Bioenergetics in glutaryl-coenzyme A dehydrogenase deficiency: a role for glutaryl-coenzyme A

Mechanistic effects of amino acids and glucose in a novel glutaric aciduria type 1 cell model

Acute neurological crises involving striatal degeneration induced by a deficiency of glutaryl-CoA dehydrogenase (GCDH) and the accumulation of glutaric (GA) and 3-hydroxyglutaric acid (3-OHGA) are considered to be the most striking features of glutaric aciduria type I (GA1). In the present study, we investigated the mechanisms of apoptosis and energy metabolism impairment in our novel GA1 neuronal model. We also explored the effects of appropriate amounts of amino acids (2 mM arginine, 2 mM homoarginine, 0.45 g/L tyrosine and 10 mM leucine) and 2 g/L glucose on these cells. Our results revealed that the novel GA1 neuronal model effectively simulates the hypermetabolic state of GA1. We found that leucine, tyrosine, arginine, homoarginine or glucose treatment of the GA1 model cells reduced the gene expression of caspase-3, caspase-8, caspase-9, bax, fos, and jun and restored the intracellular NADH and ATP levels. Tyrosine, arginine or homoarginine treatment in particular showed anti-apoptotic effects; increased α-ketoglutarate dehydrogenase complex (OGDC), fumarase (FH), and citrate synthase (CS) expression; and relieved the observed impairment in energy metabolism. To the best of our knowledge, this study is the first to investigate the protective mechanisms of amino acids and glucose in GA1 at the cellular level from the point of view of apoptosis and energy metabolism. Our data support the results of previous studies, indicating that supplementation of arginine and homoarginine as a dietary control strategy can have a therapeutic effect on GA1. All of these findings facilitate the understanding of cell apoptosis and energy metabolism impairment in GA1 and reveal new therapeutic perspectives for this disease.

Metabolism leaves its mark on the powerhouse: recent progress in post-translational modifications of lysine in mitochondria

Lysine modifications have been studied extensively in the nucleus, where they play pivotal roles in gene regulation and constitute one of the pillars of epigenetics. In the cytoplasm, they are critical to proteostasis. However, in the last decade we have also witnessed the emergence of mitochondria as a prime locus for post-translational modification (PTM) of lysine thanks, in large measure, to evolving proteomic techniques. Here, we review recent work on evolving set of PTM that arise from the direct reaction of lysine residues with energized metabolic thioester-coenzyme A intermediates, including acetylation, succinylation, malonylation, and glutarylation. We highlight the evolutionary conservation, kinetics, stoichiometry, and cross-talk between members of this emerging family of PTMs. We examine the impact on target protein function and regulation by mitochondrial sirtuins. Finally, we spotlight work in the heart and cardiac mitochondria, and consider the roles acetylation and other newly-found modifications may play in heart disease.

Unravelling the complex MRI pattern in glutaric aciduria type I using statistical models-a cohort study in 180 patients

BACKGROUND

Glutaric aciduria type I (GA-I) is a cerebral organic aciduria caused by inherited deficiency of glutaryl-CoA dehydrogenase and is characterized biochemically by an accumulation of putatively neurotoxic dicarboxylic metabolites. The majority of untreated patients develops a complex movement disorder with predominant dystonia during age 3-36 months. Magnetic resonance imaging (MRI) studies have demonstrated striatal and extrastriatal abnormalities.

AIMS/METHODS

The major aim of this study was to elucidate the complex neuroradiological pattern of patients with GA-I and to associate the MRI findings with the severity of predominant neurological symptoms. In 180 patients, detailed information about the neurological presentation and brain region-specific MRI abnormalities were obtained via a standardized questionnaire.

RESULTS

Patients with a movement disorder had more often MRI abnormalities in putamen, caudate, cortex, ventricles and external CSF spaces than patients without or with minor neurological symptoms. Putaminal MRI changes and strongly dilated ventricles were identified as the most reliable predictors of a movement disorder. In contrast, abnormalities in globus pallidus were not clearly associated with a movement disorder. Caudate and putamen as well as cortex, ventricles and external CSF spaces clearly collocalized on a two-dimensional map demonstrating statistical similarity and suggesting the same underlying pathomechanism.

CONCLUSIONS

This study demonstrates that complex statistical methods are useful to decipher the age-dependent and region-specific MRI patterns of rare neurometabolic diseases and that these methods are helpful to elucidate the clinical relevance of specific MRI findings.

Interaction of glutaric aciduria type 1-related glutaryl-CoA dehydrogenase with mitochondrial matrix proteins

Glutaric aciduria type 1 (GA1) is an inherited neurometabolic disorder caused by mutations in the GCDH gene encoding glutaryl-CoA dehydrogenase (GCDH), which forms homo- and heteromeric complexes in the mitochondrial matrix. GA1 patients are prone to the development of encephalopathic crises which lead to an irreversible disabling dystonic movement disorder. The clinical and biochemical manifestations of GA1 vary considerably and lack correlations to the genotype. Using an affinity chromatography approach we report here for the first time on the identification of mitochondrial proteins interacting directly with GCDH. Among others, dihydrolipoamide S-succinyltransferase (DLST) involved in the formation of glutaryl-CoA, and the β-subunit of the electron transfer flavoprotein (ETFB) serving as electron acceptor, were identified as GCDH binding partners. We have adapted the yellow fluorescent protein-based fragment complementation assay and visualized the oligomerization of GCDH as well as its direct interaction with DLST and ETFB in mitochondria of living cells. These data suggest that GCDH is a constituent of multimeric mitochondrial dehydrogenase complexes, and the characterization of their interrelated functions may provide new insights into the regulation of lysine oxidation and the pathophysiology of GA1.

Perioperative management of a child with glutaric aciduria type I undergoing cardiac surgery

Patients with glutaric aciduria type I are at risk for acute striatal injury precipitated by catabolic stress. Here, we report the successful interdisciplinary anesthetic and perioperative management of a child with glutaric aciduria type I undergoing cardiac surgery with extracorporeal circulation. Given the central focus on prevention of acute striatal injury, our anesthetic strategy emphasized avoiding a high protein load, high-dose inotropics, especially epinephrine (associated with impaired glucose utilization), deliberate hyperventilation, and other interventions associated with systemic inflammatory response.

Current concepts in organic acidurias: understanding intra- and extracerebral disease manifestation

This review focuses on the pathophysiology of organic acidurias (OADs), in particular, OADs caused by deficient amino acid metabolism. OADs are termed classical if patients present with acute metabolic decompensation and multiorgan dysfunction or cerebral if patients predominantly present with neurological symptoms but without metabolic crises. In both groups, however, the brain is the major target. The high energy demand of the brain, the gate-keeping function of the blood-brain barrier, a high lipid content, vulnerable neuronal subpopulations, and glutamatergic neurotransmission all make the brain particularly vulnerable against mitochondrial dysfunction, oxidative stress, and excitotoxicity. In fact, toxic metabolites in OADs are thought to cause secondary impairment of energy metabolism; some of these toxic metabolites are trapped in the brain. In contrast to cerebral OADs, patients with classical OADs have an increased risk of multiorgan dysfunction. The lack of the anaplerotic propionate pathway, synergistic inhibition of energy metabolism by toxic metabolites, and multiple oxidative phosphorylation (OXPHOS) deficiency may best explain the involvement of organs with a high energy demand. Intriguingly, late-onset organ dysfunction may manifest even under metabolically stable conditions. This might be explained by chronic mitochondrial DNA depletion, increased production of reactive oxygen species, and altered gene expression due to histone modification. In conclusion, pathomechanisms underlying the acute disease manifestation in OADs, with a particular focus on the brain, are partially understood. More work is required to predict the risk and to elucidate the mechanism of late-onset organ dysfunction, extracerebral disease manifestation, and tumorigenesis.

