Functional characterization and RNAi-mediated suppression reveals roles for hexose transporters in sugar accumulation by tomato fruit

Hexoses accumulate to high concentrations (∼ 200 mM) in storage parenchyma cells of tomato fruit. Hexoses are sourced from the fruit apoplasm as hydrolysis products of phloem-imported sucrose. Three hexose transporters (LeHT1, LeHT2, LeHT3), expressed in fruit storage parenchyma cells, may contribute to hexose uptake by these cells. An analysis of their full-length sequences demonstrated that all three transporters belong to the STP sub-family of monosaccharide transporters that localize to plasma membranes. Heterologous expression of LeHT1 (and previously LeHT2, Gear et al., 2000), but not LeHT3, rescued a hexose transport-impaired yeast mutant when raised on glucose or fructose as the sole carbon source. Biochemically, LeHT1, similarly to LeHT2, exhibited transport properties consistent with a high-affinity glucose/H(+) symporter. Significantly, LeHT1 and LeHT2 also functioned as low-affinity fructose/H(+) symporters with apparent K(m) values commensurate with those of fruit tissues. A substantial reduction (80-90%) in fruit expression levels of all LeHT genes by RNAi-mediated knockdown caused a 55% decrease in fruit hexose accumulation. In contrast, photoassimilate production by source leaves and phloem transport capacity to fruit were unaffected by transporter knockdown. Collectively, these findings demonstrate that LeHTs play key roles in driving accumulation of hexoses into storage parenchyma cells during tomato fruit development.

Remodeling of protein and mRNA expression in Leishmania mexicana induced by deletion of glucose transporter genes

Glucose is a major nutrient in the insect vector stage of Leishmania parasites. Glucose transporter null mutants of Leishmania mexicana exhibit profound phenotypic changes in both insect stage promastigotes and mammalian host stage amastigotes that reside within phagolysosomes of host macrophages. Some of these phenotypic changes could be either mediated or attenuated by changes in gene expression that accompany deletion of the glucose transporter genes. To search for changes in protein expression, the profile of proteins detected on two-dimensional gels was compared for wild type and glucose transporter null mutant promastigotes. A total of 50 spots whose intensities changed significantly and consistently in multiple experiments were detected, suggesting that a cohort of proteins is altered in expression levels in the null mutant parasites. Following identification of proteins by mass spectrometry, 3 such regulated proteins were chosen for more detailed analysis: mitochondrial aldehyde dehydrogenase, ribokinase, and hexokinase. Immunoblots employing antisera against these enzymes confirmed that their levels were upregulated, both in glucose transporter null mutants and in wild type parasites starved for glucose. Quantitative reverse transcriptase PCR (qRT-PCR) revealed that the levels of mRNAs encoding these enzymes were also enhanced. Global expression profiling using microarrays revealed a limited number of additional changes, although the sensitivity of the microarrays to detect modest changes in amplitude was less than that of two-dimensional gels. Hence, there is likely to be a network of proteins whose expression levels are altered by genetic ablation of glucose transporters, and much of this regulation may be reflected by changes in the levels of the cognate mRNAs. Some of these changes in protein expression may reflect an adaptive response of the parasites to limitation of glucose.

Comparisons of mutants lacking the Golgi UDP-galactose or GDP-mannose transporters establish that phosphoglycans are important for promastigote but not amastigote virulence in Leishmania major

