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Biomedical subjects

Simon J R Heales

Publications and source records attributed to Simon J R Heales.

At least 19 recordsLinked to original sources
PubMed2006

Tetrahydrobiopterin availability in Parkinson's and Alzheimer's disease; potential pathogenic mechanisms.

Within the central nervous system, tetrahydrobiopterin (BH4) is an essential cofactor for dopamine and serotonin synthesis. In addition, BH4 is now established to be an essential cofactor for all isoforms of nitric oxide synthase (NOS). Inborn errors of metabolism affecting BH4 availability are well documented and the clinical presentation can be attributed to a paucity of dopamine, serotonin, and nitric oxide (NO) generation. In this article, we have focussed upon the sensitivity of BH4 to oxidative catabolism and the observation that when BH4 is limiting some cellular sources of NOS may generate superoxide whilst other BH4 saturated NOS enzymes may be generating NO. Such a scenario could favor peroxynitrite generation. If peroxynitrite is not scavenged, e.g., by antioxidants such as reduced glutathione, irreversible damage to critical cellular enzymes could ensue. Such targets include components of the mitochondrial electron transport chain, alpha ketoglutarate dehydrogenase and possibly pyruvate dehydrogenase. Such a cascade of events is hypothesized, in this article, to occur in neurodegenerative conditions such as Parkinson's and Alzheimer's disease.

Alzheimer Disease↗
PubMed2006

Some observations upon biochemical causes of ataxia and a new disease entity ubiquinone, CoQ10 deficiency.

Some hereditary ataxias are treatable and the insight required for this has come from an in depth knowledge of the phenotypes and clinical biochemistry of the conditions. This has required both fundamental and translational clinical research. Prof John Blass was fortunate to begin his career at what we can now recognise as a golden era for such studies and he worked upon two important conditions; Refsum's disease and Friedreich's ataxia. More recently the mitochondrial encephalomyopathies have been described and similar investigative work has been undertaken upon them. Ubiquinone, CoQ(10), deficiency is the most recently recognised encephalomyopathy and is itself treatable. Though rare, it is becoming increasingly recognised and patients are benefiting from the same scholarly approach to its investigation as was afforded Refsums' disease and Friedreich's ataxia.

Animals↗
PubMed2006

Mutations in the gene encoding 3-hydroxyisobutyryl-CoA hydrolase results in progressive infantile neurodegeneration.

Only a single patient with 3-hydroxyisobutyryl-CoA hydrolase deficiency has been described in the literature, and the molecular basis of this inborn error of valine catabolism has remained unknown until now. Here, we present a second patient with 3-hydroxyisobutyryl-CoA hydrolase deficiency, who was identified through blood spot acylcarnitine analysis showing persistently increased levels of hydroxy-C(4)-carnitine. Both patients manifested hypotonia, poor feeding, motor delay, and subsequent neurological regression in infancy. Additional features in the newly identified patient included episodes of ketoacidosis and Leigh-like changes in the basal ganglia on a magnetic resonance imaging scan. In cultured skin fibroblasts from both patients, the 3-hydroxyisobutyryl-CoA hydrolase activity was deficient, and virtually no 3-hydroxyisobutyryl-CoA hydrolase protein could be detected by western blotting. Molecular analysis in both patients uncovered mutations in the HIBCH gene, including one missense mutation in a conserved part of the protein and two mutations affecting splicing. A carefully interpreted acylcarnitine profile will allow more patients with 3-hydroxyisobutyryl-CoA hydrolase deficiency to be diagnosed.

Base Sequence↗
PubMed2006

Glutathione depletion and increased apoptosis rate in human cystinotic proximal tubular cells.

We have determined levels of glutathione (GSH), ATP, mitochondrial complex activity and apoptosis rate in proximal tubular cells (PTCs) exfoliated from urine in cystinotic (n=9) and control (n=9) children. Intracellular GSH was significantly depleted in cystinotic PTCs compared with controls (6.8 nmol GSH/mg protein vs 11.8 nmol GSH/mg protein; P<0.001), but there were no significant differences in mitochondrial complex activities or ATP levels under basal conditions. Cystinotic PTCs showed significantly increased apoptosis rate. After PTCs had been stressed by hypoxia, there was further depletion of GSH in cystinotic and control PTCs (2.4 nmol GSH/mg protein vs 7.2 nmol GSH/mg protein; P<0.001). Hypoxic stress led to increased complex I and complex IV activities in control but not in cystinotic PTCs. ATP levels were significantly reduced in cystinotic PTCs after hypoxic stress (12.2 nmol/mg protein vs 26.9 nmol/mg protein; P<0.001). GSH depletion occurs in this in vitro model of cystinotic PTCs, is exaggerated by hypoxic stress and may contribute to reduced ATP and failure to increase complex I/IV activities. Apoptotic rate is also increased, and these mechanisms may contribute to cellular dysfunction in cultured, human cystinotic PTCs.