Effects of targeted suppression of glutaryl-CoA dehydrogenase by lentivirus-mediated shRNA and excessive intake of lysine on apoptosis in rat striatal neurons

In glutaric aciduria type 1 (GA1), glutaryl-CoA dehydrogenase (GCDH) deficiency has been shown to be responsible for the accumulation of glutaric acid and striatal degeneration. However, the mechanisms by which GA1 induces striatal degeneration remain unclear. In this study, we aimed to establish a novel neuronal model of GA1 and to investigate the effects of GCDH deficiency and lysine-related metabolites on the viability of rat striatal neurons. Thus we constructed a lentiviral vector containing short hairpin RNA targeted against the GCDH gene expression (lentivirus-shRNA) in neurons. A virus containing a scrambled short hairpin RNA construct served as a control. Addition of lysine (5 mmol/L) was used to mimic hypermetabolism. Cell viability was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. Apoptosis was assessed using Hoechst33342 staining and Annexin V-PE/7-AAD staining. The mitochondrial membrane potential (MPP) was monitored using tetramethylrhodamine methyl ester. The expression levels of caspases 3, 8, and 9 were determined by Western blotting. We found that lentivirus-shRNA induced apoptosis and decreased MMP levels in neurons, and addition of 5 mmol/L lysine enhanced this effect markedly. Lentivirus-shRNA upregulated the protein levels of caspases 3 and 9 regardless of the presence of 5 mmol/L lysine. The expression level of caspase 8 was higher in neurons co-treated with lentivirus-shRNA and 5 mmol/L lysine than in control. Benzyloxy-carbonyl-Val-Ala-Asp(OMe)-fluoromethylketone, a pan-caspase inhibitor, blocked the apoptosis induced by lentivirus-shRNA and 5 mmol/L lysine to a great extent. These results indicate that the targeted suppression of GCDH by lentivirus-mediated shRNA and excessive intake of lysine may be a useful cell model of GA1. These also suggest that GA1-induced striatal degeneration is partially caspase-dependent.

Low lysine diet in glutaric aciduria type I--effect on anthropometric and biochemical follow-up parameters

BACKGROUND

Metabolic treatment in glutaric aciduria type I (GA-I) including a low lysine diet with lysine-free, tryptophan-reduced amino acid supplements (AAS), carnitine supplementation and early start of emergency treatment during putatively threatening episodes of intermittent febrile illness dramatically improves the outcome and thus has been recommended by an international guideline group (Kölker et al, J Inherit Metab Dis 30:5-22, 2007). However, possible affection of linear growth, weight gain and biochemical follow-up monitoring has not been studied systematically.

METHODS

Thirty-three patients (n = 29 asymptomatic, n = 4 dystonic) with GA-I who have been identified by newborn screening in Germany from 1999 to 2009 were followed prospectively during the first six years of life. Dietary treatment protocols, anthropometrical and biochemical parameters were longitudinally evaluated.

RESULTS

Mean daily intake as percentage of guideline recommendations was excellent for lysine (asymptomatic patients: 101 %; dystonic patients: 103 %), lysine-free, tryptophan-reduced AAS (108 %; 104 %), energy (106 %; 110 %), and carnitine (92 %; 102 %). Low lysine diet did not affect weight gain (mean SDS 0.05) but mildly impaired linear growth in asymptomatic patients (mean SDS -0.38), while dystonic patients showed significantly reduced weight gain (mean SDS -1.32) and a tendency towards linear growth retardation (mean SDS -1.03). Patients treated in accordance with recent recommendations did not show relevant abnormalities of routine biochemical follow-up parameters.

INTERPRETATION

Low lysine diet promotes sufficient intake of essential nutrients and anthropometric development in asymptomatic children up to age 6 year, whereas individualized nutritional concepts are required for dystonic patients. Revised recommendations for biochemical monitoring might be required for asymptomatic patients.

T cell activation is driven by an ADP-dependent glucokinase linking enhanced glycolysis with mitochondrial reactive oxygen species generation

Mitochondria-originating reactive oxygen species (ROS) control T cell receptor (TCR)-induced gene expression. Here, we show that TCR-triggered activation of ADP-dependent glucokinase (ADPGK), an alternative, glycolytic enzyme typical for Archaea, mediates generation of the oxidative signal. We also show that ADPGK is localized in the endoplasmic reticulum and suggest that its active site protrudes toward the cytosol. The ADPGK-driven increase in glycolytic metabolism coincides with TCR-induced glucose uptake, downregulation of mitochondrial respiration, and deviation of glycolysis toward mitochondrial glycerol-3-phosphate dehydrogenase(GPD) shuttle; i.e., a metabolic shift to aerobic glycolysis similar to the Warburg effect. The activation of respiratory-chain-associated GPD2 results in hyperreduction of ubiquinone and ROS release from mitochondria. In parallel, mitochondrial bioenergetics and ultrastructure are altered. Downregulation of ADPGK or GPD2 abundance inhibits oxidative signal generation and induction of NF-κB-dependent gene expression, whereas overexpression of ADPGK potentiates them.

Reduction of Na+, K+-ATPase activity and expression in cerebral cortex of glutaryl-CoA dehydrogenase deficient mice: a possible mechanism for brain injury in glutaric aciduria type I

Mitochondrial dysfunction has been proposed to play an important role in the neuropathology of glutaric acidemia type I (GA I). However, the relevance of bioenergetics disruption and the exact mechanisms responsible for the cortical leukodystrophy and the striatum degeneration presented by GA I patients are not yet fully understood. Therefore, in the present work we measured the respiratory chain complexes activities I-IV, mitochondrial respiratory parameters state 3, state 4, the respiratory control ratio and dinitrophenol (DNP)-stimulated respiration (uncoupled state), as well as the activities of α-ketoglutarate dehydrogenase (α-KGDH), creatine kinase (CK) and Na+, K+-ATPase in cerebral cortex, striatum and hippocampus from 30-day-old Gcdh-/- and wild type (WT) mice fed with a normal or a high Lys (4.7%) diet. When a baseline (0.9% Lys) diet was given, we verified mild alterations of the activities of some respiratory chain complexes in cerebral cortex and hippocampus, but not in striatum from Gcdh-/- mice as compared to WT animals. Furthermore, the mitochondrial respiratory parameters and the activities of α-KGDH and CK were not modified in all brain structures from Gcdh-/- mice. In contrast, we found a significant reduction of Na(+), K(+)-ATPase activity associated with a lower degree of its expression in cerebral cortex from Gcdh-/- mice. Furthermore, a high Lys (4.7%) diet did not accentuate the biochemical alterations observed in Gcdh-/- mice fed with a normal diet. Since Na(+), K(+)-ATPase activity is required for cell volume regulation and to maintain the membrane potential necessary for a normal neurotransmission, it is presumed that reduction of this enzyme activity may represent a potential underlying mechanism involved in the brain swelling and cortical abnormalities (cortical atrophy with leukodystrophy) observed in patients affected by GA I.

Marked reduction of Na(+), K(+)-ATPase and creatine kinase activities induced by acute lysine administration in glutaryl-CoA dehydrogenase deficient mice

Glutaric acidemia type I (GA I) is an inherited neurometabolic disorder caused by a severe deficiency of the mitochondrial glutaryl-CoA dehydrogenase activity leading to accumulation of predominantly glutaric (GA) and 3-hydroxyglutaric (3HGA) acids in the brain and other tissues. Affected patients usually present with hypotonia and brain damage and acute encephalopathic episodes whose pathophysiology is not yet fully established. In this study we investigated important parameters of cellular bioenergetics in brain, heart and skeletal muscle from 15-day-old glutaryl-CoA dehydrogenase deficient mice (Gcdh(-/-)) submitted to a single intra-peritoneal injection of saline (Sal) or lysine (Lys - 8 μmol/g) as compared to wild type (WT) mice. We evaluated the activities of the respiratory chain complexes II, II-III and IV, α-ketoglutarate dehydrogenase (α-KGDH), creatine kinase (CK) and synaptic Na(+), K(+)-ATPase. No differences of all evaluated parameters were detected in the Gcdh(-/-) relatively to the WT mice injected at baseline (Sal). Furthermore, mild increases of the activities of some respiratory chain complexes (II-III and IV) were observed in heart and skeletal muscle of Gcdh(-/-) and WT mice after Lys administration. However, the most marked effects provoked by Lys administration were marked decreases of the activities of Na(+), K(+)-ATPase in brain and CK in brain and skeletal muscle of Gcdh(-/-) mice. In contrast, brain α-KGDH activity was not altered in WT and Gcdh(-/-) injected with Sal or Lys. Our results demonstrate that reduction of Na(+), K(+)-ATPase and CK activities may play an important role in the pathogenesis of the neurodegenerative changes in GA I.