Abundant surface Leishmania phosphoglycans (PGs) containing [Gal(beta1,4)Man(alpha1-PO(4))]-derived repeating units are important at several points in the infectious cycle of this protozoan parasite. PG synthesis requires transport of activated nucleotide-sugar precursors from the cytoplasm to the Golgi apparatus. Correspondingly, null mutants of the L. major GDP-mannose transporter LPG2 lack PGs and are severely compromised in macrophage survival and induction of acute pathology in susceptible mice, yet they are able to persist indefinitely and induce protective immunity. However, lpg2(-) L. mexicana amastigotes similarly lacking PGs but otherwise normal in known glycoconjugates remain able to induce acute pathology. To explore this further, we tested the infectivity of a new PG-null L. major mutant, which is inactivated in the two UDP-galactose transporter genes LPG5A and LPG5B. Surprisingly this mutant did not recapitulate the phenotype of L. major lpg2(-), instead resembling the L. major lipophosphoglycan-deficient lpg1(-) mutant. Metacyclic lpg5A(-)/lpg5B(-) promastigotes showed strong defects in the initial steps of macrophage infection and survival. However, after a modest delay, the lpg5A(-)/lpg5B(-) mutant induced lesion pathology in infected mice, which thereafter progressed normally. Amastigotes recovered from these lesions were fully infective in mice and in macrophages despite the continued absence of PGs. This suggests that another LPG2-dependent metabolite is responsible for the L. major amastigote virulence defect, although further studies ruled out cytoplasmic mannans. These data thus resolve the distinct phenotypes seen among lpg2(-) Leishmania species by emphasizing the role of glycoconjugates other than PGs in amastigote virulence, while providing further support for the role of PGs in metacyclic promastigote virulence.

The location and characterisation of the O-linked glycans of the human insulin receptor

O-linked glycosylation is a post-translational and post-folding event involving exposed S/T residues at beta-turns or in regions with extended conformation. O-linked sites are difficult to predict from sequence analyses compared to N-linked sites. Here we compare the results of chemical analyses of isolated glycopeptides with the prediction using the neural network prediction method NetOGlyc3.1, a procedure that has been reported to correctly predict 76% of O-glycosylated residues in proteins. Using the heavily glycosylated human insulin receptor as the test protein six sites of mucin-type O-glycosylation were found at residues T744, T749, S757, S758, T759, and T763 compared to the three sites (T759 and T763- correctly, T756- incorrectly) predicted by the neural network method. These six sites occur in a 20 residue segment that begins nine residues downstream from the start of the insulin receptor beta-chain. This region which also includes N-linked glycosylation sites at N742 and N755, is predicted to lack secondary structure and is followed by residues 765-770, the known linear epitope for the monoclonal antibody 18-44.

Leukocyte adhesion deficiency II. Advances and open questions

Leukocyte adhesion deficiency II (LAD II) belongs to a group of human congenital diseases in which the interactions of leukocytes with the vascular endothelium are strongly impaired. LAD II is based on a defect in the synthesis of fucosylated glycostructures. This leads to an immunodeficiency owing to the absence of functional selectin ligands and to strong psychomotor defects, as a result of as-yet unknown reasons. In this review we focused on the current controversies, and open questions that have arisen from recent studies on the genetic defect, therapy and the basis of psychomotor defects in LAD II.

Silencing of the human microsomal glucose-6-phosphate translocase induces glioma cell death: Potential new anticancer target for curcumin

G6P translocase (G6PT) is thought to play a crucial role in transducing intracellular signaling events in brain tumor-derived cancer cells. In this report, we investigated the contribution of G6PT to the control of U-87 brain tumor-derived glioma cell survival using small interfering RNA (siRNA)-mediated suppression of G6PT. Three siRNA constructs were generated and found to suppress up to 91% G6PT gene expression. Flow cytometry analysis of propidium iodide/annexin-V-stained cells indicated that silencing the G6PT gene induced necrosis and late apoptosis. The anticancer agent curcumin, also inhibited G6PT gene expression by more than 90% and triggered U-87 glioma cells death. Overexpression of recombinant G6PT rescued the cells from curcumin-induced cell death. Targeting G6PT expression may provide a new mechanistic rationale for the action of chemopreventive drugs and lead to the development of new anti-cancer strategies.

