Glucocerebrosidase deficiency in substantia nigra of parkinson disease brains

Objective

Mutations in the glucocerebrosidase gene (GBA) represent a significant risk factor for developing Parkinson disease (PD). We investigated the enzymatic activity of glucocerebrosidase (GCase) in PD brains carrying heterozygote GBA mutations (PD+GBA) and sporadic PD brains.

Methods

GCase activity was measured using a fluorescent assay in cerebellum, frontal cortex, putamen, amygdala, and substantia nigra of PD+GBA (n = 9–14) and sporadic PD brains (n = 12–14). Protein expression of GCase and other lysosomal proteins was determined by western blotting. The relation between GCase, α-synuclein, and mitochondria function was also investigated in vitro.

Results

A significant decrease in GCase activity was observed in all PD+GBA brain areas except the frontal cortex. The greatest deficiency was in the substantia nigra (58% decrease; p < 0.01). GCase activity was also significantly decreased in the substantia nigra (33% decrease; p < 0.05) and cerebellum (24% decrease; p < 0.05) of sporadic PD brains. GCase protein expression was lower in PD+GBA and PD brains, whereas increased C/EBP homologous protein and binding immunoglobulin protein levels indicated that the unfolded protein response was activated. Elevated α-synuclein levels or PTEN-induced putative kinase 1 deficiency in cultured cells had a significant effect on GCase protein levels.

Interpretation

GCase deficiency in PD brains with GBA mutations is a combination of decreased catalytic activity and reduced protein levels. This is most pronounced in the substantia nigra. Biochemical changes involved in PD pathogenesis affect wild-type GCase protein expression in vitro, and these could be contributing factors to the GCase deficiency observed in sporadic PD brains. ANN NEUROL 2012;72:455–463.

Depletion of glutathione does not affect electron transport chain complex activity in brain mitochondria: Implications for Parkinson disease and postmortem studies

Glutathione is an important antioxidant in the brain that appears to be decreased, in conjunction with mitochondrial complex I activity, in Parkinson disease patients. In postmortem analysis, measurement of glutathione levels and complex I activity can be delayed up to 20h. We investigated whether depletion of glutathione in the preweanling rat induces a reduction in complex I activity in brain mitochondria and the effects that postmortem delay has on glutathione levels and electron transport chain activity. After injection with the glutamate-cysteine ligase inhibitor, buthionine sulfoximine (L-BSO), glutathione levels were decreased by 53% compared to the control values in whole-brain homogenates. During postmortem delay of 24h, in which animals were kept at 4°C, the levels of glutathione decreased in the control group by 58% and in the L-BSO-treated group by 79%. However, during this period, there were no changes in mitochondrial electron transport chain complex I, II-III, or IV activity in either group. These results suggest that a preexisting deficiency of glutathione or a loss of glutathione during postmortem delay does not influence mitochondrial respiratory chain activity in the brain.

An intragenic duplication in guanosine triphosphate cyclohydrolase-1 gene in a dopa-responsive dystonia family

BACKGROUND

Autosomal dominant dopa-responsive dystonia is commonly caused by mutations in the guanosine triphosphate cyclohydrolase-1 gene.

METHODS

We report a British family that has been followed for more than 20 years in which no mutations were previously identified.

RESULTS

Reanalysis of this pedigree detected a duplication of guanosine triphosphate cyclohydrolase-1 exon 2 in affected family members. mRNA analysis showed a mutant transcript with a tandem exon 2 duplication. Four family members developed dopa-responsive dystonia, with onset in their late teens, and subsequently developed restless leg syndrome and migraine.

CONCLUSIONS

This is the first report of an intragenic guanosine triphosphate cyclohydrolase-1 duplication in a dopa-responsive dystonia family.

