Defective glucose transport across the blood-brain barrier as a cause of persistent hypoglycorrhachia, seizures, and developmental delay

Glucose transporter type1 (GLUT-1) deficiency

Glucose transporter type1 (GLUT-1) deficiency may be rare, but it is a preventable cause of severe learning difficulties; and therefore there is an urgency in making an early diagnosis. Suspicions must be roused when intractable seizures occur in infancy. These may be associated with acquired microcephaly and developmental delay. The finding of low glucose sugar levels in the cerebrospinal fluid, but not in the blood will identify the condition. The gene encoding the GLUT-1 protein is located on the short arm of chromosome 1, and inheritance is by a dominant trait. Patients with this syndrome can have heterozygous mutations, with one allele being a normal wild type and one being mutant. An efficient transport of glucose across the blood-brain barrier is essential as it is such an important fuel for the brain, and this is provided by glucose transporter type1 in the endothelial cells of the brain capillaries. Another minor contribution to the symptomatology of GLUT-1 may be impaired transport of an oxidised form of vitamin C. Treatment with anti-epileptic drugs may be needed, and the ketogenic diet may reduce symptoms, as ketosis can provide an alternative source of fuel for the brain. It has also been suggested that antioxidant thioctic acid may be of benefit. Substances such as caffeine and phenobarbitone should be avoided as they inhibit glucose transport.

Reversible infantile hypoglycorrhachia: possible transient disturbance in glucose transport?

Facilitated glucose transporter isoform 1 deficiency syndrome (GLUT1 DS), caused by impaired GLUT1-mediated glucose transport into the brain, is characterized by hypoglycorrhachia. The defect in the facilitative glucose transporter isoform 1 (GLUT1) can be confirmed by functional, quantitative, and molecular analyses. Diagnostic difficulties arise when these analyses are normal and hypoglycorrhachia remains unexplained. Three infants presenting with seizures and hypoglycorrhachia at 2, 4, and 6 weeks of age, which suggests GLUT1 deficiency syndrome, are reported. The seizures responded to a ketogenic diet in Patients 1 and 3 and phenobarbitone in Patient 2. Repeated GLUT1 analyses were normal. When treatment was discontinued, all patients remained seizure-free and developed normally. Subsequent lumbar punctures showed the return to normoglycorrhachia. We conclude that these cases might represent a transient disturbance in GLUT1-mediated glucose transport. The biomolecular basis for this clinical observation remains unknown. Though no treatment is required, clinical follow-up and repeated lumbar punctures are necessary to distinguish this benign condition from the original GLUT1 deficiency syndrome.

Molecular features, regulation, and function of monocarboxylate transporters: implications for drug delivery

The diffusion of monocarboxylates such as lactate and pyruvate across the plasma membrane of mammalian cells is facilitated by a family of integral membrane transport proteins, the monocarboxylate transporters (MCTs). Currently, at least eight unique members of the MCT family have been discovered and orthologs to each have been identified in a variety of species. Four MCTs (MCT1-MCT4) have been functionally characterized. Each isoform possesses unique biochemical properties such as kinetic constants and sensitivity to known MCT inhibitors. Several fold changes in the expression of MCTs may be evoked by altered physiological conditions, yet the molecular mechanisms underlying the regulation of MCTs are poorly understood. Post-translational regulation of MCT1 and MCT4 occurs, in part, by interaction with CD147, an accessory protein that is necessary for trafficking, localization, and functional expression of these transporters. Because of the physiological importance of monocarboxylates to the overall maintenance of metabolic homeostasis, the function of MCTs is significant to several pathologies that occur with disease, such as ischemic stroke and cancer. Finally, the expression of MCT1 in the epithelium of the small intestine and colon and in the blood-brain barrier may provide routes for the intestinal and blood to brain transfer of carboxylated pharmaceutical agents and other exogenous monocarboxylates.

Neuroglycopenia in normoglycaemic patients, and the potential benefit of ketosis

We report a patient with recurrent symptoms of neuroglycopenia due to a defective glucose transport into brain. The potential benefit of ketosis in neuroglycopenia is discussed from the therapeutic concept of a ketogenic diet in GLUT1-deficiency syndrome.

Seizure characterization and electroencephalographic features in Glut-1 deficiency syndrome

PURPOSE

To characterize seizure types and electroencephalographic features of glucose transporter type 1 deficiency syndrome (Glut-1 DS).

METHODS

Twenty children with clinical and laboratory features of Glut-1 DS were evaluated. Age at seizure diagnosis, seizure classification, and response to treatment were determined by chart review. Thirty-two continuous 24-h EEG monitoring sessions and reports of 42 routine EEG studies were assessed.

RESULTS

Age at seizure diagnosis was between 4 weeks and 18 months (mean, 5 months). Seizure types were generalized tonic or clonic (14), absence (10), partial (nine), myoclonic (six), or astatic (four). During 24-h EEGs, background activity showed generalized 2.5- to 4-Hz spike-wave discharges (41%), generalized slowing or attenuation (34%), no abnormalities (34%), focal epileptiform discharges (13%), or focal slowing or attenuation (9%). No seizures were captured during 69% of the studies; the remainder had absence (19%), myoclonic (9%), or partial seizures (3%). On evaluation of routine and 24-h EEG studies, focal epileptiform discharges (24%) and slowing (11%) were more frequent in ages 0-24 months. In older children (2-8 years), generalized epileptiform discharges (37.5%) and slowing (21%) were more common.

CONCLUSIONS

In all ages, a normal interictal EEG was the most common EEG finding. When abnormalities occurred, focal slowing or epileptiform discharges were more prevalent in the infant. In older children (2 years or older), a generalized 2.5- to 4-Hz spike-wave pattern emerged. Seizure types observed included, absence, myoclonic, partial, and astatic.