Adipose-specific overexpression of GLUT4 reverses insulin resistance and diabetes in mice lacking GLUT4 selectively in muscle

Adipose tissue plays an important role in glucose homeostasis and affects insulin sensitivity in other tissues. In obesity and type 2 diabetes, glucose transporter 4 (GLUT4) is downregulated in adipose tissue, and glucose transport is also impaired in muscle. To determine whether overexpression of GLUT4 selectively in adipose tissue could prevent insulin resistance when glucose transport is impaired in muscle, we bred muscle GLUT4 knockout (MG4KO) mice to mice overexpressing GLUT4 in adipose tissue (AG4Tg). Overexpression of GLUT4 in fat not only normalized the fasting hyperglycemia and glucose intolerance in MG4KO mice, but it reduced these parameters to below normal levels. Glucose infusion rate during a euglycemic clamp study was reduced 46% in MG4KO compared with controls and was restored to control levels in AG4Tg-MG4KO. Similarly, insulin action to suppress hepatic glucose production was impaired in MG4KO mice and was restored to control levels in AG4Tg-MG4KO. 2-deoxyglucose uptake during the clamp was increased approximately twofold in white adipose tissue but remained reduced in skeletal muscle of AG4Tg-MG4KO mice. AG4Tg and AG4Tg-MG4KO mice have a slight increase in fat mass, a twofold elevation in serum free fatty acids, an approximately 50% increase in serum leptin, and a 50% decrease in serum adiponectin. In MG4KO mice, serum resistin is increased 34% and GLUT4 overexpression in fat reverses this. Overexpression of GLUT4 in fat also reverses the enhanced clearance of an oral lipid load in MG4KO mice. Thus overexpression of GLUT4 in fat reverses whole body insulin resistance in MG4KO mice without restoring glucose transport in muscle. This effect occurs even though AG4Tg-MG4KO mice have increased fat mass and low adiponectin and is associated with normalization of elevated resistin levels.

Serum retinol binding protein 4 contributes to insulin resistance in obesity and type 2 diabetes

In obesity and type 2 diabetes, expression of the GLUT4 glucose transporter is decreased selectively in adipocytes. Adipose-specific Glut4 (also known as Slc2a4) knockout (adipose-Glut4(-/-)) mice show insulin resistance secondarily in muscle and liver. Here we show, using DNA arrays, that expression of retinol binding protein-4 (RBP4) is elevated in adipose tissue of adipose-Glut4(-/-) mice. We show that serum RBP4 levels are elevated in insulin-resistant mice and humans with obesity and type 2 diabetes. RBP4 levels are normalized by rosiglitazone, an insulin-sensitizing drug. Transgenic overexpression of human RBP4 or injection of recombinant RBP4 in normal mice causes insulin resistance. Conversely, genetic deletion of Rbp4 enhances insulin sensitivity. Fenretinide, a synthetic retinoid that increases urinary excretion of RBP4, normalizes serum RBP4 levels and improves insulin resistance and glucose intolerance in mice with obesity induced by a high-fat diet. Increasing serum RBP4 induces hepatic expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) and impairs insulin signalling in muscle. Thus, RBP4 is an adipocyte-derived 'signal' that may contribute to the pathogenesis of type 2 diabetes. Lowering RBP4 could be a new strategy for treating type 2 diabetes.

Clinical presentation, EEG studies, and novel mutations in two cases of GLUT1 deficiency syndrome in Japan

We report the first two Japanese children diagnosed with glucose transporter type 1 (GLUT1) deficiency syndrome. Both boys had been treated under the initial diagnosis of epilepsy and were reinvestigated for previously unexplainable hypoglycorrhachia. Myoclonic seizures developed at 4 months of age in Patient #1 (7 years old), and at 2 months of age in Patient #2 (11 years old), followed by cerebellar ataxia, spastic diplegia, and mental retardation. Both patients had hypoglycorrhachia, and the symptoms were more severe in the latter. CSF and serum glucose levels determined simultaneously showed a CSF/serum glucose ratio of below 0.4 in both patients. In mildly affected Patient #1, the postprandial waking EEG showed improvement in the background activity, as compared to that recorded after overnight fasting, while no significant changes were observed in severely affected Patient #2. In both patients, the functional GLUT1 defect was confirmed by 3-O-methyl-D-glucose uptake into erythrocytes. Molecular analyses identified heterozygous novel mutations in both patients, within exons 6 and 2 of the GLUT1 gene, respectively. The ketogenic diet was refused in Patient #1, but started in Patient #2 with significant clinical benefit. Fasting CSF analysis and pre-/postprandial EEG changes in children with epileptic seizures and unexplainable neurological deterioration help in diagnosing this potentially treatable disorder.