Apoptosis↗
PubMed2005

Induction of mitochondrial oxidative stress in astrocytes by nitric oxide precedes disruption of energy metabolism.

Inhibition of the mitochondrial electron transport chain (ETC) ultimately limits ATP production and depletes cellular ATP. However, the individual complexes of the ETC in brain mitochondria need to be inhibited by approximately 50% before causing significant depression of ATP synthesis. Moreover, the ETC is the key site for the production of intracellular reactive oxygen species (ROS) and inhibition of one or more of the complexes of the ETC may increase the rate of mitochondrial ROS generation. We asked whether partial inhibition of the ETC, to a degree insufficient to perturb oxidative phosphorylation, might nonetheless induce ROS production. Chronic increase in mitochondrial ROS might then cause oxidative damage to the ETC sufficient to produce prolonged changes in ETC function and so compound the defect. We show that the exposure of astrocytes in culture to low concentrations of nitric oxide (NO) induces an increased rate of O2*- generation that outlasts the presence of NO. No effect was seen on oxygen consumption, lactate or ATP content over the 4-6 h that the cells were exposed to NO. These data suggest that partial ETC inhibition by NO may initially cause oxidative stress rather than ATP depletion, and this may subsequently induce irreversible changes in ETC function providing the basis for a cycle of damage.

Adenosine Triphosphate↗
PubMed2005

Neonatal epileptic encephalopathy caused by mutations in the PNPO gene encoding pyridox(am)ine 5'-phosphate oxidase.

In the mouse, neurotransmitter metabolism can be regulated by modulation of the synthesis of pyridoxal 5'-phosphate and failure to maintain pyridoxal phosphate (PLP) levels results in epilepsy. This study of five patients with neonatal epileptic encephalopathy suggests that the same is true in man. Cerebrospinal fluid and urine analyses indicated reduced activity of aromatic L-amino acid decarboxylase and other PLP-dependent enzymes. Seizures ceased with the administration of PLP, having been resistant to treatment with pyridoxine, suggesting a defect of pyridox(am)ine 5'-phosphate oxidase (PNPO). Sequencing of the PNPO gene identified homozygous missense, splice site and stop codon mutations. Expression studies in Chinese hamster ovary cells showed that the splice site (IVS3-1g>a) and stop codon (X262Q) mutations were null activity mutations and that the missense mutation (R229W) markedly reduced pyridox(am)ine phosphate oxidase activity. Maintenance of optimal PLP levels in the brain may be important in many neurological disorders in which neurotransmitter metabolism is disturbed (either as a primary or as a secondary phenomenon).

Amino Acid Sequence↗
PubMed2005

Nitric oxide and neurological disorders.

This article aims to give a broad overview of some of the potential targets for nitric oxide (NO) in the brain. The relevance of NO in both physiological and pathological scenarios is considerable. There is substantial evidence that neurodegenerative conditions such as Alzheimer's disease and Parkinson's disease, involve NO in their pathogenesis. Here we describe a number of cellular components which may be affected by NO, with particular relevance to neurological diseases. As the mitochondrion (in particular the electron transport chain) would appear to be of importance when considering the deleterious effects of NO, this review has a particular emphasis on that organelle. Cellular and mitochondrial antioxidants such as glutathione and ubiquinone are also discussed. In addition, the pivotal role of the astrocyte in both neuroprotection or neurodegeneration are examined.

Alzheimer Disease↗
PubMed2005

The effect of HMG-CoA reductase inhibitors on coenzyme Q10: possible biochemical/clinical implications.