The ketogenic diet for the treatment of epilepsy: a challenge for nutritional neuroscientists

The ketogenic diet (KD) is a high-fat, low-carbohydrate, adequate-protein diet that has been used for more than eight decades for the treatment of refractory epilepsy in children. Despite this long history, the mechanisms by which the KD exerts its anti-seizure action are not fully understood. Questions remain regarding several aspects of KD action, including its effects on brain biochemistry and energetics, neuronal membrane function and cellular network behavior. With the explosion of the KD use in the last 10 years, it is now imperative that we understand these factors in greater detail, in order to optimize the formulation, administration and fine-tuning of the diet. This review discusses what is known and what remains to be learned about the KD, with emphasis on clinical questions that can be approached in the laboratory. We encourage scientists with a primary interest in nutritional neuroscience to join with those of us in the epilepsy research community to address these urgent questions, for the benefit of children ravaged by intractable seizures.

[Ketogenic regime as anti-epileptic treatment: its use in 29 epileptic children]

INTRODUCTION

The ketogenic diet is a treatment option for patient with intractable or refractory epilepsy. It is a high-fat, low protein, low carbohydrate diet developed in 1920s. Recent research publications and media interest have renewed debate on the merits of ketogenic diet.

POPULATION

We report our experience with 29 children suffering from refractory epilepsy, treated with the ketogenic diet. No surgical option was available. Modalities are explained.

RESULTS

The ketogenic diet improved seizure control in 12/29 cases. It appeared effective in infants with infantile spasms. Refractory-status epilepticus responded to the ketogenic diet (3/6 cases). Migrating partial seizures in infancy were always refractory to the diet. Compliance with the diet was good. Adverse effects must be compared with the toxicity of antiepileptic drugs. One child had hypokaliemia with cardiac complication.

CONCLUSION

The ketogenic diet should be continued during one or 2 years when it is effective. It should be considered as an alternative therapy for children with refractory epilepsy.

GLUT-1 deficiency without epilepsy--an exceptional case

The GLUT-1 deficiency is a metabolic disorder caused by a defect in glucose transport across the blood-brain barrier as a result of a defect in the glucose-transport protein. Patients present with epileptic seizures, delayed development, ataxia and hypotonia, and in many cases acquired microcephaly. In most patients, treatment with a ketogenic diet proved to be successful in controlling the epilepsy. We report a 9-year-old boy with retardation and ataxia, but without epilepsy, caused by GLUT-1 deficiency, proven biochemically and by DNA analysis. Treatment with a medium-chain triglyceride ketogenic diet had a beneficial effect.

Hypoglycemic injury to the immature brain

Despite the fact that hypoglycemia is an extremely common disorder of the newborn, consensus has been difficult to reach regarding definition, diagnosis, outcome, and treatment. With improved neuroradiologic techniques, such as MRI and PET scanning becoming increasingly available, studies to determine the correlation between hypoglycemia and outcome will help to clarify issues surrounding the effects of hypoglycemia on brain pathology. Long-term epidemiologic studies correlating the severity and duration of hypoglycemia with neurologic consequences are required, and can be complemented by appropriate parallel investigations in animal models of neonatal hypoglycemia.

Intractable pediatric epilepsy: vagal nerve stimulation and the ketogenic diet

The KD has been proven an effective alternative epilepsy treatment in children refractory to standard anticonvulsants. Children to be placed on the diet must be carefully selected, monitored, and followed. The diet is to be regarded as a strict medical regimen and requires a comprehensive medical team approach in concert with intensive parental involvement. With better understanding of the scientific principles underlying brain ketosis, we should be able to optimize the KD to achieve even better results.

Imaging the metabolic footprint of Glut1 deficiency on the brain

Cerebral 18F-fluorodeoxyglucose positron emission tomography in 14 patients with microcephaly, developmental delay, seizures, and mutations of the glucose transporter Glut1 (Glut1 deficiency syndrome) showed distinct abnormalities. Within a global context of diminished cortical uptake, more severe hypometabolism was found in the mesial temporal regions and thalami, accentuating a relative signal increase in the basal ganglia. In contrast, the structure of the brain appeared preserved in patients additionally investigated by magnetic resonance imaging. This metabolic footprint was relatively constant in all patients regardless of age, seizure history, or therapies and therefore constitutes a radiological signature of the disease. The full expression of the signature in the youngest patient (aged 19 months) indicates that the state of haploinsufficiency caused by Glut1 mutation leaves a permanent footprint on the nervous system from its earlier postnatal stages of development. The potential benefit of prompt diagnosis, aided by 18F-fluorodeoxyglucose positron emission tomography, and early initiation of available therapies is underscored by our results.

Changes in glucose transport and water permeability resulting from the T310I pathogenic mutation in Glut1 are consistent with two transport channels per monomer

We studied glucose and water passage across wild type (WT) glucose transporter Glut1 and its T310I pathogenic mutant, expressing them in Xenopus laevis oocytes. We found that the T310I mutation produced a 8-fold decrease in glucose transport (zero-trans influx, 13 +/- 2% compared with WT), accompanied by a 2.8-fold increase in the osmotic water permeability (P(f) 280 +/- 40% compared with WT), and no change in the diffusional water permeability (P(d)). The dependence of glucose and water transports on the amounts of mutant cRNA injected was identical exponential buildups (k = 19.7 ng), suggesting that they depend similarly on the quaternary structure. The E(a) values for P(f) were 16 +/- 0.4 (WT) and 11 +/- 1 kcal mol(-1) (T310I). We report for the first time that 10 mm d-glucose and l-glucose inhibit P(f) by approximately 45% in the WT but not in the T310I mutant. In addition, 10 mm maltose reduces P(f) (15-20%) in both cases. However, 5 mm l-glucose increased the P(f) of T310I, consistent with a cooperative effect. These experimental observations and an analysis of our three-dimensional model strongly suggest the presence of two channels per Glut1 monomer, one of which can be blocked by the mutation T310I.