Glut-1 deficiency syndrome: clinical, genetic, and therapeutic aspects

Impaired glucose transport across the blood-brain barrier results in Glut-1 deficiency syndrome (Glut-1 DS, OMIM 606777), characterized by infantile seizures, developmental delay, acquired microcephaly, spasticity, ataxia, and hypoglycorrhachia. We studied 16 new Glut-1 deficiency syndrome patients focusing on clinical and laboratory features, molecular genetics, genotype-phenotype correlation, and treatment. These patients were classified phenotypically into three groups. The mean cerebrospinal fluid glucose concentration was 33.1 +/- 4.9mg/dl equal to 37% of the simultaneous blood glucose concentration. The mean cerebrospinal fluid lactate concentration was 1.0 +/- 0.3mM, which was less than the normal mean value of 1.63mM. The mean V(max) for the 3-O-methyl-D-glucose uptake into erythrocytes was 996 fmol/10(6) red blood cells per second, significantly less (54 +/- 11%; t test, p < 0.05) than the mean control value of 1,847. The mean Km value for the patient group (1.4 +/- 0.5mM) was similar to the control group (1.7 +/- 0.5mM; t test, p > 0.05). We identified 16 rearrangements, including seven missense, one nonsense, one insertion, and seven deletion mutations. Fourteen were novel mutations. There were no obvious correlations between phenotype, genotype, or biochemical measures. The ketogenic diet produced good seizure control.

The effects of phenytoin and its metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin on cellular glucose transport

Glucose is the principal fuel for brain metabolism and its movement across the blood-brain barrier depends on Glut1. Impaired glucose transport to the brain may have deleterious consequences. For example, Glut1 deficiency syndrome (Glut1DS) is the result of heterozygous loss of function Glut1 mutation leading to energy failure of the brain and subsequently, epileptic encephalopathy. To preserve the integrity of the energy supply to the brain in patients with compromised glucose transport function, consumption of compounds with glucose transport inhibiting properties should be avoided. Phenytoin is a widely used anticonvulsant that affects carbohydrate metabolism. In this study, the hypothesis that phenytoin and its metabolite 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH) affect cellular glucose transport was tested. With a focus on Glut1, the effects of phenytoin and HPPH on cellular glucose transport were studied. Glucose uptake assay measuring the zero-trans influx of radioactive-labeled glucose analogues showed that phenytoin and HPPH did not exert immediate effects on erythrocyte Glut1 activity or glucose transport in Hs68 control fibroblasts, Glut1DS primary fibroblasts isolated from two patients, or in rat primary astrocytes. Prolonged exposure to the two compounds could stimulate glucose transport by up to 30-60% over the control level (p <0.05) in Hs68 and Glut1DS fibroblasts as well as in rat astrocytes. The stimulation of glucose transport by HPPH was dose-dependent and accompanied by an up-regulation of GLUT1 mRNA expression (p <0.05). In conclusion, phenytoin and HPPH do not compromise cellular glucose transport. Prolonged exposure to these compounds can modify carbohydrate homeostasis by up-regulating glucose transport in both normal and Glut1DS conditions in vitro.

Potential application of a glucose-transport-deficient mutant of Schizosaccharomyces pombe for removing gluconic acid from grape must

Musts from rotten grapes typically contain high levels of gluconic acid, which can raise severe problems in winemaking processes. In this work, the ability of the glucose-transport-deficient mutant YGS-5 of Schizosaccharomyces pombe to completely or partly remove gluconic acid from a synthetic glucose-containing medium and the potential use of this yeast strain for the same purpose in musts and wines were examined. Surprisingly, the S. pombe YGS-5 strain successfully removed 93% of the initial gluconic acid (2.5 gL(-1)) and 80% of the initial malic acid (1.0 gL(-1)) within 30 h after inoculation. Also, the yeast strain produced no volatile compounds other than those obtained in fermentations conducted with the wine yeast Saccharomyces cerevisiae. S. pombe YGS-5 could thus be used to remove gluconic acid present in musts from rotten grapes. On the basis of these results, various ways of using S. pombe YGS-5 to treat musts containing gluconic acid in order to solve the problems due to the high gluconic acid concentrations in botrytized grape must are proposed.