The HMG-CoA reductase inhibitors, also known as statins, have an enviable safety profile; however, myotoxicity and to a lesser extent hepatotoxicity have been noted in some patients following treatment. Statins target several tissues, depending upon their lipophilicity, where they competitively inhibit HMG-CoA reductase, the rate-limiting enzyme for mevalonic acid synthesis and subsequently cholesterol biosynthesis. HMG-CoA reductase is also the first committed rate-limiting step for the synthesis of a range of other compounds including steroid hormones and ubidecarenone (ubiquinone), otherwise known as coenzyme Q(10) (CoQ(10)). Recent interest has focused on the possible role CoQ(10) deficiency may have in the pathophysiology of the rare adverse effects of statin treatment. Currently, there is insufficient evidence from human studies to link statin therapy unequivocally to pathologically significantly decreased tissue CoQ(10) levels. Although statin treatment has been reported to lower plasma/serum CoQ(10) status, few human studies have assessed tissue CoQ(10) status. The plasma/serum CoQ(10) level is influenced by a number of physiological factors and, therefore, has limited value as a means of assessing intracellular CoQ(10) status. In those limited studies that have assessed the effect of statin treatment upon tissue CoQ(10) levels, none have shown evidence of a fall in CoQ(10) levels. This may reflect the doses of statins used, since many appear to have been used at doses below those recommended for their maximum therapeutic effects. Moreover, the poor bioavailability in those peripheral tissues tested may not reflect the effects the agents are having in liver and muscle, the tissues commonly affected in those patients who do not tolerate statins. This article reviews the biochemistry of CoQ(10), its role in cellular metabolism and the available evidence linking possible CoQ(10) deficiency to statin therapy.

Animals↗
PubMed2004

Mitochondrial disease: a historical, biochemical, and London perspective.

Roland Luft is credited with describing the first truly mitochondrial disorder in the late 1950s and early 1960s. Cases such as his have proven to be exceptionally rare. Some years later, methods of mitochondrial analysis--enzymatic, polarographic, and spectroscopic, which had been developed primarily by groups in Philadelphia--were applied to the study of mitochondria isolated from skeletal muscle biopsies of patients thought to have defects of oxidative phosphorylation. In the vanguard of these investigations were groups in New York and London. John Clark led the latter group. Application of biochemical studies, more recently supplemented by molecular mtDNA and nuclear DNA studies, have revealed that mitochondrial disorders are among the most common of all metabolic disorders.

DNA, Mitochondrial↗
PubMed2004

Neurodegeneration or neuroprotection: the pivotal role of astrocytes.

Formation of nitric oxide (NO), by astrocytes, has been suggested to contribute, via impairment of mitochondrial function, to the neurodegnerative process. Thus co-culture of neuronal cells with NO-generating astrocytes leads to a loss of mitochondrial function, as reflected by diminished activities of complexes IV and II+III. However, such damage may in the first instance be limited due to upregulation of neuronal glutathione metabolism as a result of metabolic trafficking of glutathione from the astrocyte to neurone. Furthermore, exposure of astrocytes to NO leads to increased glutathione metabolism resulting in the preservation of glutathione precursors for neuronal utilization. Failure of glutathione trafficking could render neuronal cells particularly susceptible to NO, leading to cell death. In addition, depletion with time of the nitric oxide synthase cofactor, tetrahydrobiopterin, may result in the astrocytic generation of more potent oxidizing species, which could contribute to the neurodegenerative process.

Alzheimer Disease↗
PubMed2003

Nitric oxide-induced mitochondrial dysfunction: implications for neurodegeneration.

Excessive generation of nitric oxide (NO) has been implicated in the pathogenesis of several neurodegenerative disorders. Damage to the mitochondrial electron transport chain has also been implicated in these disorders. NO and its toxic metabolite peroxynitrite (ONOO(-)) can inhibit the mitochondrial respiratory chain, leading to energy failure and ultimately cell death. There appears to be a differential susceptibility of brain cell types to NO/ONOO(-), which may be influenced by factors including cellular antioxidant status and the ability to maintain energy requirements in the face of marked respiratory chain damage. Although formation of NO/ONOO(-) following cytokine exposure does not affect astrocyte survival, these molecules may diffuse out and cause mitochondrial damage to neighboring NO/ONOO(-)-sensitive cells such as neurons. Evidence suggests that NO/ONOO(-) causes release of neuronal glutamate, leading to glutamate-induced activation of neuronal NO synthase and generation of further damaging species. While neurons appear able to recover from short-term exposure to NO/ONOO(-), extending the period of exposure results in persistent damage to the respiratory chain and cell death ensues. These findings have important implications for acute infection vs. chronic neuroinflammatory disease states. The evidence for NO/ONOO(-)-mediated mitochondrial damage in neurodegenerative disorders is reviewed and potential therapeutic strategies are discussed.