Bezafibrate induced increase in mitochondrial electron transport chain complex IV activity in human astrocytoma cells: Implications for mitochondrial cytopathies and neurodegenerative diseases

Mitochondrial encephalomyopathies resulting from electron transport chain (ETC) dysfunction can present with a wide spectrum of clinical manifestations having significant neuropathology and a progressive nature. Despite advances in diagnosis of ETC disorders, treatment still remains inadequate. A recent study in fibroblasts and myoblasts revealed the ability of fibrate treatment to correct ETC enzyme deficiencies. Therefore, fibrates may represent potential therapeutic agents to correct the neurological ETC impairment responsible for the encephalopathic presentation of these disorders. Consequently, this study assessed the effect of bezafibrate on human astrocytoma (HA) 1321N cell ETC activity and coenzyme Q(10) (CoQ(10) ) status. HA cells were incubated for 72 H with 300 μM or 500 μM bezafibrate and for 7 days with only 500 μM bezafibrate. A significant effect on ETC activity was observed after 7 days incubation with 500 μM bezafibrate yielding a 130% (P < 0.05) increase in complex IV activity, accompanied by a 33% (P < 0.05) increase in cellular ATP level and a 25% (P < 0.001) decrease in extracellular lactate/pyruvate ratio compared to control levels. Following 7 days culture with bezafibrate, the CoQ(10) status of the HA cells appeared to increase although this was not found to be significant. The results of this study have indicated evidence of a bezafibrate induced increase in ETC complex IV activity. Further studies are required to assess the ability of bezafibrate treatment to correct neurological ETC impairment in available animal models of ETC dysfunction before the therapeutic efficacy of this pharmacological agent can be further considered in the treatment of the encephalopathic presentation of ETC disorders.

Decreased ubiquinone availability and impaired mitochondrial cytochrome oxidase activity associated with statin treatment

In order to investigate the potential involvement of mitochondrial electron transport chain (ETC) dysfunction in myotoxicity associated with 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor (statin) treatment, assessment was made of ETC activity and ubiquinone status in two patients experiencing myopathy following treatment with simvastatin (40 mg/day) and cyclosporin (patient 1) and simvastatin (40 mg/day) and itraconazole (patient 2). Analysis of skeletal muscle biopsies revealed a decreased ubiquinone status (77 and 132; reference range: 140-580 pmol/mg) and cytochrome oxidase (complex IV) activity (0.006 and 0.007 reference range: 0.014-0.034). To assess statin treatment in the absence of possible pharmacological interference from cyclosporin or itraconazole, primary astrocytes were cultured with lovastatin (100 microM). Lovastatin treatment resulted in a decrease in ubiquinone (97.9 +/- 14.9; control: 202.9 +/- 18.4 pmol/mg; p < 0.05), and complex IV activity (0.008 +/- 0.001; control: 0.011 +/- 0.001; p < 0.05) relative to control. These data, coupled with the patient findings, indicate a possible association between statin treatment, decreased ubiquinone status, and loss of complex IV activity.

Pyridoxal 5'-phosphate deficiency causes a loss of aromatic L-amino acid decarboxylase in patients and human neuroblastoma cells, implications for aromatic L-amino acid decarboxylase and vitamin B(6) deficiency states

Pyridoxal 5'-phosphate, the active form of vitamin B(6), is an essential cofactor for multiple enzymes, including aromatic l-amino acid decarboxylase that catalyses the final stage in the production of the neurotransmitters dopamine and serotonin. In two patients with inherited disorders of vitamin B(6) metabolism, we observed reductions in plasma aromatic l-amino acid decarboxylase activity. In one patient, this change was related to an increase in K(m) for pyridoxal 5'-phosphate. Furthermore, pyridoxal 5'-phosphate-deficient human SH-SY5Y neuroblastoma cells were found to exhibit reduced levels of aromatic l-amino acid decarboxylase activity and protein but with no alteration in expression. Further reductions in activity and protein were observed with the addition of the vitamin B(6) antagonist 4-deoxypyridoxine, which also reduced aromatic l-amino acid decarboxylase mRNA levels. Neither pyridoxal 5'-phosphate deficiency nor the addition of 4-deoxypyridoxine affected aromatic l-amino acid decarboxylase stability over 8 h with protein synthesis inhibited. Increasing extracellular availability of pyridoxal 5'-phosphate was not found to have any significant effect on intracellular pyridoxal 5'-phosphate concentrations or on aromatic l-amino acid decarboxylase. These findings suggest that maintaining adequate pyridoxal 5'-phosphate availability may be important for optimal treatment of aromatic l-amino acid decarboxylase deficiency and l-dopa-responsive conditions.

