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J B Clark

Publications and source records attributed to J B Clark.

At least 37 records · Page 2Linked to original sources
PubMed1999

A P element-homologous sequence in the house fly, Musca domestica.

Sequences homologous to the P transposable element have been identified in Musca domestica. Sequence analysis of a genomic clone (Md-P1) indicates that, although the house fly P element has lost its coding capacity, the basic general structure of drosophilid P elements is present. The house fly P element sequence shares a number of structural features with that from the blow fly, Lucilia cuprina, including a large intron separating exons 1 and 2, two additional introns interrupting exon 2 and the apparent absence of inverted repeat termini. Within a relatively well-conserved central region, the house fly sequence shows 59% similarity to the D. melanogaster P element, but distal regions are more diverged. Southern blot analysis of several strains indicated the presence of at least four P element copies.

Amino Acid Sequence↗
PubMed1999

Stimulation of the brain NO/cyclic GMP pathway by peripheral administration of tetrahydrobiopterin in the hph-1 mouse.

Mutations in GTP-cyclohydrolase I (GTP-CH) have been identified as causing a range of inborn errors of metabolism, including dopa-responsive dystonia. GTP-CH catalyses the first step in the biosynthesis of tetrahydrobiopterin (BH4), a cofactor necessary for the synthesis of catecholamines and serotonin. Current therapy based on monoamine neurotransmitter replacement may be only partially successful in correcting the neurological deficits. The reason might be that BH4 is also a cofactor for nitric oxide synthase. Using a strain of mutant GTP-CH-deficient (hph-1) mice, we demonstrate that in addition to impaired monoamine metabolism, BH4 deficiency is also associated with diminished nitric oxide synthesis in the brain (as evaluated by measuring the levels of cyclic GMP), when compared with wild-type animals. We have found a decline in the levels of BH4 with age in all animals, but no gender-related differences. We found a strong association between the levels of BH4 and cyclic GMP in hph-1 mice but not in wild-type animals. We also demonstrate that acute peripheral administration of BH4 (100 micromol/kg s.c.) in hph-1 mice significantly elevated the brain BH4 concentration and subsequently cyclic GMP levels in cerebellum, with peaks at 2 and 3 h, respectively. We suggest that BH4 administration should be considered in BH4 deficiency states in addition to monoamine replacement therapy.

Animals↗
PubMed1999

Noise-induced neurologic disturbances in divers exposed to intense water-borne sound: two case reports.

Divers may be exposed to intense noise underwater. Two cases of neurologic disturbances during experimental exposures to 15 min of continuous underwater sound are described. Sound exposure in the first case consisted of a warble tone with center frequency of 240 Hz and a sound pressure level of 160 dB re 1 microPa. Symptoms during exposure consisted of somnolence, lightheadedness, and an inability to concentrate. No apparent effect on hearing was noted. In the second case, a center frequency of 1,000 Hz at 181 dB was used. Lightheadedness, inability to concentrate, agitation, and head vibrations were noted during the exposure. The diver also exhibited a temporary auditory threshold shift of 19.2 dB. In both cases, overt symptoms resolved within 30 min after exposure, but both divers reported recurrent symptoms days to weeks after the exposures. Medical histories and examinations, assessment of dive profiles, and breathing gas analysis failed to support a source other than the sound exposures to account for the symptoms observed. Potential mechanisms for the described symptoms are discussed.

Adult↗
PubMed1998

Increased inducible nitric oxide synthase protein but limited nitric oxide formation occurs in astrocytes of the hph-1 (tetrahydrobiopterin deficient) mouse.

It has been suggested that decreased tetrahydrobiopterin (BH4) availability may be a useful tool for limiting excessive nitric oxide (NO) formation. In order to test this hypothesis we utilised cultured astrocytes derived from the brain of the hph-1 (BH4 deficient) mouse. In response to treatment with lipopolysaccharide and interferon-gamma (LPS/gammaIFN) levels of BH4 doubled in both wild type and hph-1 astrocytes. However, levels of BH4 in hph-1 astrocytes remained only 25% of the wild type astrocytes. Nitric oxide formation, measured with an NO-electrode, was 45% less from LPS/gammaIFN stimulated hph-1 astrocytes compared with wild type stimulated astrocytes. In contrast, iNOS specific activity and iNOS protein were enhanced in hph-1 stimulated astrocytes by 40 and 60%, respectively when compared with wild type. In conclusion it appears that whilst a decrease in BH4 may limit NO release per se, the possibility and consequences of long term 'over' induction of iNOS protein requires further consideration.