Complementary dietary treatment using lysine-free, arginine-fortified amino acid supplements in glutaric aciduria type I - A decade of experience

The cerebral formation and entrapment of neurotoxic dicarboxylic metabolites (glutaryl-CoA, glutaric and 3-hydroxyglutaric acid) are considered to be important pathomechanisms of striatal injury in glutaric aciduria type I (GA-I). The quantitatively most important precursor of these metabolites is lysine. Recommended therapeutic interventions aim to reduce lysine oxidation (low lysine diet, emergency treatment to minimize catabolism) and to enhance physiologic detoxification of glutaryl-CoA via formation of glutarylcarnitine (carnitine supplementation). It has been recently shown in Gcdh(-/-) mice that cerebral lysine influx and oxidation can be modulated by arginine which competes with lysine for transport at the blood-brain barrier and the inner mitochondrial membrane [Sauer et al., Brain 134 (2011) 157-170]. Furthermore, short-term outcome of 12 children receiving arginine-fortified diet showed very promising results [Strauss et al., Mol. Genet. Metab. 104 (2011) 93-106]. Since lysine-free, arginine-fortified amino acid supplements (AAS) are commercially available and used in Germany for more than a decade, we evaluated the effect of arginine supplementation in a cohort of 34 neonatally diagnosed GA-I patients (median age, 7.43 years; cumulative follow-up period, 221.6 patient years) who received metabolic treatment according to a published guideline [Kölker et al., J. Inherit. Metab. Dis. 30 (2007) 5-22]. Patients used one of two AAS product lines during the first year of life, resulting in differences in arginine consumption [group 1 (Milupa Metabolics): mean=111 mg arginine/kg; group 2 (Nutricia): mean=145 mg arginine/kg; p<0.001]. However, in both groups the daily arginine intake was increased (mean, 137 mg/kg body weight) and the dietary lysine-to-arginine ratio was decreased (mean, 0.7) compared to infants receiving human milk and other natural foods only. All other dietary parameters were in the same range. Despite significantly different arginine intake, the plasma lysine-to-arginine ratio did not differ in both groups. Frequency of dystonia was low (group 1: 12.5%; group 2: 8%) compared with patients not being treated according to the guideline, and gross motor development was similar in both groups. In conclusion, the development of complementary dietary strategies exploiting transport competition between lysine and arginine for treatment of GA-I seems promising. More work is required to understand neuroprotective mechanisms of arginine, to develop dietary recommendations for arginine and to evaluate the usefulness of plasma monitoring for lysine and arginine levels as predictors of cerebral lysine influx.

The unsolved puzzle of neuropathogenesis in glutaric aciduria type I

Glutaric aciduria type I (GA-I) is a cerebral organic aciduria caused by deficiency of glutaryl-Co-A dehydrogenase (GCDH). GCDH deficiency leads to accumulation of glutaric acid (GA) and 3-hydroxyglutaric acid (3-OHGA), two metabolites that are believed to be neurotoxic, in brain and body fluids. The disorder usually becomes clinically manifest during a catabolic state (e.g. intercurrent illness) with an acute encephalopathic crisis that results in striatal necrosis and in a permanent dystonic-dyskinetic movement disorder. The results of numerous in vitro and in vivo studies have pointed to three main mechanisms involved in the metabolite-mediated neuronal damage: excitotoxicity, impairment of energy metabolism and oxidative stress. There is evidence that during a metabolic crisis GA and its metabolites are produced endogenously in the CNS and accumulate because of limiting transport mechanisms across the blood-brain barrier. Despite extensive experimental work, the relative contribution of the proposed pathogenic mechanisms remains unclear and specific therapeutic approaches have yet to be developed. Here, we review the experimental evidence and try to delineate possible pathogenetic models and approaches for future studies.

Severe dysfunction of respiratory chain and cholesterol metabolism in Atp7b(-/-) mice as a model for Wilson disease

Wilson disease (WD) is caused by mutations of the WD gene ATP7B resulting in copper accumulation in different tissues. WD patients display hepatic and neurological disease with yet poorly understood pathomechanisms. Therefore, we studied age-dependent (3, 6, 47weeks) biochemical and bioenergetical changes in Atp7b(-/-) mice focusing on liver and brain. Mutant mice showed strongly elevated copper and iron levels. Age-dependently decreasing hepatic reduced glutathione levels along with increasing oxidized to reduced glutathione ratios in liver and brain of 47weeks old mice as well as elevated hepatic and cerebral superoxide dismutase activities in 3weeks old mutant mice highlighted oxidative stress in the investigated tissues. We could not find evidence that amino acid metabolism or beta-oxidation is impaired by deficiency of ATP7B. In contrast, sterol metabolism was severely dysregulated. In brains of 3week old mice cholesterol, 8-dehydrocholesterol, desmosterol, 7-dehydrocholesterol, and lathosterol were all highly increased. These changes reversed age-dependently resulting in reduced levels of all previously increased sterol metabolites in 47weeks old mice. A similar pattern of sterol metabolite changes was found in hepatic tissue, though less pronounced. Moreover, mitochondrial energy production was severely affected. Respiratory chain complex I activity was increased in liver and brain of mutant mice, whereas complex II, III, and IV activities were reduced. In addition, aconitase activity was diminished in brains of Atp7b(-/-) mice. Summarizing, our study reveals oxidative stress along with severe dysfunction of mitochondrial energy production and of sterol metabolism in Atp7b(-/-) mice shedding new light on the pathogenesis of WD.

Disruption of mitochondrial homeostasis in organic acidurias: insights from human and animal studies

Organic acidurias or organic acidemias constitute a group of inherited disorders caused by deficient activity of specific enzymes of amino acids, carbohydrates or lipids catabolism, leading to large accumulation and excretion of one or more carboxylic (organic) acids. Affected patients usually present neurologic symptoms and abnormalities, sometimes accompanied by cardiac and skeletal muscle alterations, whose pathogenesis is poorly known. However, in recent years growing evidence has emerged indicating that mitochondrial dysfunction is directly or indirectly involved in the pathology of various organic acidemias. Mitochondrial impairment in some of these diseases are generally due to mutations in nuclear genes of the tricarboxylic acid cycle or oxidative phosphorylation, while in others it seems to result from toxic influences of the endogenous organic acids to the mitochondrion. In this minireview, we will briefly summarize the present knowledge obtained from human and animal studies showing that disruption of mitochondrial homeostasis may represent a relevant pathomechanism of tissue damage in selective organic acidemias. The discussion will focus on mitochondrial alterations found in patients affected by organic acidemias and by the deleterious effects of the accumulating organic acids on mitochondrial pathways that are crucial for ATP formation and transfer. The elucidation of the mechanisms of toxicity of these acidic compounds offers new perspectives for potential novel adjuvant therapeutic strategies in selected disorders of this group.