Persistent mitochondrial damage by nitric oxide and its derivatives: neuropathological implications

Approximately 15 years ago we reported that cytochrome c oxidase (CcO) was persistently inhibited as a consequence of endogenous induction and activation of nitric oxide (^•NO) synthase-2 (NOS2) in astrocytes. Furthermore, the reactive nitrogen species implicated was peroxynitrite. In contrast to the reversible inhibition by ^•NO, which occurs rapidly, in competition with O₂, and has signaling regulatory implications, the irreversible CcO damage by peroxynitrite is progressive in nature and follows and/or is accompanied by damage to other key mitochondrial bioenergetic targets. In purified CcO it has been reported that the irreversible inhibition occurs through a mechanism involving damage of the heme a₃-Cu_B binuclear center leading to an increase in the K_m for oxygen. Astrocyte survival, as a consequence of peroxynitrite exposure, is preserved due to their robust bioenergetic and antioxidant defense mechanisms. However, by releasing peroxynitrite to the neighboring neurons, whose antioxidant defense can, under certain conditions, be fragile, activated astrocytes trigger bioenergetic stress leading to neuronal cell death. Thus, such irreversible inhibition of CcO by peroxynitrite may be a plausible mechanism for the neuronal death associated with neurodegenerative diseases, in which the activation of astrocytes plays a crucial role.

Consequences of long-term oral administration of the mitochondria-targeted antioxidant MitoQ to wild-type mice

The mitochondria-targeted quinone MitoQ protects mitochondria in animal studies of pathologies in vivo and is being developed as a therapy for humans. However, it is unclear whether the protective action of MitoQ is entirely due to its antioxidant properties, because long-term MitoQ administration may alter whole-body metabolism and gene expression. To address this point, we administered high levels of MitoQ orally to wild-type C57BL/6 mice for up to 28 weeks and investigated the effects on whole-body physiology, metabolism, and gene expression, finding no measurable deleterious effects. In addition, because antioxidants can act as pro-oxidants under certain conditions in vitro, we examined the effects of MitoQ administration on markers of oxidative damage. There were no changes in the expression of mitochondrial or antioxidant genes as assessed by DNA microarray analysis. There were also no increases in oxidative damage to mitochondrial protein, DNA, or cardiolipin, and the activities of mitochondrial enzymes were unchanged. Therefore, MitoQ does not act as a pro-oxidant in vivo. These findings indicate that mitochondria-targeted antioxidants can be safely administered long-term to wild-type mice.

Altered skeletal muscle insulin signaling and mitochondrial complex II-III linked activity in adult offspring of obese mice

We recently reported insulin resistance in adult offspring of obese C57BL/6J mice. We have now evaluated whether parameters of skeletal muscle structure and function may play a role in insulin resistance in this model of developmental programming. Obesity was induced in female mice by feeding a highly palatable sugar and fat-rich diet for 6 wk prior to pregnancy, and during pregnancy and lactation. Offspring of obese dams were weaned onto standard laboratory chow. At 3 mo of age, skeletal muscle insulin signaling protein expression, mitochondrial electron transport chain activity (ETC), muscle fiber type, fiber density, and fiber cross-sectional area were compared with that of offspring of control dams weaned onto the chow diet. Female offspring of obese dams demonstrated decreased skeletal muscle expression of p110beta, the catalytic subunit of PI3K (P < 0.01), as well as reduced Akt phosphorylation at Serine residue 473 compared with control offspring. Male offspring of obese dams demonstrated increased skeletal muscle Akt2 and PKCzeta expression (P < 0.01; P < 0.001, respectively). A decrease in mitochondrial-linked complex II-III was observed in male offspring of obese dams (P < 0.01), which was unrelated to CoQ deficiency. This was not observed in females. There were no differences in muscle fiber density between offspring of obese dams and control offspring in either sex. Sex-related alterations in key insulin-signaling proteins and in mitochondrial ETC may contribute to a state of insulin resistance in offspring of obese mice.