Animals↗
PubMed1998

Decreased endothelial cell glutathione and increased sensitivity to oxidative stress in an in vitro blood-brain barrier model system.

Using a cell culture model of the blood-brain barrier (BBB) we have evaluated the role of endothelial cell glutathione in protecting barrier integrity against nitric oxide (NO)-induced oxidative stress. The co-culture of human umbilical vein endothelial cells (ECV304) with rat (C6) glioma cells, or incubation with glioma cell or primary astrocytic conditioned medium, resulted in a decline in endothelial cell glutathione. Exposure to a single addition of NO gas induced a rapid breakdown in model barrier integrity in endothelial/glioma co-cultures. Addition of NO gas or tumour necrosis factor-alpha (TNF-alpha) also resulted in a loss of membrane integrity, as measured by an enhanced release of lactate dehydrogenase, only from endothelial cells treated with glioma conditioned medium. Furthermore, assessment of viability in endothelial cells grown alone or treated with glioma conditioned medium, by propidium iodide labelled flow cytometry. demonstrated no difference in the number of positively stained cells after NO exposure. These results indicate that when enhanced endothelial monolayer barrier formation occurs via astrocytic-endothelial interactions, cellular glutathione levels are decreased. This renders the barrier cells, under these conditions, more susceptible to oxidative stress but does no necessarily lead to greater cell death.

Animals↗
PubMed1998

Energy thresholds in brain mitochondria. Potential involvement in neurodegeneration.

Decreases in mitochondrial respiratory chain complex activities have been implicated in neurodegenerative disorders such as Parkinson's disease, Huntington's disease, and Alzheimer's disease. However, the extent to which these decreases cause a disturbance in oxidative phosphorylation and energy homeostasis in the brain is not known. We therefore examined the relative contribution of individual mitochondrial respiratory chain complexes to the control of NAD-linked substrate oxidative phosphorylation in synaptic mitochondria. Titration of complex I, III, and IV activities with specific inhibitors generated threshold curves that showed the extent to which a complex activity could be inhibited before causing impairment of mitochondrial energy metabolism. Complex I, III, and IV activities were decreased by approximately 25, 80, and 70%, respectively, before major changes in rates of oxygen consumption and ATP synthesis were observed. These results suggest that, in mitochondria of synaptic origin, complex I activity has a major control of oxidative phosphorylation, such that when a threshold of 25% inhibition is exceeded, energy metabolism is severely impaired, resulting in a reduced synthesis of ATP. Additionally, depletion of glutathione, which has been reported to be a primary event in idiopathic Parkinson's disease, eliminated the complex I threshold in PC12 cells, suggesting that antioxidant status is important in maintaining energy thresholds in mitochondria. The implications of these findings are discussed with respect to neurodegenerative disorders and energy metabolism in the synapse.

Adenosine Triphosphate↗
PubMed1998

Comparison of mitochondrial respiratory chain enzyme activities in rodent astrocytes and neurones and a human astrocytoma cell line.

This study has found that mitochondrial NADH-CoQ1 reductase (complex I) activity is significantly lower in C57 mice astrocytes compared with Wistar and Sprague-Dawley rat astrocytes, and a human astrocytoma cell line. In addition, complex I activity is 4-fold greater in Sprague-Dawley neurones when compared to Wistar or C57 neurones. These findings have important implications for mitochondrial studies involving rodent or human cell line systems, and in particular, indicate the importance of choosing an appropriate model when investigating the mitochondrial respiratory chain.

Animals↗
PubMed1998

Molecular cloning of rat mitochondrial 3-hydroxy-3-methylglutaryl-CoA lyase and detection of the corresponding mRNA and of those encoding the remaining enzymes comprising the ketogenic 3-hydroxy-3-methylglutaryl-CoA cycle in central nervous system of suckling rat.