EEG features of glut-1 deficiency syndrome

PURPOSE

Glut-1 deficiency syndrome (Glut-1 DS) is caused by the deficiency of the major glucose transporter in cerebral microvessels.

METHODS

We performed pre- and postprandial EEG recordings in two unrelated children with Glut-1 DS with developmental delay and seizures predominantly in the morning before breakfast.

RESULTS

Extensive epileptiform discharges observed in the fasting state were improved markedly by food intake, as documented in EEG recordings 1 and 2 h after a meal. The ratio of cerebrospinal fluid glucose to blood glucose was decreased in both children. Glut-1 deficiency was confirmed by biochemical and molecular genetic investigations.

CONCLUSIONS

Pre- and postprandial EEG recordings offer a simple screening test for Glut-1 DS.

Autosomal dominant glut-1 deficiency syndrome and familial epilepsy

Glut-1 deficiency syndrome was first described in 1991 as a sporadic clinical condition, later shown to be the result of haploinsufficiency. We now report a family with Glut-1 deficiency syndrome affecting 5 members over 3 generations. The syndrome behaves as an autosomal dominant condition. Affected family members manifested mild to severe seizures, developmental delay, ataxia, hypoglycorrhachia, and decreased erythrocyte 3-O-methyl-D-glucose uptake. Seizure frequency and severity were aggravated by fasting, and responded to a carbohydrate load. Glut-1 immunoreactivity in erythrocyte membranes was normal. A heterozygous R126H missense mutation was identified in the 3 patients available for testing, 2 brothers (Generation 3) and their mother (Generation 2). The sister and her father were clinically and genotypically normal. In vitro mutagenesis studies in Xenopus laevis oocytes demonstrated significant decreases in the transport of 3-O-methyl-D-glucose and dehydroascorbic acid. Xenopus oocyte membranes expressed high amounts of the R126H mutant Glut-1. Kinetic analysis indicated that replacement of arginine-126 by histidine in the mutant Glut-1 resulted in a lower Vmax. These studies demonstrate the pathogenicity of the R126H missense mutation and transmission of Glut-1 deficiency syndrome as an autosomal dominant trait.

Cerebral glucose transport implies individualized glial cell function

Previous positron emission tomography (PET) measurements of cerebral glucose transport using [11C]-3-O-methylglucose (CMG) suggested an interindividual variation in the values of the rate constant of tracer outflow (k2) larger than that for the clearance rate of inflow (K1). These two parameters were examined in healthy cerebral cortex by dynamic PET in 4 men and 2 women (aged 24 to 73 years) without neurologic disease, and in 1 man (42 years) with a recent left hemispheric cerebral infarction under normoglycemia (average blood plasma d-glucose concentration, 5.44 +/- 1.94 micromol/mL) and again under hyperglycemia (average, 10.24 +/- 1.44 micromol/mL). Time-radioactivity curves were obtained from healthy cortex (grey matter) and plasma and analyzed for the values of K1 and k2 by two graphical approaches and two fitting procedures. Both K1 and k2 significantly declined with increasing plasma glucose levels. A highly significant interindividual but not intraindividual variability for k2 was found at normoglycemia and hyperglycemia. The interindividual variability of K1, although borderline significant, was less than that of k2. Accordingly variable were the distribution volumes K1/k2. These data suggest individualized glial cell function and may be relevant to pathogenesis of neuropsychiatric disease.

Glucose transporter type 1 deficiency syndrome (Glut1DS): methylxanthines potentiate GLUT1 haploinsufficiency in vitro

Methylxanthines such as caffeine and theophylline are known to inhibit glucose transport. We have studied such inhibition in the glucose transporter type 1 deficiency syndrome (Glut1DS) by erythrocyte glucose transport assays. Data from four patients with individual mutations in the GLUT1 gene are discussed: patient 1 (hemizygosity), 3 (S66F), 15 (368Ins23), and 17 (R333W). Zero-trans influx of (14)C-labeled 3-O-methyl glucose (3-OMG) into erythrocytes of patients is reduced (patient 1, 51%; 3, 45%; 15, 31%; 17, 52%) compared with maternal controls. Inhibition studies on patients 1, 3, 17, and maternal controls show an IC(50) for caffeine of approximately 1.5 mM both in controls (n = 3) and patients (n = 3) at 5 mM 3-OMG concentration. In the same two groups, kinetic studies show that 3 mM caffeine significantly decreases V(max) (p < 0.005), whereas the decrease in K(m) is significant (p < 0.01) only in the three controls and one patient (patient 3). Kinetic data from individual patients permit us to speculate that the interactions between caffeine and Glut1 are influenced by the mutation. Three mM caffeine also inhibits the transport of dehydroascorbic acid (DHA), another substrate for Glut1. The combined effects of caffeine (3 mM) and phenobarbital (10 mM) on glucose transport, as determined in patient 15 and the maternal control, show no additive or synergistic inhibition. These data indicate that caffeine and phenobarbital have similar Glut1 inhibitory properties in these two subjects. Our study suggests that Glut1DS patients may have a reduced safety margin for methylxanthines. Consumption of methylxanthine-containing products may aggravate the neurologic symptoms associated with the Glut1DS.

Experience with the ketogenic diet in infants

OBJECTIVE

To evaluate the effectiveness, tolerability, and adverse effects of the ketogenic diet in infants with refractory epilepsy.

METHODS

A retrospective review of 32 infants who had been treated with the ketogenic diet at a large metropolitan institution.