Homozygous loss-of-function mutations in the gene encoding the dopamine transporter are associated with infantile parkinsonism-dystonia

Genetic variants of the SLC6A3 gene that encodes the human dopamine transporter (DAT) have been linked to a variety of neuropsychiatric disorders, particularly attention deficit hyperactivity disorder. In addition, the homozygous Slc6a3 knockout mouse displays a hyperactivity phenotype. Here, we analyzed 2 unrelated consanguineous families with infantile parkinsonism-dystonia (IPD) syndrome and identified homozygous missense SLC6A3 mutations (p.L368Q and p.P395L) in both families. Functional studies demonstrated that both mutations were loss-of-function mutations that severely reduced levels of mature (85-kDa) DAT while having a differential effect on the apparent binding affinity of dopamine. Thus, in humans, loss-of-function SLC6A3 mutations that impair DAT-mediated dopamine transport activity are associated with an early-onset complex movement disorder. Identification of the molecular basis of IPD suggests SLC6A3 as a candidate susceptibility gene for other movement disorders associated with parkinsonism and/or dystonic features.

A new perspective on the treatment of aromatic L-amino acid decarboxylase deficiency

The final step in production of the neurotransmitters dopamine and serotonin is catalyzed by aromatic l-amino acid decarboxylase (AADC). AADC deficiency is a debilitating genetic condition that results in a deficit in these neurotransmitters, and manifests in infancy as a severe movement disorder with developmental delay. Response to current treatments is often disappointing. We have reviewed the literature to look for improvements to the current treatment strategy and also for new directions for AADC deficiency treatment. There may be differences in the mode of action, side-effect risk and effectiveness between different dopamine agonists and monoamine oxidase inhibitors currently used for AADC deficiency treatment. The range of these drugs used requires re-evaluation as some may have greater efficacy than others. Pyridoxal 5'-phosphate, the AADC cofactor may stabilize AADC and could increase AADC activity. Pyridoxal 5'-phosphate could have advantages as a treatment instead of pyridoxine. Atypical neuroleptics and peripheral AADC inhibitors both increase AADC activity in vivo and could be a future direction for AADC deficiency treatment and related conditions. Parkinson's disease gene therapy to deliver and express the human AADC gene in striatum is being tested in humans. Consequently gene therapy for AADC deficiency could be a realistic aim however an animal model of AADC deficiency is important for further progression.

Mitochondrial cytochrome c release: a factor to consider in mitochondrial disease?

The pathogenesis of mitochondrial disorders has largely focused on the impairment of cellular energy metabolism. However, mitochondrial dysfunction has also been implicated as a factor in the initiation of apoptosis due to the translocation of cytochrome c, from mitochondria to the cytosol, and the subsequent cleavage of pro-caspase 3. In this study, we determined the cytochrome c content of cytosols (skeletal muscle) prepared from 22 patients with evidence of compromised mitochondrial electron transport chain enzyme activity and 26 disease controls. The cytochrome c content of the mitochondrial electron transport chain-deficient group was found to be significantly (p < 0.02) elevated when compared with the control group (63.7 +/- 15.5 versus 27.7 +/- 2.5 ng/mg protein). Furthermore, a relationship between the cytosolic cytochrome c content of skeletal muscle and complex I and complex IV activities was demonstrated. Such data raise the possibility that mitochondrial cytochrome c release may be a feature of mitochondrial disorders, particularly for those patients with marked deficiencies of respiratory chain enzymes. Whether initiation of apoptosis occurs as a direct consequence of this cytochrome c release has not been fully evaluated here. However, for one patient with the greatest documented cytosolic cytochrome c content, caspase 3 could be demonstrated in the cytosolic preparation. Further work is required in order to establish whether a relationship also exists between caspase 3 formation and the magnitude of respiratory chain deficiency.