Glutaric aciduria type 1 metabolites impair the succinate transport from astrocytic to neuronal cells

The inherited neurodegenerative disorder glutaric aciduria type 1 (GA1) results from mutations in the gene for the mitochondrial matrix enzyme glutaryl-CoA dehydrogenase (GCDH), which leads to elevations of the dicarboxylates glutaric acid (GA) and 3-hydroxyglutaric acid (3OHGA) in brain and blood. The characteristic clinical presentation of GA1 is a sudden onset of dystonia during catabolic situations, resulting from acute striatal injury. The underlying mechanisms are poorly understood, but the high levels of GA and 3OHGA that accumulate during catabolic illnesses are believed to play a primary role. Both GA and 3OHGA are known to be substrates for Na(+)-coupled dicarboxylate transporters, which are required for the anaplerotic transfer of the tricarboxylic acid cycle (TCA) intermediate succinate between astrocytes and neurons. We hypothesized that GA and 3OHGA inhibit the transfer of succinate from astrocytes to neurons, leading to reduced TCA cycle activity and cellular injury. Here, we show that both GA and 3OHGA inhibit the uptake of [(14)C]succinate by Na(+)-coupled dicarboxylate transporters in cultured astrocytic and neuronal cells of wild-type and Gcdh(-/-) mice. In addition, we demonstrate that the efflux of [(14)C]succinate from Gcdh(-/-) astrocytic cells mediated by a not yet identified transporter is strongly reduced. This is the first experimental evidence that GA and 3OHGA interfere with two essential anaplerotic transport processes: astrocytic efflux and neuronal uptake of TCA cycle intermediates, which occur between neurons and astrocytes. These results suggest that elevated levels of GA and 3OHGA may lead to neuronal injury and cell death via disruption of TCA cycle activity.

Iron regulatory proteins secure mitochondrial iron sufficiency and function

Mitochondria supply cells with ATP, heme, and iron sulfur clusters (ISC), and mitochondrial energy metabolism involves both heme- and ISC-dependent enzymes. Here, we show that mitochondrial iron supply and function require iron regulatory proteins (IRP), cytosolic RNA-binding proteins that control mRNA translation and stability. Mice lacking both IRP1 and IRP2 in their hepatocytes suffer from mitochondrial iron deficiency and dysfunction associated with alterations of the ISC and heme biosynthetic pathways, leading to liver failure and death. These results uncover a major role of the IRPs in cell biology: to ensure adequate iron supply to the mitochondrion for proper function of this critical organelle.

Induction of S100B secretion in C6 astroglial cells by the major metabolites accumulating in glutaric acidemia type I

Glutaryl-CoA dehydrogenase deficiency or glutaric acidemia type I (GA I) is an inherited neurometabolic disorder biochemically characterized by tissue accumulation of predominantly glutaric (GA) and 3-hydroxyglutaric (3OHGA) acids and clinically by severe neurological symptoms and structural brain abnormalities, manifested as progressive cerebral atrophy and acute striatum degeneration following encephalopathic crises, whose pathophysiology is still in debate. Considering that reactive astrogliosis is a common finding in brain of GA I patients, in the present study we investigated the effects of GA and 3OHGA on glial activity determined by S100B release by rat C6-glioma cells. We also evaluated the effects of these organic acids on some parameters of oxidative stress in these astroglial cells. We observed that GA and 3OHGA significantly increased S100B secretion and thiobarbituric acid-reactive substances (lipid peroxidation), whereas GA markedly decreased reduced glutathione levels in these glioma cells. This is the first report demonstrating that the major metabolites accumulating in GA I activate S100B secretion in astroglial cells, indicating activation of these cells. We also showed that GA and 3OHGA induced oxidative stress in C6 lineage cells, confirming previous findings observed in brain fresh tissue. It is therefore presumed that reactive glial cells and oxidative damage may underlie at least in part the neuropathology of GA I.

Mitochondrial reactive oxygen species control T cell activation by regulating IL-2 and IL-4 expression: mechanism of ciprofloxacin-mediated immunosuppression

This article shows that T cell activation-induced expression of the cytokines IL-2 and -4 is determined by an oxidative signal originating from mitochondrial respiratory complex I. We also report that ciprofloxacin, a fluoroquinolone antibiotic, exerts immunosuppressive effects on human T cells suppressing this novel mechanism. Sustained treatment of preactivated primary human T cells with ciprofloxacin results in a dose-dependent inhibition of TCR-induced generation of reactive oxygen species (ROS) and IL-2 and -4 expression. This is accompanied by the loss of mitochondrial DNA and a resulting decrease in activity of the complex I. Consequently, using a complex I inhibitor or small interfering RNA-mediated downregulation of the complex I chaperone NDUFAF1, we demonstrate that TCR-triggered ROS generation by complex I is indispensable for activation-induced IL-2 and -4 expression and secretion in resting and preactivated human T cells. This oxidative signal (H(2)O(2)) synergizes with Ca(2+) influx for IL-2/IL-4 expression and facilitates induction of the transcription factors NF-kappaB and AP-1. Moreover, using T cells isolated from patients with atopic dermatitis, we show that inhibition of complex I-mediated ROS generation blocks disease-associated spontaneous hyperexpression and TCR-induced expression of IL-4. Prolonged ciprofloxacin treatment of T cells from patients with atopic dermatitis also blocks activation-induced expression and secretion of IL-4. Thus, our work shows that the activation phenotype of T cells is controlled by a mitochondrial complex I-originated oxidative signal.

Glutaric aciduria type I and methylmalonic aciduria: simulation of cerebral import and export of accumulating neurotoxic dicarboxylic acids in in vitro models of the blood-brain barrier and the choroid plexus

Intracerebral accumulation of neurotoxic dicarboxylic acids (DCAs) plays an important pathophysiological role in glutaric aciduria type I and methylmalonic aciduria. Therefore, we investigated the transport characteristics of accumulating DCAs - glutaric (GA), 3-hydroxyglutaric (3-OH-GA) and methylmalonic acid (MMA) - across porcine brain capillary endothelial cells (pBCEC) and human choroid plexus epithelial cells (hCPEC) representing in vitro models of the blood-brain barrier (BBB) and the choroid plexus respectively. We identified expression of organic acid transporters 1 (OAT1) and 3 (OAT3) in pBCEC on mRNA and protein level. For DCAs tested, transport from the basolateral to the apical site (i.e. efflux) was higher than influx. Efflux transport of GA, 3-OH-GA, and MMA across pBCEC was Na(+)-dependent, ATP-independent, and was inhibited by the OAT substrates para-aminohippuric acid (PAH), estrone sulfate, and taurocholate, and the OAT inhibitor probenecid. Members of the ATP-binding cassette transporter family or the organic anion transporting polypeptide family, namely MRP2, P-gp, BCRP, and OATP1B3, did not mediate transport of GA, 3-OH-GA or MMA confirming the specificity of efflux transport via OATs. In hCPEC, cellular import of GA was dependent on Na(+)-gradient, inhibited by NaCN, and unaffected by probenecid suggesting a Na(+)-dependent DCA transporter. Specific transport of GA across hCPEC, however, was not found. In conclusion, our results indicate a low but specific efflux transport for GA, 3-OH-GA, and MMA across pBCEC, an in vitro model of the BBB, via OAT1 and OAT3 but not across hCPEC, an in vitro model of the choroid plexus.