RESULTS

Most infants (71%) were able to maintain strong ketosis. The overall effectiveness of the diet in infants was similar to that reported in the literature for older children; 19.4% became seizure-free, and an additional 35.5% had >50% reduction in seizure frequency. The diet was particularly effective for patients with infantile spasms/myoclonic seizures. There were concomitant reductions in antiepileptic medications. The majority of parents reported improvements in seizure frequency and in their child's behavior and function, particularly with respect to attention/alertness, activity level, and socialization. The diet generally was well-tolerated, and 96.4% maintained appropriate growth parameters. Adverse events, all reversible and occurring in one patient each, included renal stone, gastritis, ulcerative colitis, alteration of mentation, and hyperlipidemia.

CONCLUSION

The ketogenic diet should be considered safe and effective treatment for infants with intractable seizures.

Functional consequences of the autosomal dominant G272A mutation in the human GLUT1 gene

The first autosomal dominant missense mutation (G272A) reported within the human GLUT1 gene and shared by three affected family members was investigated in respect to functional consequences. Substitution of glycine-91 by site-directed mutagenesis with either aspartate or alanine resulted in a significant decrease in transport activity of GLUT1 expressed in Xenopus oocytes. Expression of mutant transporters was confirmed by immunoblot, 2-deoxy-glucose uptake and confocal laser microscopy. The data agree with 3-O-methyl-glucose uptake into patient erythrocytes and indicate that the loss of glycine rather than a hydrophilic side chain (Gly91Asp) defines the functional consequences of this mutation.

Diet-induced ketosis increases monocarboxylate transporter (MCT1) levels in rat brain

Monocarboxylate transporter (MCT1) levels in brains of adult Long-Evans rats on a high-fat (ketogenic) diet were investigated using light and electron microscopic immunocytochemical methods. Rats given the ketogenic diet (91% fat and 9% protein) for up to 6 weeks had increased levels of the monocarboxylate transporter MCT1 (and of the glucose transporter GLUT1) in brain endothelial cells and neuropil compared to rats on a standard diet. In ketonemic rats, electron microscopic immunogold methods revealed an 8-fold greater MCT1 labeling in the brain endothelial cells at 4 weeks. Abluminal endothelial membranes were twice as heavily labeled as luminal membranes. In controls, luminal and abluminal labeling was not significantly different. The endothelial cytoplasmic compartment was sparsely labeled (<8% of total endothelial labeling) in all brains. Neuropil MCT1 staining was more intense throughout the brain in ketonemic rats, especially in neuropil of the molecular layer of the cerebellum, as revealed by avidin-biotin immunocytochemistry. This study demonstrates that adult rats retain the capacity to upregulate brain MCT1 levels. Furthermore, their brains react to a diet that increases monocarboxylate levels in the blood by enhancing their capability to take up both monocarboxylates (MCT1 upregulation) and glucose (GLUT1 upregulation). This may have important implications for delivery of fuel to the brain under stressful and pathological conditions, such as epilepsy and GLUT1 deficiency syndrome.

Vagus nerve stimulation and the ketogenic diet

Antiepileptic drugs are the primary form of treatment for patients with epilepsy. In the United States, hundreds of thousands of people do not achieve seizure control, or have significant side effects, or both. Only a minority of patients with intractable epilepsy are candidates for traditional epilepsy surgery. Vagus nerve stimulation is now the second most common treatment for epilepsy in the United States. Additionally, the ketogenic diet has established itself as a valid treatment. This article discusses the history, mechanism of action, patient selection, efficacy, initiation, complications, and advantages of vagus nerve stimulation and the ketogenic diet.

Anorectic effects of circulating cytokines: role of the vascular blood-brain barrier

Communication between the central nervous system and peripheral tissues is mediated in part by the ability of the blood-brain barrier (BBB) to transport peptides and regulatory proteins. Many cytokines with effects on appetite, including interleukins 1alpha, 1beta, and 6 and tumor necrosis factor-alpha, are transported across the BBB. Cytokines also can interact with the luminal surface of the brain endothelial cells, which constitute the BBB, to induce brain endothelial cells to release appetite-affecting substances into brain interstitial fluid. Leptin, a 16-kDa protein that binds to a cytokine receptor, is produced by fat cells and transported across the BBB by a saturable system to exert its anorectic effects. Transporter performance for appetite-related peptides and regulatory proteins can be altered by disease and under conditions associated with anorexia or obesity, a striking example being leptin transport in obesity. In mice with obesity of maturity, leptin transport is reduced by about two-thirds, showing that obesity involves a dysfunction of the BBB. That altered transport across the BBB of other substances related to feeding also might result in obesity or anorexia is a possibility that deserves investigation.

Mutational analysis of GLUT1 (SLC2A1) in Glut-1 deficiency syndrome

Fifteen children presenting with infantile seizures, acquired microcephaly, and developmental delay were found to have novel heterozygous mutations in the GLUT1 (SLC2A1). We refer to this condition as the Glut-1 Deficiency Syndrome (Glut-1 DS). The encoded protein (Glut-1), which has 12 transmembrane domains, is the major glucose transporter in the mammalian blood-brain barrier. The presence of GLUT1 mutations correlates with reduced cerebrospinal fluid glucose concentrations (hypoglycorrhachia) and reduced erythrocyte glucose transporter activities in the patients. We used Florescence in situ hybridization, PCR, single-stranded DNA conformational polymorphism, and DNA sequencing to identify novel GLUT1 mutations in 15 patients. These abnormalities include one large-scale deletion (hemizygosity), five missense mutations (S66F, R126L, E146K, K256V, R333W), three deletions (266delC, 267A>T; 904delA; 1086delG), three insertions (368-369 insTCCTGCCCACCACGCTCACCACG, 741-742insC, 888-889insG), three splice site mutations (197+1G>A; 1151+1G>T; 857T>G, 858G>A, 858+1del10), and one nonsense mutation (R330X). In addition, six silent mutations were identified in exons 2, 4, 5, 9, and 10. The K256V missense mutation involved the maternally derived allele in the patient and one allele in his mother. A spontaneous R126L missense mutation also was present in the paternally derived allele of the patient. The apparent pathogenicity of these mutations is discussed in relation to the functional domains of Glut-1.