We have investigated, by RNase protection assays in rat brain regions and primary cortical astrocyte cultures, the presence of the mRNA species encoding the three mitochondrially located enzymes acetoacetyl-CoA thiolase, mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mt. HMG-CoA synthase) and HMG-CoA lyase (HMG-CoA lyase) that together constitute the ketogenic HMG-CoA cycle. As a prerequisite we obtained a full-length cDNA encoding rat HMG-CoA lyase by degenerate oligonucleotide-primed PCR coupled to a modification of PCR-rapid amplification of cDNA ends (PCR-RACE). We report here: (1) the nucleotide sequence of rat mt. HMG-CoA lyase, (2) detection of the mRNA species encoding all three HMG-CoA cycle enzymes in all regions of rat brain during suckling, (3) approximately twice the abundance of mt. HMG-CoA synthase mRNA in cerebellum than in cortex in 11-day-old suckling rat pups, (4) significantly lower abundances of mt. HMG-CoA synthase mRNA in brain regions derived from rats weaned to a high-carbohydrate/low-fat diet compared with the corresponding regions derived from the suckling rat, and (5) the presence of mt. HMG-CoA synthase mRNA in primary cultures of neonatal cortical astrocytes at an abundance similar to that found in liver of weaned animals. These results provide preliminary evidence that certain neural cell types possess ketogenic potential and might thus have a direct role in the provision of fatty acid-derived ketone bodies during the suckling period.

Acyl Coenzyme A↗
PubMed1998

Molecular evolution of P transposable elements in the Genus drosophila. II. The obscura species group.

A phylogenetic analysis of P transposable elements in the Drosophila obscura species group is described. Multiple P sequences from each of 10 species were obtained using PCR primers that flank a conserved region of exon 2 of the transposase gene. In general, the P element phylogeny is congruent with the species phylogeny, indicating that the dominant mode of transmission has been vertical, from generation to generation. One manifestation of this is the distinction of P elements from the Old World obscura and subobscura subgroups from those of the New World affinis subgroup. However, the overall distribution of elements within the obscura species group is not congruent with the phylogenetic relationships of the species themselves. There are at least four distinct subfamilies of P elements, which differ in sequence from each other by as much as 34%, and some individual species carry sequences belonging to different subfamilies. P sequences from D. bifasciata are particularly interesting. These sequences belong to two subfamilies and both are distinct from all other P elements identified in this survey. Several mechanisms are postulated to be involved in determining phylogenetic relationships among P elements in the obscura group. In addition to vertical transmission, these include retention of ancestral polymorphisms and horizontal transfer by an unknown mating-independent mechanism.

Animals↗
PubMed1998

Alterations in transendothelial electrical resistance by vasoactive agonists and cyclic AMP in a blood-brain barrier model system.

We have previously reported that the co-culture of endothelial and glioma cell lines provides an in vitro model for investigating properties of the blood-brain barrier (BBB). To characterise the model system further we have investigated the effects of vasoactive substances implicated in increases in BBB permeability. Additionally, we have also examined whether activation of cyclic AMP signalling pathways, which elevate cerebral endothelial cell barrier function, similarly modulate our model system. ATP, histamine, bradykinin, and serotonin significantly decreased model BBB transendothelial electrical resistance and manipulations which elevate cyclic AMP enhanced culture resistance. These data indicate that our model BBB system responds in a manner characteristic of cerebral microvascular endothelial cells and the BBB in vivo. These data further emphasize the usefulness of our model system.

Blood-Brain Barrier↗
PubMed1998

The effects of carotid endarterectomy on ocular haemodynamics.

PURPOSE: To determine the profile of blood flow velocities and resistive indices in the ocular vessels of patients with atherosclerotic carotid disease and characterise the effect of endarterectomy on these parameters. METHODS: Following a preliminary study on healthy volunteers, ophthalmic colour Doppler ultrasound examinations were performed on 27 male and 11 female patients with carotid disease. These measurements were compared with central retinal artery perfusion pressures and intraoperative internal carotid artery stump pressures. RESULTS: Significant changes were seen on the endarterectomised side. The peak systolic velocity in the ophthalmic artery, and resistive indices in the ophthalmic artery, central retinal artery and nasal posterior ciliary artery, rose from pre-operative values. No correlation between colour Doppler ultrasound measurements and intraoperative internal carotid artery stump pressures was present. When compared with ophthalmodynamometry readings, a relationship was noted with maximum velocities in the central retinal vein. CONCLUSION: Carotid endarterectomy alters the haemodynamics in selected vessels of the ocular circulation as measured by colour Doppler ultrasound, but more work is required to determine the clinical utility of this investigative modality.

Adult↗