Inactivation of brain mitochondrial Lon protease by peroxynitrite precedes electron transport chain dysfunction

The accumulation of oxidatively modified proteins has been shown to be a characteristic feature of many neurodegenerative disorders and its regulation requires efficient proteolytic processing. One component of the mitochondrial proteolytic system is Lon, an ATP-dependent protease that has been shown to degrade oxidatively modified aconitase in vitro and may thus play a role in defending against the accumulation of oxidized matrix proteins in mitochondria. Using an assay system that allowed us to distinguish between basal and ATP-stimulated Lon protease activity, we have shown in isolated non-synaptic rat brain mitochondria that Lon protease is highly susceptible to oxidative inactivation by peroxynitrite (ONOO(-)). This susceptibility was more pronounced with regard to ATP-stimulated activity, which was inhibited by 75% in the presence of a bolus addition of 1mM ONOO(-), whereas basal unstimulated activity was inhibited by 45%. Treatment of mitochondria with a range of peroxynitrite concentrations (10-1000 microM) revealed that a decline in Lon protease activity preceded electron transport chain (ETC) dysfunction (complex I, II-III and IV) and that ATP-stimulated activity was approximately fivefold more sensitive than basal Lon protease activity. Furthermore, supplementation of mitochondrial matrix extracts with reduced glutathione, following ONOO(-) exposure, resulted in partial restoration of basal and ATP-stimulated activity, thus suggesting possible redox regulation of this enzyme complex. Taken together these findings suggest that Lon protease may be particularly vulnerable to inactivation in conditions associated with GSH depletion and elevated oxidative stress.

Astrocytes protect against copper-catalysed loss of extracellular glutathione

Glutathione (GSH) is one of the major antioxidants in the brain. GSH is secreted by astrocytes and this extracellular GSH is used by neurones to maintain and increase their intracellular GSH levels. For efficient GSH trafficking between astrocytes and neurones, GSH needs to be maintained in the reduced form. In model systems, GSH trafficking has been shown to be essential for neuroprotection against a variety of stress conditions. Previously we and others have shown that GSH and thiols are unstable in cell culture media and are easily oxidised. In the present study it is shown that nanomolar concentrations of copper (II) ions can cause decay of GSH in cell culture media. Increased free or redox active copper has been implicated in a variety of diseases and degradation of extracellular GSH is a possible mechanism by which it exerts its harmful effects. Rat astrocytes, a human astrocytoma cell line and astrocyte-conditioned media, in the absence of cells, are able to retard this copper-catalysed decay of GSH and maintain GSH in its reduced form. The protective effect of astrocytes appears to be a combination of copper removing and antioxidant mechanisms. The importance of these protective mechanisms is discussed with regards to neurodegenerative diseases.

Nitric oxide accelerates the degradation of tetrahydrobiopterin but not total neopterin in cerebrospinal fluid; potential implications for the assessment of tetrahydrobiopterin metabolism

Assessment of total neopterin and tetrahydrobioterin (BH4) concentrations in cerebrospinal fluid (CSF) can be used to identify potential disorders of BH4 biosynthesis. In this study, we demonstrate that exposure of CSF to nitric oxide leads to an accelerated degradation of BH4 but does not affect the total neopterin concentration. These data suggest that in those conditions associated with increased nitric oxide formation, perturbation of the total neopterin to BH4 ratio could occur. Under such circumstances a putative diagnosis of a defect in BH4 biosynthesis may erroneously be proposed. Assessment of central nitric oxide generation may therefore be a useful adjunct to the determination of CSF pterin status.

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.