Glucose transporters: structure, function and consequences of deficiency

There are two mechanisms for glucose transport across cell membranes. In the intestine and renal proximal tubule, glucose is transported against a concentration gradient by a secondary active transport mechanism in which glucose is cotransported with sodium ions. In all other cells, glucose transport is mediated by one or more of the members of the closely related GLUT family of glucose transporters. The pattern of expression of the GLUT transporters in different tissues is related to the different roles of glucose metabolism in different tissues. Primary defects in glucose transport all appear to be extremely rare and not all possible deficiencies have been identified. Deficiency of the secondary active sodium/glucose transporters result in glucose/galactose malabsorption or congenital renal glycosuria. GLUT1 deficiency produces a seizure disorder with low glucose concentration in cerebrospinal fluid and GLUT2 deficiency is the basis of the Fanconi-Bickel syndrome, which resembles type I glycogen storage disease.

The effects of morphine on blood-brain barrier disruption caused by intracarotid injection of hyperosmolar mannitol in rats

This study was performed to evaluate whether morphine could alter the degree of disruption of the blood-brain barrier (BBB) caused by hyperosmolar mannitol. Under isoflurane anesthesia, rats in a control group were infused with 25% mannitol into the internal carotid artery before measuring the transfer coefficient (Ki) of (14)C-alpha-aminoisobutyric acid. Infusion of morphine 3 mg/kg in the small-dose morphine group and 10 mg/kg in the large-dose morphine group was completed, 10 min before administering mannitol. There were no statistical differences in systemic blood pressures between these three groups of animals. In the control group, the Ki of the ipsilateral cortex where mannitol was injected, increased to 4.6 times that of the contralateral cortex (19.5 +/- 8.5 vs 4.2 +/- 1.2 microL. g(-1). min(-1), P < 0.002). The Ki of the ipsilateral cortex of the small-dose morphine group was 13.5 +/- 7.6 microL. g(-1). min(-1). The Ki of the ipsilateral cortex of the large-dose morphine group was 9.2 +/- 4.5 microL. g(-1). min(-1) and was smaller than that of control animals (P < 0.05). There was no significant difference in the Ki of the contralateral cortex among the three groups. In conclusion, morphine attenuated BBB disruption induced by hyperosmolar solution without significant effects on systemic blood pressure.

IMPLICATIONS

Our study suggests that morphine may be effective in reducing the blood-brain barrier disruption by hyperosmolar mannitol without significant effects on systemic blood pressure.

Metabolic and endocrine aspects of the ketogenic diet

The ketogenic diet (KD) is designed to simulate the biochemical effects of fasting by maintaining a state of ketosis. The complex interplay of endocrine and metabolic factors requires that a continuous ingestion of a diet high in lipid calories is necessary to achieve such a state and yet maintain body weight. The resulting condition provides for much of the cerebral energy requirements in the form of ketone bodies. We review energy metabolism with special emphasis on fatty acid oxidation to provide the readers with a foundation that facilitates identification of patients who will especially benefit from this diet, as well as to assist clinicians in screening candidates who may experience a catastrophic outcome if fasted and placed on this diet. The review includes a discussion of the role of carnitine in mitochondrial fatty acid metabolism, and the criteria for carnitine supplementation. Only limited information is available regarding the interaction of the diet with the commonly used antiepileptic drugs.

GLUT1-deficiency: barbiturates potentiate haploinsufficiency in vitro

Barbiturates are known to inhibit glucose transport mediated by the facilitative sugar transporter GLUTI. We have studied such inhibition in children with GLUT1-deficiency. Zero-trans influx of 14C-labeled 3-O-methyl glucose (3OMG) into erythrocytes of patients (n = 3) was 35% of controls (n = 6). Preincubation with 10 mM phenobarbital or pentobarbital reduced patients' 30MG influx to 17%. In patients and controls, preincubation with barbiturates significantly decreased Vmax in a dose-dependent manner (for pentobarbital, IC50 = 0.84 mM, patient 2). The apparent Km in individuals remained largely unchanged. Three-OMG influx without preincubation resulted in a stronger inhibition at lower barbiturate concentrations. The patients' data are discussed in the light of individual missense mutations (patient 1: R126L and K256V; patient 2: T310I; patient 3: S66F) in the GLUTI gene. In conclusion, in controls and patients with GLUT1-deficiency barbiturates interact with GLUT1, lowering its intrinsic activity. The use of barbiturates in this condition for anesthesia or as anticonvulsants could therefore potentially aggravate the existing glucose transport defect and may put these patients at increased risk.

GLUT1 glucose transporter gene transcription is repressed by Sp3. Evidence for a regulatory role of Sp3 during myogenesis

GLUT1 glucose transporters are highly expressed in proliferating and transformed cells as well as in tissues during fetal life. However, the mechanisms that regulate GLUT1 gene expression remain largely unknown. Here, we demonstrate that Sp3 proteins bind to the GLUT1 proximal promoter gene and inhibit transcriptional activity in muscle and non-muscle cells. Two different Sp3 translational products (110 and 74 kDa) derived from differential translational initiation were detected in nuclear extracts from myoblast cells, and both Sp3 protein species inhibited GLUT1 gene transcriptional activity. The inhibitory effect of Sp3 was dominant over the stimulatory effect of Sp1 on transcriptional activity of GLUT1 gene. Furthermore, abolition of Sp3 binding to the proximal promoter of GLUT1 gene completely blocked the response to Sp3. We provide evidence that the expression of Sp3 protein is subject to regulation in muscle cells and that this is likely to control GLUT1. Thus, Sp3 protein was up-regulated in the absence of changes in Sp1 early after the induction of IGF-II-dependent myogenesis. Furthermore, forced over-expression of MyoD caused an enhancement in the cellular Sp3/Sp1 ratio which was concomitant to a reduced GLUT1 expression. Later during myogenesis, Sp3 expression was substantial whereas Sp1 was markedly down-regulated. In summary, we provide direct evidence that the transcription factor Sp3 represses gene expression in non-muscle and muscle cells and this is likely to operate in fetal heart by binding to the GLUT1 gene promoter. This is the first description of a repressor of GLUT1 gene transcription. Furthermore, we propose that variations in the ratio of Sp3 versus Sp1 regulate GLUT1 promoter activity and this is crucial in the down-regulation of GLUT1 associated to myogenesis.