Cerebral haemodynamics in patients with glutaryl-coenzyme A dehydrogenase deficiency

In glutaric aciduria type 1, glutaryl-coenzyme A and its derivatives are produced from intracerebral lysine and entrapped at high concentrations within the brain, where they interfere with energy metabolism. Biochemical toxicity is thought to trigger stroke-like striatal degeneration in susceptible children under 2 years of age. Here, we explore vascular derangements that might also contribute to brain damage. We studied injured and non-injured Amish glutaric aciduria type 1 patients using magnetic resonance imaging (n = 26), transcranial Doppler ultrasound (n = 35) and perfusion computed tomography (n = 6). All glutaric aciduria type 1 patients had wide middle cerebral, internal carotid and basilar arteries. In non-injured patients, middle cerebral artery velocities were 18-26% below control values throughout late infancy and early childhood, whereas brain-injured children had an early velocity peak (18 months) and low values thereafter. Perfusion scans from six patients showed that tissue blood flow did not undergo a normal developmental surge. We observed four different perfusion patterns. (i) Three children (two non-injured) had low cerebral blood flow, prolonged mean transit time, elevated cerebral blood volume and high mean transit time/cerebral blood flow and cerebral blood volume/cerebral blood flow ratios. This pattern optimizes substrate extraction at any given flow rate but indicates low perfusion pressure and limited autoregulatory reserve. (ii) Ten hours after the onset of striatal necrosis in an 8-month-old infant, mean transit time and cerebral blood volume were low relative to cerebral blood flow, which varied markedly from region to region. This pattern indicates disturbed autoregulation, regional perfusion pressure gradients, or redistribution of flow from functional capillaries to non-exchanging vessels. (iii) In an infant with atrophic putaminal lesions, striatal flow was normal but mean transit time and cerebral blood volume were low, consistent with perfusion in excess of metabolic demand. (iv) Finally, a brain-injured adult with glutaric aciduria type 1 had regional perfusion values within the normal range, but the putamina, which normally have the highest regional perfusion, had cerebral blood flow values 24% below cortical grey matter. Although metabolic toxicity appears central to the pathophysiology of striatal necrosis, cerebrovascular changes probably also contribute to the process. These changes may be the primary cause of expanded cerebrospinal fluid volume in newborns, intracranial and retinal haemorrhages in infants and interstitial white matter oedema in children and adults. This pilot study suggests important new areas for clinical investigation.

Long-term exposure of human proximal tubule cells to hydroxycobalamin[c-lactam] as a possible model to study renal disease in methylmalonic acidurias

Dysfunction of proximal tubules resulting in tubulointerstitial nephritis and chronic renal failure is a frequent long-term complication of methylmalonic acidurias. However, the underlying pathomechanisms have not yet been extensively studied owing to the lack of suitable in vitro and in vivo models. Application of hydroxycobalamin[c-lactam] has been shown to inhibit the metabolism of hydroxycobalamin and, thereby, to induce methylmalonic aciduria in rats, oligodendrocytes, and rat hepatocytes. Our study characterizes the biochemical and bioenergetic effects of long-term exposure of human proximal tubule cells to hydroxycobalamin[c-lactam], aiming to establish a novel in vitro model for the renal pathogenesis of methylmalonic acidurias. Incubation of human proximal tubule cells with hydroxycobalamin[c-lactam] and propionic acid resulted in a strong, time-dependent intra- and extracellular accumulation of methylmalonic acid. Bioenergetic studies of respiratory chain enzyme complexes revealed an increase of complex II-IV activity after 2 weeks and an increase of complex I and IV activity as well as a decrease of complex II and III activity after 3 weeks of incubation. In addition, human proximal tubule cells displayed reduced glutathione content after the exposure to hydroxycobalamin[c-lactam] and propionic acid.

Dynamic changes of striatal and extrastriatal abnormalities in glutaric aciduria type I

In glutaric aciduria type I, an autosomal recessive disease of mitochondrial lysine, hydroxylysine and tryptophan catabolism, striatal lesions are characteristically induced by acute encephalopathic crises during a finite period of brain development (age 3-36 months). The frequency of striatal injury is significantly less in patients diagnosed as asymptomatic newborns by newborn screening. Most previous studies have focused on the onset and mechanism of striatal injury, whereas little is known about neuroradiological abnormalities in pre-symptomatically diagnosed patients and about dynamic changes of extrastriatal abnormalities. Thus, the major aim of the present retrospective study was to improve our understanding of striatal and extrastriatal abnormalities in affected individuals including those diagnosed by newborn screening. To this end, we systematically analysed magnetic resonance imagings (MRIs) in 38 patients with glutaric aciduria type I diagnosed before or after the manifestation of neurological symptoms. To identify brain regions that are susceptible to cerebral injury during acute encephalopathic crises, we compared the frequency of magnetic resonance abnormalities in patients with and without such crises. Major specific changes after encephalopathic crises were found in the putamen (P < 0.001), nucleus caudatus (P < 0.001), globus pallidus (P = 0.012) and ventricles (P = 0.001). Analysis of empirical cumulative distribution frequencies, however, demonstrated that isolated pallidal abnormalities did not significantly differ over time in both groups (P = 0.544) suggesting that isolated pallidal abnormalities are not induced by acute crises--in contrast to striatal abnormalities. The manifestation of motor disability was associated with signal abnormalities in putamen, caudate, pallidum and ventricles. In addition, we found a large number of extrastriatal abnormalities in patients with and without preceding encephalophatic crises. These abnormalities include widening of anterior temporal and sylvian CSF spaces, pseudocysts, signal changes of substantia nigra, nucleus dentatus, thalamus, tractus tegmentalis centralis and supratentorial white matter as well as signs of delayed maturation (myelination and gyral pattern). In contrast to the striatum, extrastriatal abnormalities were variable and could regress or even normalize with time. This includes widening of sylvian fissures, delayed maturation, pallidal signal changes and pseudocysts. Based on these results, we hypothesize that neuroradiological abnormalities and neurological symptoms in glutaric aciduria type I can be explained by overlaying episodes of cerebral alterations including maturational delay of the brain in utero, acute striatal injury during a vulnerable period in infancy and chronic progressive changes that may continue lifelong. This may have widespread consequences for the pathophysiological understanding of this disease, long-term outcomes and therapeutic considerations.

Genetic mapping of glutaric aciduria, type 3, to chromosome 7 and identification of mutations in c7orf10

While screening Old Order Amish children for glutaric aciduria type 1 (GA1) between 1989 and 1993, we found three healthy children who excreted abnormal quantities of glutaric acid but low 3-hydroxyglutaric acid, a pattern consistent with glutaric aciduria type 3 (GA3). None of these children had the GCDH c.1262C-->T mutation that causes GA1 among the Amish. Using single-nucleotide polymorphism (SNP) genotypes, we identified a shared homozygous 4.7 Mb region on chromosome 7. This region contained 25 genes including C7orf10, an open reading frame with a putative mitochondrial targeting sequence and coenzyme-A transferase domain. Direct sequencing of C7orf10 revealed that the three Amish individuals were homozygous for a nonsynonymous sequence variant (c.895C-->T, Arg299Trp). We then sequenced three non-Amish children with GA3 and discovered two nonsense mutations (c.322C-->T, Arg108Ter, and c.424C-->T, Arg142Ter) in addition to the Amish mutation. Two pathogenic alleles were identified in each of the six patients. There was no consistent clinical phenotype associated with GA3. In affected individuals, urine molar ratios of glutarate to its derivatives (3-hydroxyglutarate, glutarylcarnitine, and glutarylglycine) were elevated, suggesting impaired formation of glutaryl-CoA. These observations refine our understanding of the lysine-tryptophan degradation pathway and have important implications for the pathophysiology of GA1.

Impact of short- and medium-chain organic acids, acylcarnitines, and acyl-CoAs on mitochondrial energy metabolism

Accumulation of organic acids as well as their CoA and carnitine esters in tissues and body fluids is a common finding in organic acidurias, beta-oxidation defects, Reye syndrome, and Jamaican vomiting sickness. Pathomechanistic approaches for these disorders have been often focused on the effect of accumulating organic acids on mitochondrial energy metabolism, whereas little is known about the pathophysiologic role of short- and medium-chain acyl-CoAs and acylcarnitines. Therefore, we investigated the impact of short- and medium-chain organic acids, acylcarnitines, and acyl-CoAs on central components of mitochondrial energy metabolism, namely alpha-ketoglutarate dehydrogenase complex, pyruvate dehydrogenase complex, and single enzyme complexes I-V of respiratory chain. Although at varying degree, all acyl-CoAs had an inhibitory effect on pyruvate dehydrogenase complex and alpha-ketoglutarate dehydrogenase complex activity. Effect sizes were critically dependent on chain length and number of functional groups. Unexpectedly, octanoyl-CoA was shown to inhibit complex III. The inhibition was noncompetitive regarding reduced ubiquinone and uncompetitive regarding cytochrome c. In addition, octanoyl-CoA caused a blue shift in the gamma band of the absorption spectrum of reduced complex III. This effect may play a role in the pathogenesis of medium-chain and multiple acyl-CoA dehydrogenase deficiency, Reye syndrome, and Jamaican vomiting sickness which are inherited and acquired conditions of intracellular accumulation of octanoyl-CoA.