Tetrahydrobiopterin availability, nitric oxide metabolism and glutathione status in the hph-1 mouse; implications for the pathogenesis and treatment of tetrahydrobiopterin deficiency states

Tetrahydrobiopterin (BH4) is an essential cofactor for all isoforms of nitric oxide synthase. While it is well established that BH4 deficiency states are associated with impairment of dopamine, serotonin and phenylalanine metabolism, less is known with regard to the effects of deficiency of the cofactor upon nitric oxide (NO) metabolism. In this study, we have evaluated the effects of partial BH4 deficiency upon (a) tissue availability of the antioxidant glutathione, (b) basal NO production and (c) NO generation following exposure to lipopolysaccharide (LPS), which is known to increase expression of the inducible form of nitric oxide synthase. Using the hph-1 mouse, which displays a partial BH4 deficiency owing to impaired activity of GTP cyclohydrolase, we report decreased levels of glutathione in brain and kidney and evidence for decreased basal generation of nitric oxide in the periphery (as judged by the plasma nitrate plus nitrite concentration). Following LPS administration, peripheral NO generation increases. However, the concentration of plasma nitrate plus nitrite achieved was significantly decreased in the hph-1 mouse. Furthermore, LPS administration caused loss of glutathione in both wild-type and hph-1 liver and kidney. It is concluded that cofactor replacement, sufficient to fully correct a cellular BH4 deficiency, may be of benefit to patients with inborn errors of BH4 metabolism.

Inhibition of mitochondrial complex IV leads to secondary loss complex II-III activity: implications for the pathogenesis and treatment of mitochondrial encephalomyopathies

Mitochondrial encephalomyopathies, arising from deficiencies of the electron transport chain (ETC) give rise to a wide clinical spectrum of presentation and are often progressive in nature. The aetiology of mitochondrial encephalomyopathies have yet to be fully elucidated, however, a successive loss of ETC function may contribute to the progressive nature of these disorders. The possibility arises that as a consequence of a primary impairment of ETC activity, secondary damage to the ETC may occur. In order to investigate this hypothesis, we established a model of cytochrome oxidase (Complex IV) deficiency in cultured human astrocytoma 1321N cells. Potassium cyanide (KCN, 1mM) resulted in a sustained 50% (p<0.01) loss of complex IV. At 24h activities of the other ETC complexes were unaffected. However, at 72h significant loss of succinate-cytochrome c reductase (complex II-III) activity expressed as a ratio to the mitochondrial marker, citrate synthase was observed. (KCN treated; 0.065+/-0.011 vs controls; 0.118+/-0.017 mean+/-SEM, n=8, p<0.05). These results provide a possible mechanism for the progressive nature of ETC defects and why in some patients multiple patterns of ETC deficiencies can be demonstrated.

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.

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.

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.

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.

Co-culture of neurones with glutathione deficient astrocytes leads to increased neuronal susceptibility to nitric oxide and increased glutamate-cysteine ligase activity

The antioxidant glutathione (GSH) plays an important role in protecting the mitochondrial electron transport chain (ETC) from damage by oxidative stress in astrocytes and neurones. Neurones co-cultured with astrocytes have greater GSH levels, compared to neurones cultured alone, leading to the hypothesis that astrocytes play a key role in brain GSH metabolism by supplying essential GSH precursors to neurones. A previous study has postulated that damage to the ETC following exposure to reactive nitrogen species (RNS) is less in co-cultured neurones, compared to neurones cultured alone, because of the greater GSH levels in the former cells. To investigate this further, primary culture rat neurones were co-cultured with either rat astrocytes activated with IFN-gamma and LPS to produce NO, or NO-generating astrocytes that had been depleted of intracellular GSH by 87% following incubation with the GSH synthesis inhibitor L-buthionine-S,R-sulfoximine (L-BSO). Neurones incubated with NO-generating astrocytes depleted of GSH were unable to elevate GSH levels, unlike neurones co-cultured with NO-generating astrocytes. Complexes II + III and IV of the neuronal ETC were significantly inhibited following exposure to NO-generating astrocytes depleted of GSH. No ETC damage was observed in neurones co-cultured with NO-generating astrocytes. Although neurones co-cultured with GSH depleted astrocytes did not increase cellular GSH levels, the activity of glutamate cysteine ligase (GCL), the rate-limiting enzyme of GSH synthesis, was increased by 218%, compared to neurones cultured with control astrocytes. This suggests that neuronal GCL activity could be modulated when GSH metabolism is inhibited in neighboring astrocytes.

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).