Value of lumbar puncture in the diagnosis of genetic metabolic encephalopathies

Diagnostic testing for genetically determined metabolic disease has for many years relied heavily on the use of generalized screening tests that analyze groups of related compounds in easily accessible peripheral fluids such as plasma and urine. Organic acid profiles in urine and amino acid analysis in plasma are two of the most commonly requested tests; these, together with other protocols that examine peripheral fluids, have been and continue to be invaluable tools. There is, however, an emerging realization that many metabolic encephalopathies do not arise secondary to peripheral metabolic changes but rather have their origins within the central nervous system. In these cases, testing of peripheral fluids might be uninformative. This review is designed to examine the role of cerebrospinal fluid analyses in the investigation of infants and children with undefined encephalopathies. The aims are to review the conditions in which measurement of metabolites in cerebrospinal fluid is critical if a diagnosis is to be made, and to emphasize that considerable forethought is often required to ensure correct collection and handling of cerebrospinal fluid. Thus, fidelity of the diagnostic analytic procedures is maintained. This review will help the pediatric neurologist establish practical diagnostic guidelines that in turn will help in the recognition of recently described conditions. Those conditions can, in general, be identified only after specialized cerebrospinal fluid testing.

Identification of a novel R642C mutation in Na/Cl cotransporter with Gitelman's syndrome

Gitelman's syndrome, a variant of Bartter's syndrome, is an inherited disorder characterized by hypokalemic metabolic alkalosis, hypomagnesemia, and hypocalciuria, and these abnormalities have recently been linked to the thiazide-sensitive Na/Cl cotransporter (TSC) gene. We evaluated three unrelated patients affected with this syndrome whose diagnosis was made based on clinical and biochemical features. The data of clearance studies in these patients were compatible with Gitelman's syndrome. We then investigated possible mutations of the TSC gene. In one patient whose parents are consanguineous, we identified a novel missense mutation in the TSC gene, which causes alteration of arginine to cysteine at codon 642 (R642C mutation) located in the cytoplasmic tail of the product. This mutation results in the loss of an MspI site in exon 15 of the TSC gene. MspI digestion analysis of genomic DNA fragments from the family was consistent with the autosomal recessive inheritance of the disorder, and presence of this mutation correlated with the clinical manifestations. Such mutation was not detected in 47 normal healthy subjects. In the second patient, we found another missense mutation in one allele of the TSC gene, which results in alteration of arginine to glutamine at codon 955. In the third patient, no mutation causing amino acid substitution was found in the TSC gene. These results indicate that the R642C mutation in TSC is critically important for impairment of this cotransporter function and also suggest the necessity of further investigations in the genetic background of Gitelman's syndrome.

Classification of hypoglycemic disorders

The classification of symptoms of hypoglycemia, namely, reactive versus fasting, based on segregation by timing in relation to meals is no longer useful from a clinical point of view. Every patient with neuroglycopenic symptoms, regardless of relation to food ingestion, requires comprehensive evaluation. Identification of the possible cause of hypoglycemia and selection of diagnostic modalities are facilitated by considering whether the patient appears to be healthy (with or without compensated coexistent disease) or ill (with a disease known to have a proclivity to develop hypoglycemia, or is hospitalized). Medications may mediate hypoglycemia in anybody, whether they appear healthy or ill.

Hypoglycemia in infants and children

Hypoglycemia is more common in the pediatric patient than in adults. This article discusses the many diagnoses that can be associated with hypoglycemia in infancy and childhood. A guide to help practitioners evaluate such patients and suggested treatments for many of these disorders are provided. As genetic diagnosis continues to develop, it is anticipated that the list of specific disorders associated with hypoglycemia in infancy and childhood will increase.

Folinic acid-responsive neonatal seizures

We report three cases of folinic acid-responsive intractable neonatal seizures. All patients were born at term following normal gestation and delivery. In the first infant, seizures began on the 5th day of life and were unresponsive to phenobarbital, pyridoxine, and valproate, but stopped within 24 hours of initiation of folinic acid treatment at the age of 6 months. Her sibling had died at age 6 months with intractable seizures. In the second infant, seizures began in the 2nd hour of life. These were initially controlled with phenobarbital; however, at 3 months of age she developed status epilepticus refractory to anticonvulsants, steroids, and pyridoxine and she required repeated induction of pentobarbital coma. Seizures stopped within 24 hours of starting folinic acid. Seizures and encephalopathy were noted in the third infant on the 2nd day of life. These were controlled with phenobarbital, but at 8 weeks of age seizures recurred and were difficult to control despite the addition of phenytoin. Immediately after folinic acid was initiated the seizures stopped. Breakthrough seizures in all patients have responded to increases in folinic acid; two of the three remain on standard anticonvulsants. All patients have global developmental delay. Cranial magnetic resonance imaging in the second patient shows diffuse atrophy, and in the third patient shows increased signal on T2 images in the white matter of the frontal and parietal lobes. Analysis of cerebrospinal fluid from these patients using high-performance liquid chromatography with electrochemical detection has consistently revealed an as-yet unidentified compound, which can be used as a marker for this condition. We suggest that cerebrospinal fluid be analyzed for the presence of this compound and a trial of folinic acid be considered in neonates with unexplained early onset intractable seizures.