Pathogenesis of CNS involvement in disorders of amino and organic acid metabolism

Inherited disorders of amino and organic acid metabolism have a high cumulative frequency, and despite heterogeneous aetiology and varying clinical presentation, the manifestation of neurological disease is common. It has been demonstrated for some of these diseases that accumulating pathological metabolites are directly involved in the manifestation of neurological disease. Various pathomechanisms have been suggested in different in vitro and in vivo models including an impairment of brain energy metabolism, an imbalance of excitatory and inhibitory neurotransmission, altered transport across the blood-brain barrier and between glial cells and neurons, impairment of myelination and disturbed neuronal efflux of metabolic water. This review summarizes recent knowledge on pathomechanisms involved in phenylketonuria, glutaric aciduria type I, succinic semialdehyde dehydrogenase deficiency and aspartoacylase deficiency with examples, highlighting general as well as disease-specific concepts and their putative impact on treatment.

Biochemistry and bioenergetics of glutaryl-CoA dehydrogenase deficiency

Glutaryl-CoA dehydrogenase (GCDH) is a central enzyme in the catabolic pathway of L-tryptophan, L-lysine, and L-hydroxylysine which catalyses the oxidative decarboxylation of glutaryl-CoA to crotonyl-CoA and CO2. Glutaryl-CoA dehydrogenase deficiency (GDD) is an autosomal recessive disease characterized by the accumulation of glutaric and 3-hydroxyglutaric acids in tissues and body fluids. Untreated patients commonly present with severe striatal degeneration during encephalopathic crises. Previous studies have highlighted primary excitotoxicity as a trigger of striatal degeneration. The aim of this PhD study was to investigate in detail tissue-specific bioenergetic and biochemical parameters of GDD in vitro, post mortem, and in Gcdh-/- mice. The major bioenergetic finding was uncompetitive inhibition of alpha-ketoglutarate dehydrogenase complex by glutaryl-CoA. It is suggested that a synergism of primary and secondary excitotoxic effects in concert with age-related physiological changes in the developing brain underlie acute and chronic neurodegenerative changes in GDD patients. The major biochemical findings were highly elevated cerebral concentrations of glutaric and 3-hydroxyglutaric acid despite low permeability of the blood-brain barrier for these dicarboxylic acids. It can be postulated that glutaric and 3-hydroxyglutaric acids are synthesized de novo and subsequently trapped in the brain. In this light, neurological disease in GDD is not 'transported' to the brain in analogy with phenylketonuria or hepatic encephalopathy as suggested previously but is more likely to be induced by the intrinsic biochemical properties of the cerebral tissue and the blood-brain barrier.

Age and brain structural related effects of glutaric and 3-hydroxyglutaric acids on glutamate binding to plasma membranes during rat brain development

(1) In the present study we determined the effects of glutaric (GA, 0.01-1 mM) and 3-hydroxyglutaric (3-OHGA, 1.0-100 microM) acids, the major metabolites accumulating in glutaric acidemia type I (GA I), on Na(+)-independent and Na(+)-dependent [(3)H]glutamate binding to synaptic plasma membranes from cerebral cortex and striatum of rats aged 7, 15 and 60 days. (2) GA selectively inhibited Na(+)-independent [(3)H]glutamate binding (binding to receptors) in cerebral cortex and striatum of rats aged 7 and 15 days, but not aged 60 days. In contrast, GA did not alter Na(+)-dependent glutamate binding (binding to transporters) to synaptic membranes from brain structures of rats at all studied ages. Furthermore, experiments using the glutamatergic antagonist CNQX indicated that GA probably binds to non-NMDA receptors. In addition, GA markedly inhibited [(3)H]kainate binding to synaptic plasma membranes in cerebral cortex of 15-day-old rats, indicating that this effect was probably directed towards kainate receptors. On the other hand, experiments performed with 3-OHGA revealed that this organic acid did not change Na(+)-independent [(3)H]glutamate binding to synaptic membranes from cerebral cortex and striatum of rats from all ages, but inhibited Na(+)-dependent [(3)H]glutamate binding to membranes in striatum of 7-day-old rats, but not in striatum of 15- and 60-day-old rats and in cerebral cortex of rats from all studied ages. We also provided some evidence that 3-OHGA competes with the glutamate transporter inhibitor L-trans-pyrrolidine-2,4-dicarboxylate, suggesting a possible interaction of 3-OHGA with glutamate transporters on synaptic membranes. (3) These results indicate that glutamate binding to receptors and transporters can be inhibited by GA and 3-OHGA in cerebral cortex and striatum in a developmentally regulated manner. It is postulated that a disturbance of glutamatergic neurotransmission caused by the major metabolites accumulating in GA I at early development may possibly explain, at least in part, the window of vulnerability of striatum and cerebral cortex to injury in patients affected by this disorder.

Evidence for a synergistic action of glutaric and 3-hydroxyglutaric acids disturbing rat brain energy metabolism

Glutaric acidemia type I is an inherited metabolic disorder caused by a severe deficiency of the mitochondrial glutaryl-CoA dehydrogenase activity leading to accumulation of predominantly glutaric and 3-hydroxyglutaric acids in the brain tissue of the affected patients. Considering that a toxic role was recently postulated for quinolinic acid in the neuropathology of glutaric acidemia type I, in the present work we investigated whether the combination of quinolinic acid with glutaric or 3-hydroxyglutaric acids or the mixture of glutaric plus 3-hydroxyglutaric acids could alter brain energy metabolism. The parameters evaluated in cerebral cortex from young rats were glucose utilization, lactate formation and (14)CO(2) production from labeled glucose and acetate, as well as the activities of pyruvate dehydrogenase and creatine kinase. We first observed that glutaric (5 mM), 3-hydroxyglutaric (1 mM) and quinolinic acids (0.1 microM) per se did not alter these parameters. Similarly, no change of these parameters occurred when combining glutaric with quinolinic acids or 3-hydroxyglutaric with quinolinic acids. In contrast, co-incubation of glutaric plus 3-hydroxyglutaric acids increased glucose utilization, decreased (14)CO(2) generation from glucose, inhibited pyruvate dehydrogenase activity as well as total and mitochondrial creatine kinase activities. The glutaric plus 3-hydroxyglutaric acids-induced inhibitory effects on creatine kinase were prevented by the antioxidants glutathione and catalase plus superoxide dismutase, indicating the participation of reactive oxygen species. Our data indicate a synergic action of glutaric and 3-hydroxyglutaric acids disturbing energy metabolism in cerebral cortex of young rats.

Advancing translational research with the Semantic Web

BACKGROUND

A fundamental goal of the U.S. National Institute of Health (NIH) "Roadmap" is to strengthen Translational Research, defined as the movement of discoveries in basic research to application at the clinical level. A significant barrier to translational research is the lack of uniformly structured data across related biomedical domains. The Semantic Web is an extension of the current Web that enables navigation and meaningful use of digital resources by automatic processes. It is based on common formats that support aggregation and integration of data drawn from diverse sources. A variety of technologies have been built on this foundation that, together, support identifying, representing, and reasoning across a wide range of biomedical data. The Semantic Web Health Care and Life Sciences Interest Group (HCLSIG), set up within the framework of the World Wide Web Consortium, was launched to explore the application of these technologies in a variety of areas. Subgroups focus on making biomedical data available in RDF, working with biomedical ontologies, prototyping clinical decision support systems, working on drug safety and efficacy communication, and supporting disease researchers navigating and annotating the large amount of potentially relevant literature.

RESULTS

We present a scenario that shows the value of the information environment the Semantic Web can support for aiding neuroscience researchers. We then report on several projects by members of the HCLSIG, in the process illustrating the range of Semantic Web technologies that have applications in areas of biomedicine.