GLUT1-deficient mice exhibit impaired endothelium-dependent vascular relaxation

We tested the hypothesis that decreased glucose transporter 1 (GLUT1) expression alters endothelial function. Nitric oxide-dependent endothelial relaxation, but not endothelium-independent relaxation, was significantly reduced in aortas from transgenic mice expressing GLUT1 antisense mRNA, compared to aortas from nontransgenic littermates. These data suggest that GLUT1-dependent glucose metabolism may play an important role in regulating endothelial function.

Alterations in rheological properties and erythrocyte membrane proteins in cats with diabetes mellitus

Many studies have shown that diabetes mellitus is associated with increased whole and blood viscosity and decreased erythrocyte deformability. It has been suggested that these abnormalities in blood rheology may play a causative role in the pathogenesis of diabetic vascular complications. However, less is known about the content and quality of membrane proteins which may contribute to abnormalities in membrane dynamic and decreased erythrocyte deformability. In the present study we analysed various rheological parameters (blood and plasma viscosity, erythrocyte deformability, haemotological parameters), in cats with non-insulin dependent diabetes mellitus (NIDDM). We also investigated alterations in erythrocyte membrane protein content by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). We found that erythrocyte rigidity and plasma and whole blood viscosities were significantly higher in cats with NIDDM compared to controls. SDS-PAGE revealed that the band 5 corresponding to actin was weaker while band 4.5 corresponding to integral membrane proteins (glycophorin A, B and C) had disappeared. Also, band 4.9, which is composed of dematin (a protein with actin-bundling capacity) was lost. We suggest that the observed abnormalities in membrane proteins may play a role in reduced erythrocyte deformability associated with diabetes mellitus.

Improved insulin sensitivity and adipose tissue dysregulation after short-term treatment with pioglitazone in non-diabetic, insulin-resistant subjects

AIMS/HYPOTHESIS

We examined whether short-term treatment with a thiazolidinedione improves insulin sensitivity in non-obese but insulin-resistant subjects and whether this is associated with an improvement in dysregulated adipose tissue (reduced expression of IRS-1, GLUT4, PPARgamma co-activator 1 and markers of terminal differentiation) that we have previously documented to be associated with insulin resistance.

METHODS

Ten non-diabetic subjects, identified as having low IRS-1 and GLUT-4 protein in adipose cells as markers of insulin resistance, underwent 3 weeks of treatment with pioglitazone. The euglycaemic-hyperinsulinaemic clamp technique was used to measure insulin sensitivity before and after treatment. Serum samples were analysed for glucose, insulin, lipids, total and high-molecular-weight (HMW) adiponectin levels. Biopsies from abdominal subcutaneous adipose tissue were taken, cell size measured, mRNA and protein extracted and quantified using real-time RT-PCR and Western blot.

RESULTS

Insulin sensitivity was improved after 3 weeks treatment and circulating total as well as HMW adiponectin increased in all subjects, while no effect was seen on serum lipids. In the adipose cells, gene and protein expression of IRS-1 and PPARgamma co-activator 1 remained unchanged, while adiponectin, adipocyte P 2, uncoupling protein 2, GLUT4 and liver X receptor-alpha increased. Insulin-stimulated tyrosine phosphorylation and p-ser-PKB/Akt increased, while no significant effect of thiazolidinedione treatment was seen on the inflammatory status of the adipose tissue in these non-obese subjects.

CONCLUSIONS/INTERPRETATION

Short-term treatment with pioglitazone improved insulin sensitivity in the absence of any changes in circulating NEFA or lipid levels. Several markers of adipose cell differentiation, previously shown to be reduced in insulin resistance, were augmented, supporting the concept that insulin resistance in these individuals is associated with impaired terminal differentiation of the adipose cells.