Animals↗
PubMed2002

Activated T cells mediate direct blood-brain barrier endothelial cell death and dysfunction.

Neuro-inflammation is characterized by immune cell infiltration across the blood-brain barrier, a process instrumental in neuronal cell death. In neuro-inflammation the blood-brain barrier is also damaged and the consequences of activated lymphocytes on the integrity of the blood-brain barrier is not well characterized. Utilizing an blood-brain barrier model we demonstrate that endothelial cell viability and barrier integrity are directly altered following lymphocyte exposure. The effect of activated lymphocytes is cell number dependent, mostly mediated by direct contact, and is not associated with the pro-inflammatory cytokine TNF-alpha. For the successful treatment of neuro-inflammatory disease, intervention of this direct effect at the blood-brain barrier is warranted.

Blood-Brain Barrier↗
PubMed2002

Evaluation of the role of P-glycoprotein in inflammation induced blood-brain barrier damage.

In this study we evaluated the role of the multi-drug transporter p-glycoprotein (Pgp) in the process of activated T lymphocyte-mediated blood-brain barrier dysfunction as described previously. Lymphocyte exposure induced significant endothelial cell death and there was an elevation of the expression of Pgp in the surviving cells. Inhibition of Pgp function using the antibody MRK16 and verapamil displayed a dose-dependent prevention of T cell mediated endothelial cell death and barrier breakdown. These data suggest that the activity of Pgp at the blood-brain barrier may play a role in lymphocyte induced barrier cell damage and a role as a possible survival mechanism to prevent further endothelial cell death in later stages of inflammation.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2002

Determination of glutamate-cysteine ligase (gamma-glutamylcysteine synthetase) activity by high-performance liquid chromatography and electrochemical detection.

The tripeptide glutathione (gamma-glutamylcysteinylglycine; GSH) is the predominant low molecular mass thiol in cells. The function of GSH is of considerable interest, with the molecule being implicated in numerous cellular processes in addition to being a major cellular antioxidant. The enzyme glutamate-cysteine ligase (GCL) is the rate-limiting step in GSH synthesis. The GCL assay described here is based on high-performance liquid chromatography and exploits the electrochemically active nature of gamma-glutamylcysteine (gamma-GC), the product of GCL activity. This method allows for the direct detection of gamma-GC rather than relying on derivatization of the molecule or linked assays. The sensitivity of the assay is sufficient to allow for the measurement of GCL activity in cultured cells. The specific activity of GCL in rat primary culture astrocytes was 9.7 +/- 1.7 nmol gamma-GC synthesized/min/mg protein.

Animals↗
PubMed2002

Further case of paroxysmal exercise-induced dystonia and some insights into pathogenesis.

Cerebrospinal fluid (CSF) analysis of pterin and monamine metabolites was performed before and after an attack in a patient with paroxysmal exercise-induced dystonia. A twofold increase in CSF homovanillic acid and 5-hydroxyindoleacetic acid after an attack was measured. This finding lends support to the hypothesis that increased dopaminergic transmission contributes to the clinical features of the hyperkinetic movement disorders.

Adult↗
PubMed2002

Impairment of brain mitochondrial function by reactive nitrogen species: the role of glutathione in dictating susceptibility.

Mitochondrial enzymes involved in energy metabolism display varying degrees of sensitivity towards reactive nitrogen species such as peroxynitrite (ONOO-). With regards to the electron transport chain, cytochrome oxidase appears particularly sensitive. Inhibition of this component may lead to an increase in mitochondrial superoxide formation, exacerbation of cellular oxidative stress and further mitochondrial damage. Impairment of the electron transport chain may lead to a loss of membrane potential, ATP deficiency, opening of the permeability transition pore and the release of factors capable of initiating apoptosis. Reduced glutathione will react, via a number of diverse reactions, with reactive nitrogen species and hence is capable of limiting mitochondiral damage. Loss of brain glutathione may therefore be an important factor in those neurological conditions in which there is evidence of excessive nitric oxide formation and mitochondrial damage.

Animals↗