CONCLUSION

Semantic Web technologies present both promise and challenges. Current tools and standards are already adequate to implement components of the bench-to-bedside vision. On the other hand, these technologies are young. Gaps in standards and implementations still exist and adoption is limited by typical problems with early technology, such as the need for a critical mass of practitioners and installed base, and growing pains as the technology is scaled up. Still, the potential of interoperable knowledge sources for biomedicine, at the scale of the World Wide Web, merits continued work.

Multimodal imaging of striatal degeneration in Amish patients with glutaryl-CoA dehydrogenase deficiency

Despite early diagnosis, one-third of Amish infants with glutaryl-CoA dehydrogenase deficiency (GA1) develop striatal lesions that leave them permanently disabled. To better understand mechanisms of striatal degeneration, we retrospectively studied imaging results from 25 Amish GA1 patients homozygous for 1296C>T mutations in GCDH. Asymptomatic infants had reduced glucose tracer uptake and increased blood volume throughout gray matter, which may signify a predisposition to brain injury. Nine children (36%) developed striatal lesions: three had sudden motor regression during infancy whereas six had insidious motor delay associated with striatal lesions of undetermined onset. Acute striatal necrosis consisted of three stages: (1) an acute stage, within 24 h of motor regression, characterized by cytotoxic oedema within the basal ganglia, cerebral oligemia, and rapid transit of blood throughout gray matter; (2) a sub-acute stage, 4-5 days after the onset of clinical signs, characterized by reduced striatal perfusion and glucose uptake, and supervening vasogenic oedema; and (3) a chronic stage of striatal atrophy. Apparent diffusion coefficient maps revealed that at least two of the six patients with insidious motor delay suffered striatal injuries before or shortly after birth, followed by latent periods of several months before disability was apparent. Thus, acute and insidious presentations may occur by similar mechanisms, and differ only with regard to the timing of injury. Intravenous fluid and dextrose therapy for illnesses during the first 2 years of life was the only intervention that was clearly neuroprotective in this cohort (odds ratio for brain injury = 0.04, 95% confidence interval = 0.01-0.34; P < 0.001).

Energy metabolism is compromised in skeletal muscle of rats chronically-treated with glutaric acid

Glutaric acidemia type I (GA I) (GA I, McKusick 23167; OMIM # 231670) is an autosomal recessive metabolic disorder caused by glutaryl-CoA dehydrogenase deficiency (EC 1.3.99.7). Clinically, the disease is characterized by macrocephaly, hypotonia, dystonia and diskinesia. Since the pathophysiology of this disorder is not yet well established, in the present investigation we determined a number of energy metabolism parameters, namely (14)CO(2) production, the activities of the respiratory chain complexes I-IV and of creatine kinase, in tissues of rats chronically exposed to glutaric acid (GA). High tissue GA concentrations (0.6 mM in the brain, 4 mM in skeletal muscle and 6 mM in plasma) were induced by three daily subcutaneous injections of saline-buffered GA (5 micromol x g(-1) body weight) to Wistar rats from the 5th to the 21st day of life. The parameters were assessed 12 h after the last GA injection in cerebral cortex and middle brain, as well as in skeletal muscle homogenates of GA-treated rats. GA administration significantly inhibited the activities of the respiratory chain complexes I-III and II and induced a significant increase of complex IV activity in skeletal muscle of rats. Furthermore, creatine kinase activity was also inhibited by GA treatment in skeletal muscle. In contrast, these measurements were not altered by GA administration in the brain structures studied. Taken together, it was demonstrated that chronic GA administration induced an impairment of energy metabolism in rat skeletal muscle probably due to a higher tissue concentration of this organic acid that may be possibly associated to the muscle weakness occurring in glutaric acidemic patients.

The aetiology of neurological complications of organic acidaemias--a role for the blood-brain barrier

The blood-brain barrier (BBB) metabolically isolates the central nervous system (CNS) from the circulation and protects it against fluctuations of hydrophilic nutrients in plasma and from intoxication. Recent studies have shown that dicarboxylic acids (DCAs) are transported across the blood-brain barrier at very low rates. In organic acidaemias, neurological complications are common. We hypothesize that, as a result of the very limited efflux, in certain organic acidaemias there is pathological accumulation of DCAs (e.g. glutarate, 3-hydroxyglutarate, D-2- and L-2-hydroxyglutarate, methylmalonate) in the brain secondary to the metabolic block. At high concentrations some of these compounds may become neurotoxic. Treatment should be aimed at preventing the accumulation of these compounds using our understanding of the properties of the BBB.

Secondary mitochondrial dysfunction in propionic aciduria: a pathogenic role for endogenous mitochondrial toxins

Mitochondrial dysfunction during acute metabolic crises is considered an important pathomechanism in inherited disorders of propionate metabolism, i.e. propionic and methylmalonic acidurias. Biochemically, these disorders are characterized by accumulation of propionyl-CoA and metabolites of alternative propionate oxidation. In the present study, we demonstrate uncompetitive inhibition of PDHc (pyruvate dehydrogenase complex) by propionyl-CoA in purified porcine enzyme and in submitochondrial particles from bovine heart being in the same range as the inhibition induced by acetyl-CoA, the physiological product and known inhibitor of PDHc. Evaluation of similar monocarboxylic CoA esters showed a chain-length specificity for PDHc inhibition. In contrast with CoA esters, non-esterified fatty acids did not inhibit PDHc activity. In addition to PDHc inhibition, analysis of respiratory chain and tricarboxylic acid cycle enzymes also revealed an inhibition by propionyl-CoA on respiratory chain complex III and alpha-ketoglutarate dehydrogenase complex. To test whether impairment of mitochondrial energy metabolism is involved in the pathogenesis of propionic aciduria, we performed a thorough bioenergetic analysis in muscle biopsy specimens of two patients. In line with the in vitro results, oxidative phosphorylation was severely compromised in both patients. Furthermore, expression of respiratory chain complexes I-IV and the amount of mitochondrial DNA were strongly decreased, and ultrastructural mitochondrial abnormalities were found, highlighting severe mitochondrial dysfunction. In conclusion, our results favour the hypothesis that toxic metabolites, in particular propionyl-CoA, are involved in the pathogenesis of inherited disorders of propionate metabolism, sharing mechanistic similarities with propionate toxicity in micro-organisms.

Natural history, outcome, and treatment efficacy in children and adults with glutaryl-CoA dehydrogenase deficiency

Glutaryl-CoA dehydrogenase (GCDH) deficiency is a rare inborn disorder of L-lysine, L-hydroxylysine, and L-tryptophan metabolism complicated by striatal damage during acute encephalopathic crises. Three decades after its description, the natural history and how to treat this disorder are still incompletely understood. To study which variables influenced the outcome, we conducted an international cross-sectional study in 35 metabolic centers. Our main outcome measures were onset and neurologic sequelae of acute encephalopathic crises. A total of 279 patients (160 male, 119 female) were included who were diagnosed clinically after clinical presentation (n = 218) or presymptomatically by neonatal screening (n = 23), high-risk screening (n = 24), or macrocephaly (n = 14). Most symptomatic patients (n = 185) had encephalopathic crises, characteristically resulting in bilateral striatal damage and dystonia, secondary complications, and reduced life expectancy. First crises usually occurred during infancy (95% by age 2 y); the oldest age at which a repeat crisis was reported was 70 mo. In a few patients, neurologic disease developed without a reported crisis. Differences in the diagnostic criteria and therapeutic protocols for patients with GCDH deficiency resulted in a huge variability in the outcome worldwide. Recursive partitioning demonstrated that timely diagnosis in neurologically asymptomatic patients followed by treatment with L-carnitine and a lysine-restricted diet was the best predictor of good outcome, whereas treatment efficacy was low in patients diagnosed after the onset of neurologic disease. Notably, the biochemical phenotype did not predict the clinical phenotype. Our study proves GCDH deficiency to be a treatable disorder and a good candidate for neonatal screening.