Effect of primary congenital hypothyroidism upon expression of genes mediating murine brain glucose uptake

Using hyt/hyt mice that exhibit naturally occurring primary hypothyroidism (n = 72) and Balb/c controls (n = 66), we examined the mRNA, protein, and activity of brain glucose transporters (Glut 1 and Glut 3) and hexokinase I enzyme at various postnatal ages (d 1, 7, 14, 21, 35, and 60). The hyt/hyt mice showed an age-dependent decline in body weight (p < 0.04) and an increase in serum TSH levels (p < 0.001) at all ages. An age-dependent translational/posttranslational 40% decline in Glut 1 (p = 0.02) with no change in Glut 3 levels was observed. These changes were predominant during the immediate neonatal period (d 1). A posttranslational 70% increase in hexokinase enzyme activity was noted at d 1 alone (p < 0.05) with no concomitant change in brain 2-deoxy-glucose uptake. This was despite a decline in the hyt/hyt glucose production rate. We conclude that primary hypothyroidism causes a decline in brain Glut 1 associated with no change in Glut 3 levels and a compensatory increase in hexokinase enzyme activity. These changes are pronounced only during the immediate neonatal period and disappear in the postweaned stages of development. These hypothyroid-induced compensatory changes in gene products mediating glucose transport and phosphorylation ensure an adequate supply of glucose to the developing brain during transition from fetal to neonatal life.

Defective glucose transport across brain tissue barriers: a newly recognized neurological syndrome

Impaired glucose transport across brain tissue barriers causes infantile seizures, developmental delay and acquired microcephaly. Since the first report in 1991 (De Vivo et al, NEJM, 1991) 17 patients have been identified with the glucose transporter protein syndrome (GTPS). The diagnostic feature of the syndrome is an unexplained hypoglycorrhachia in the clinical setting of an infantile epileptic encephalopathy. We review our clinical experience by highlighting one illustrative case: a 6-year old girl who presented at age 2 months with infantile seizures and hypoglycorrhachia. The CSF/blood glucose ratio was 0.33. DNA sequencing identified a missense mutation in exon 7 (C1108T). Erythrocyte GLUT1 immunoreactivity was normal. The time course of 3-O-methyl-glucose (3OMG) uptake by erythrocytes of the patient was 46% that of mother and father. The apparent Km was similar in all cases (2-4 mmol/L), but the apparent Vmax in the patient was only 28% that of the parents (500 versus 1,766 fmol/s/10(6)RBC; p < 0.004). In addition, a 3-month trial of oral thioctic acid also benefited the patient and increased the Vmax to 935 fmol/s/10(6) RBC (p < 3 x 10(-7)). Uptake of dehydroascorbic acid by erythrocytes of the patient was impaired to the same degree as that of 3OMG (Vmax was 38% of that of the mother's), which supports previous observations of GLUT1 being multifunctional. These studies confirm the molecular basis of the GTPS and the multifunctional role of GLUT1. The need for more effective treatment is compelling.

Protein interactions with the glucose transporter binding protein GLUT1CBP that provide a link between GLUT1 and the cytoskeleton

Subcellular targeting and the activity of facilitative glucose transporters are likely to be regulated by interactions with cellular proteins. This report describes the identification and characterization of a protein, GLUT1 C-terminal binding protein (GLUT1CBP), that binds via a PDZ domain to the C terminus of GLUT1. The interaction requires the C-terminal four amino acids of GLUT1 and is isoform specific because GLUT1CBP does not interact with the C terminus of GLUT3 or GLUT4. Most rat tissues examined contain both GLUT1CBP and GLUT1 mRNA, whereas only small intestine lacked detectable GLUT1CBP protein. GLUT1CBP is also expressed in primary cultures of neurons and astrocytes, as well as in Chinese hamster ovary, 3T3-L1, Madin-Darby canine kidney, Caco-2, and pheochromocytoma-12 cell lines. GLUT1CBP is able to bind to native GLUT1 extracted from cell membranes, self-associate, or interact with the cytoskeletal proteins myosin VI, alpha-actinin-1, and the kinesin superfamily protein KIF-1B. The presence of a PDZ domain places GLUT1CBP among a growing family of structural and regulatory proteins, many of which are localized to areas of membrane specialization. This and its ability to interact with GLUT1 and cytoskeletal proteins implicate GLUT1CBP in cellular mechanisms for targeting GLUT1 to specific subcellular sites either by tethering the transporter to cytoskeletal motor proteins or by anchoring the transporter to the actin cytoskeleton.

Transportadores de glicose

A glicose, principal fonte de energia celular, é transportada na maioria das células por difusão facilitada, através de proteínas transportadoras presentes na membrana plasmática. Está caracterizada a existencia de urna familia de transportadores (GLUT1-GLUT7), com características funcionáis e distribuição tecidual distintas. Por outro lado, em epitelios intestinal e tubular renal, o transporte é contra gradiente e acoplado ao Na+ na membrana apical das células através de cotransportadores (SGLT1-SGLT2), com posterior difusão para o intersticio através de GLUTs presentes na membrana basolateral. As alterações fisiopatológicas do transporte de glicose passaram a ser investigadas através da análise dos transportadores, objetivando futuras abordagens preventivas ou terapêuticas. Uma mutação em um aminoácido do SGLT1 já foi descrita na malabsorção de glicose/galactose. Na glicosúria renal familiar, a participação do SGLT2 e do SGLT1 parece ser fundamental, seja por perda da capacidade de transporte, seja por diminuição na afinidade do transportador. A síndrome de De Vivo, descrita em recém-nascidos com quadro convulsivo, e hipoglicorraquia na vigência de normoglicemia, foi atribuída a uma redução no conteúdo de GLUT1, nas células endoteliais da barreira hematoencefálica. Extensas investigações têm sido conduzidas para avaliar o papel do GLUT4 em alterações de sensibilidade insulínica, tais como diabete melito tipo 2 (DM2). Os estudos revelam que no DM2, o GLUT4 reduz-se dramaticamente o que desempenha um importante papel na resistência insulínica. Na obesidade, o conteúdo de GLUT4 não está diminuído enquanto a sensibilidade à insulina estiver preservada. É plausível propor-se que a modulação do GLUT4 seja acionada por uma conjunção de fatores que expressam a sensibilidade celular à insulina. Além disso, o DM altera o conteúdo de GLUT 1 e GLUT2 no túbulo renal, mas o papel dessa modulação no processo de reabsorção da glicose ainda é desconhecido.

