plasmid revealed the energetics and structures of the left-handed helices. Also, the presence of supercoil-induced altered DNA conformations within the inserts of topoisomer populations of the plasmids was detected by reaction with S1 nuclease followed by restriction mapping of the cleavage sites. We conclude that consecutive T.A base pairs, whether alternating (TATA) or contiguous (TTTT), can adopt a left-handed conformation (presumably Z) when flanked by reasonably short runs of alternating (C-G)n (n = 3-5). Thus, these results substantially broaden the range of DNA sequences that can adopt left-handed Z conformations.

Adenine↗
PubMed1986

Anticonvulsant drugs: mechanisms of action.

A variety of the anticonvulsant drugs, including carbamazepine, phenytoin, primidone, phenobarbital, clonazepam, valproic acid, and ethosuximide, are available for use in the treatment of patients with seizure disorders. These anticonvulsants vary in their efficacy against experimental seizures in animals and against seizures in humans. The mechanistic basis for this variability in anticonvulsant drug action remains uncertain, but numerous mechanisms of action have been proposed. We have used mouse neurons in primary dissociated cell culture to study the action of these anticonvulsant drugs on several aspects of membrane excitability and synaptic transmission. We have proposed that the anticonvulsant drugs can be classified according to their actions on sustained high frequency repetitive firing (SRF) of action potentials and on postsynaptic gamma-aminobutyric acid (GABA) responses. Phenytoin and carbamazepine were both effective against SRF but did not modify postsynaptic GABA responses at therapeutically relevant concentrations. Phenobarbital, benzodiazepines, and valproic acid modified both SRF and postsynaptic GABA responses. Ethosuximide had no effect on SRF or GABAergic mechanisms. Based on these results, we have proposed that blockade of SRF may underlie the action of phenytoin, carbamazepine, phenobarbital, valproic acid, and benzodiazepines against generalized tonic-clonic seizures in humans and maximal electroshock seizures in animals. Enhancement of GABAergic synaptic transmission may underlie efficacy of benzodiazepines and valproic-acid drugs against generalized absence seizures in humans and pentylenetetrazol-induced seizures in experimental animals. The mechanism of action of ethosuximide against generalized absence seizures in humans and pentylenetetrazol-induced seizures in experimental animals may be by a third, as yet unknown, mechanism.

Action Potentials↗
PubMed1986

Carbamazepine and 10,11-epoxycarbamazepine produce use- and voltage-dependent limitation of rapidly firing action potentials of mouse central neurons in cell culture.

Effects of the anticonvulsant, carbamazepine (CBZ), on mouse central neurons in cell culture were examined using intracellular recording techniques. Spinal cord and cortical neurons demonstrated sustained repetitive firing (SRF) of action potentials (APs) at high frequency in response to depolarizing current pulses. Hippocampal neurons fired rapidly only in the pressure of the calcium channel blocker verapamil (1 microM). Concentrations of CBZ equivalent to therapeutic anticonvulsant levels in cerebrospinal fluid (greater than 4.2 microM) limited firing to a few action potentials in all three cell types in parallel with use-dependent reduction of AP maximal rate of rise (Vmax). Resting membrane properties and postsynaptic responses to ionophoretically applied gamma-aminobutyric acid and glutamate were unaffected. The CBZ metabolite, 10,11-epoxycarbamazepine, limited SRF at a concentration (4 microM) encountered at therapeutic CBZ levels. Another major CBZ metabolite, the 10,11-diol, limited SRF only at concentrations greater than those encountered with therapeutic serum CBZ levels. In control medium, APs could be evoked at a maximal rate of 450 Hz while in CBZ (10.6 microM) AP frequency was limited to 100 to 200 Hz. Limitation of SRF was voltage-dependent and could be partially reversed, or prevented, by membrane hyperpolarization. In addition, recovery of Vmax of sodium-dependent action potentials from inactivation was slowed, suggesting an effect on voltage-dependent sodium channels. CBZ and phenytoin effects of SRF were similar. We suggest that limitation of sustained high frequency repetitive firing may be a significant anticonvulsant action of carbamazepine. The 10,11-epoxy derivative of CBZ, but not the 10,11-diol, may contribute to anticonvulsant efficacy by the same mechanism.

Action Potentials↗