Riboflavin-responsive glutaryl CoA dehydrogenase deficiency

We report here riboflavin responsiveness in a patient with glutaryl CoA dehydrogenase (GCDH) deficiency, compound heterozygous for the S139L and P248L mutations and with 20% residual GCDH enzyme activity in vitro. Our results suggest the mitochondrial GCDH homotetramer remains intact with one of these mutations associated with the binding site of the single FAD cofactor and that pharmacological doses of the cofactor precursor may be sufficient to induce an increase in activity in the mutant GCDH enzyme, although not sufficient to normalise urinary organic acid excretion. Serine139 is one of nine conserved amino acid residues that line the binding site of the protein and is in close proximity to both substrate and FAD cofactor. It is possible that steric alterations caused by substitution of serine with leucine at this position may be overcome with high cofactor concentrations. P248L is also associated with some residual GCDH activity in other patients and the unique combination of S139L with P248L may also explain the results in our patient. Responsiveness to riboflavin in our patient has been compared with two other patients with glutaric aciduria type 1 and minimal residual GCDH activity, one with homozygosity for the R257Q mutation and one with heterozygosity for the G354S mutation and a novel G156V mutation. A low lysine diet reduced glutaric acid excretion in our riboflavin-responsive GCDH-deficient patient almost to control values. She is now 21 years of age and clinically and neurologically normal.

Intracerebral accumulation of glutaric and 3-hydroxyglutaric acids secondary to limited flux across the blood-brain barrier constitute a biochemical risk factor for neurodegeneration in glutaryl-CoA dehydrogenase deficiency

Glutaric acid (GA) and 3-hydroxyglutaric acids (3-OH-GA) are key metabolites in glutaryl co-enzyme A dehydrogenase (GCDH) deficiency and are both considered to be potential neurotoxins. As cerebral concentrations of GA and 3-OH-GA have not yet been studied systematically, we investigated the tissue-specific distribution of these organic acids and glutarylcarnitine in brain, liver, skeletal and heart muscle of Gcdh-deficient mice as well as in hepatic Gcdh-/- mice and in C57Bl/6 mice following intraperitoneal loading. Furthermore, we determined the flux of GA and 3-OH-GA across the blood-brain barrier (BBB) using porcine brain microvessel endothelial cells. Concentrations of GA, 3-OH-GA and glutarylcarnitine were significantly elevated in all tissues of Gcdh-/- mice. Strikingly, cerebral concentrations of GA and 3-OH-GA were unexpectedly high, reaching similar concentrations as those found in liver. In contrast, cerebral concentrations of these organic acids remained low in hepatic Gcdh-/- mice and after intraperitoneal injection of GA and 3-OH-GA. These results suggest limited flux of GA and 3-OH-GA across the BBB, which was supported in cultured porcine brain capillary endothelial cells. In conclusion, we propose that an intracerebral de novo synthesis and subsequent trapping of GA and 3-OH-GA should be considered as a biochemical risk factor for neurodegeneration in GCDH deficiency.

A diet-induced mouse model for glutaric aciduria type I

In the autosomal recessive human disease, glutaric aciduria type I (GA-1), glutaryl-CoA dehydrogenase (GCDH) deficiency disrupts the mitochondrial catabolism of lysine and tryptophan. Affected individuals accumulate glutaric acid (GA) and 3-hydroxyglutaric acid (3-OHGA) in the serum and often suffer acute striatal injury in childhood. Prior attempts to produce selective striatal vulnerability in an animal model have been unsuccessful. We hypothesized that acute striatal injury may be induced in GCDH-deficient (Gcdh-/-) mice by elevated dietary protein and lysine. Here, we show that high protein diets are lethal to 4-week-old and 8-week-old Gcdh-/- mice within 2-3 days and 7-8 days, respectively. High lysine alone resulted in vasogenic oedema and blood-brain barrier breakdown within the striatum, associated with serum and tissue GA accumulation, neuronal loss, haemorrhage, paralysis, seizures and death in 75% of 4-week-old Gcdh-/- mice after 3-12 days. In contrast, most 8-week-old Gcdh-/- mice survived on high lysine, but developed white matter lesions, reactive astrocytes and neuronal loss after 6 weeks. Thus, the Gcdh-/- mouse exposed to high protein or lysine may be a useful model of human GA-1 including developmentally dependent striatal vulnerability.

Glutaric acid moderately compromises energy metabolism in rat brain

Glutaric acidemia type I is an inherited metabolic disorder biochemically characterized by tissue accumulation of predominantly glutaric acid (GA). Affected patients present frontotemporal hypotrophy, as well as caudate and putamen injury following acute encephalopathic crises. Considering that the underlying mechanisms of basal ganglia damage in this disorder are poorly known, in the present study we tested the effects of glutaric acid (0.2-5mM) on critical enzyme activities of energy metabolism, namely the respiratory chain complexes I-IV, succinate dehydrogenase and creatine kinase in midbrain of developing rats. Glutaric acid significantly inhibited creatine kinase activity (up to 26%) even at the lowest dose used in the assays (0.2mM). We also observed that CK inhibition was prevented by pre-incubation of the homogenates with reduced glutathione, suggesting that the inhibitory effect of GA was possibly mediated by oxidation of essential thiol groups of the enzyme. In addition, the activities of the respiratory chain complex I-III and of succinate dehydrogenase were also significantly inhibited by 20 and 30%, respectively, at the highest glutaric acid concentration tested (5mM). In contrast, complexes II-III and IV activities of the electron transport chain were not affected by the acid. The effect of glutaric acid on the rate of oxygen consumption in intact mitochondria from the rat cerebrum was also investigated. Glutaric acid (1mM) significantly lowered the respiratory control ratio (state III/state IV) up to 40% in the presence of the respiratory substrates glutamate/malate or succinate. Moreover, state IV respiration linked to NAD and FAD substrates was significantly increased in GA-treated mitochondria while state III was significantly diminished. The results indicate that the major metabolite accumulating in glutaric acidemia type I moderately compromises brain energy metabolism in vitro.

3-Hydroxyglutaric acid moderately impairs energy metabolism in brain of young rats

3-Hydroxyglutaric acid (3HGA) accumulates in the inherited neurometabolic disorder known as glutaryl-CoA dehydrogenase deficiency. The disease is clinically characterized by severe neurological symptoms, frontotemporal atrophy and striatum degeneration. Because of the pathophysiology of the brain damage in glutaryl-CoA dehydrogenase deficiency is not completed clear, we investigated the in vitro effect of 3HGA (0.01-5.0mM) on critical enzyme activities of energy metabolism, including the respiratory chain complexes I-V, creatine kinase isoforms and Na(+),K(+)-ATPase in cerebral cortex and striatum from 30-day-old rats. Complex II activity was also studied in rat C6-glioma cells exposed to 3HGA. The effect of 3HGA was further investigated on the rate of oxygen consumption in mitochondria from rat cerebrum. We observed that 1.0mM 3HGA significantly inhibited complex II in cerebral cortex and C6 cells but not the other activities of the respiratory chain complexes. Creatine kinase isoforms and Na(+),K(+)-ATPase were also not affected by the acid. Furthermore, no inhibition of complex II activity occurred when mitochondrial preparations from cerebral cortex or striatum homogenates were used. In addition, 3HGA significantly lowered the respiratory control ratio in the presence of glutamate/malate and succinate under stressful conditions or when mitochondria were permeabilized with digitonin. Since 3HGA stimulated oxygen consumption in state IV and compromised ATP formation, it can be presumed that this organic acid might act as an endogenous uncoupler of mitochondria respiration. Finally, we observed that 3HGA changed C6 cell morphology from a round flat to a spindle-differentiated shape, but did not alter cell viability neither induced apoptosis. The data provide evidence that 3HGA provokes a moderate impairment of brain energy metabolism and do not support the view that 3HGA-induced energy failure would solely explain the characteristic brain degeneration observed in glutaryl-CoA dehydrogenase deficiency patients.