Deficient transport of dehydroascorbic acid in the glucose transporter protein syndrome

The glucose transporter protein syndrome (GTPS) is caused by defective transport of glucose across the blood-brain barrier via the glucose transporter GLUT1, resulting in hypoglycorrhachia, infantile seizures, and developmental delay. Recent reports indicated that GLUT1 is a multifunctional transporter. We investigated the transport of vitamin C in its oxidized form (dehydroascorbic acid) via GLUT1 into erythrocytes of 2 patients with GTPS. In both patients, uptake of oxidized vitamin C was 61% of the mothers' values. Our findings are consistent with recent observations that vitamin C is transported in its oxidized form via GLUT1. We speculate that impaired transport of this substrate and perhaps other substrates in GTPS might contribute to the pathophysiology of this condition.

The effects of pentobarbital on blood-brain barrier disruption caused by intracarotid injection of hyperosmolar mannitol in rats

This study was performed to evaluate both the effects of pentobarbital on disruption of the blood-brain barrier (BBB) by hyperosmolar mannitol and the relationship between its effect on blood pressure and the integrity of the BBB. Under isoflurane anesthesia, rats in the control group were infused with 25% mannitol into the internal carotid artery before measuring the transfer coefficient (Ki) of 14C alpha-aminoisobutyric acid. Ten minutes before the administration of mannitol, rats received an infusion of pentobarbital: 20 mg/kg in the small-dose group and 50 mg/kg in the large-dose group. In another group of animals (hydralazine group), hydralazine was administered to maintain the mean arterial blood pressure (MAP) at 65 mm Hg during the experimental period. The MAP of the control group (113 +/- 14 mm Hg) was significantly higher (P < 0.002) than that of the small-dose pentobarbital group (78 +/- 13 mm Hg) or the large-dose pentobarbital group (68 +/- 14 mm Hg). In the control group, the Ki of the cortex ipsilateral to the mannitol injection was increased to 4.5 times that of the contralateral cortex (14.5 +/- 7.7 vs 3.2 +/- 0.6 microL x g(-1) x min(-1); P < 0.002). The Ki of the ipsilateral cortex of the small-dose pentobarbital group was 9.7 +/- 5.6 microL x g(-1) x min(-1). The Ki of the ipsilateral cortex of the large-dose pentobarbital group was 5.5 +/- 2.9 microL x g(-1) x min(-1), and lower (-9.0 microL x g(-1) x min(-1)) than that of the control animals (P < 0.05). There was no significant difference in the Ki of the contralateral cortex among any of the three groups of animals. At the same MAP, the Ki of the ipsilateral cortex of the large-dose pentobarbital group was lower (-4.3 microL x g(-1) x min(-1)) than that of the hydralazine group (9.8 +/- 4.6 microL x g(-1) x min(-1)) (P < 0.05). Pentobarbital attenuated the BBB disruption induced by hyperosmolar mannitol. This may be attributed, at least in part, to the blood pressure effect of pentobarbital.

IMPLICATIONS

When the blood-brain barrier (BBB) was disrupted by a hyperosmolar solution, pentobarbital attenuated the degree of leakage of the BBB. Systemic hypotension caused by pentobarbital played a significant role in decreasing the leakage. Our study suggests that when the BBB is disrupted, pentobarbital may be effective in protecting the BBB. Furthermore, systemic blood pressure plays an important role in determining the degree of disruption.

GLUT-1 deficiency syndrome caused by haploinsufficiency of the blood-brain barrier hexose carrier

The high metabolic requirements of the mammalian central nervous system require specialized structures for the facilitated transport of nutrients across the blood-brain barrier. Stereospecific high-capacity carriers, including those that recognize glucose, are key components of this barrier, which also protects the brain against noxious substances. Facilitated glucose transport in vertebrates is catalyzed by a family of carriers consisting of at least five functional isoforms with distinct tissue distributions, subcellular localizations and transport kinetics. Several of these transporters are expressed in the mammalian brain. GLUT-1, whose sequence was originally deduced from cDNAs cloned from human hepatoma and rat brain, is present at high levels in primate erythrocytes and brain endothelial cells. GLUT1 has been cloned and positionally mapped to the short arm of chromosome 1 (1p35-p31.3; refs 6-8). Despite substantial metabolic requirements of the central nervous system, no genetic disease caused by dysfunctional blood-brain barrier transport has been identified. Several years ago, we described two patients with infantile seizures, delayed development and acquired microcephaly who have normal circulating blood glucose, low-to-normal cerebrospinal fluid (CSF) lactate, but persistent hypoglycorrachia (low CSF glucose) and diminished transport of hexose into isolated red blood cells (RBC). These symptoms suggested the existence of a defect in glucose transport across the blood brain barrier. We now report two distinct classes of mutations as the molecular basis for the functional defect of glucose transport: hemizygosity of GLUT1 and nonsense mutations resulting in truncation of the GLUT-1 protein.