Die ketogene Diät in den deutschsprachigen Ländern im Jahr 2003: eine Standortbestimmung

BACKGROUND

The ketogenic diet has been used for decades to treat intractable childhood epilepsies. It is also the treatment of choice for GLUT1 deficiency syndrome and pyruvate-dehydrogenase-complex-deficiency. Recent studies have once again confirmed the efficiacy of the diet, but the diet is hardly known in Europe and has never been quite accepted as an effective treatment of childhood epilepsy.

PATIENTS

We report retrospective data on 146 children treated with the ketogenic diet in Austria, Switzerland, and Germany.

METHOD

In 2000 and 2002, standardized questionaires were sent to 13 neuropediatric departments to evaluate indications, effects and side effects.

RESULTS

In children with refractory epilepsy (n = 111), 8 % became seizure-free on the diet. Seizure reduction of > 90 % was achieved in additional 9 % of patients, a seizure reduction of 50-90 % in additional 14 % of patients. There was a great variability between epilepsy departments. All patients with GLUT1 deficiency syndrome (n = 18) and pyruvate-dehydrogenase-complex-deficiency (n = 15) showed clinical improvement. In GLUT1 deficiency syndrome, complete seizure control was achieved in 94 % of patients. Compliance was good in 82 % of all patients regardless of the indication for the diet.

CONCLUSION

In contrast to the general restraint towards the ketogenic diet in Europe, our data supports its effectiveness as the treatment of choice for GLUT1-deficiency syndrome und pyruvate-dehydrogenase-complex-deficiency. In children with refractory epilepsy, the ketogenic diet matched the effect of most anticonvulsants and was well tolerated. These data and two workshops resulted in recommendations for the use of the ketogenic diet in children as a basis for a general diagnostic and therapeutic standards to compare and improve the use of the ketogenic diet in Europe.

Regulation of insulin-responsive aminopeptidase expression and targeting in the insulin-responsive vesicle compartment of glucose transporter isoform 4-deficient cardiomyocytes

In adipocytes and cardiac or skeletal muscle, glucose transporter isoform 4 (GLUT4) is targeted to insulin-responsive intracellular membrane vesicles (IRVs) that contain several membrane proteins, including insulin-responsive aminopeptidase (IRAP) that completely colocalizes with GLUT4 in basal and insulin-treated cells. Cardiac GLUT4 content is reduced by 65-85% in IRAP knockout mice, suggesting that IRAP may regulate the targeting or degradation of GLUT4. To determine whether GLUT4 is required for maintenance of IRAP content within IRVs, we studied the expression and cellular localization of IRAP and other GLUT4 vesicle-associated proteins, in hearts of mice with cardiac-specific deletion of GLUT4 (G4H-/-). In G4H-/- hearts, IRAP content was reduced by 60%, but the expression of other vesicle-associated proteins, namely cellugyrin, IGF-II/mannose-6-phosphate, and transferrin receptors, secretory carrier-associated membrane proteins and vesicle-associated membrane protein were unchanged. Using sucrose gradient centrifugation and cell surface biotinylation, we found that IRAP content in 50-80S vesicles where GLUT4 vesicles normally sediment was markedly depleted in G4H-/- hearts, and the remaining IRAP was found in the heavy membrane fraction. Although insulin caused a discernible increase in cell surface IRAP content of G4H-/- cardiomyocytes, cell surface IRAP remained 70% lower than insulin-stimulated controls. Immunoabsorption of intracellular vesicles with anticellugyrin antibodies revealed that IRAP content was reduced by 70% in both cellugyrin-positive and cellugyrin-negative vesicles. Endosomal recycling, as measured by transferrin receptor recycling was normal. Thus, GLUT4 and IRAP content of early endosome-derived sorting vesicles and of IRVs are coordinately regulated, and both proteins are required for maintenance of key constituents of these compartments in cardiac muscle cells in vivo.

GLUT1 deficiency and other glucose transporter diseases

We review the three genetically determined disorders of glucose transport across cell membranes. Diseases such as glucose-galactose malabsorption, Fanconi-Bickel syndrome and De Vivo disease (GLUT1 deficiency syndrome (GLUT1DS)) arise from heritable mutations in transporter-encoding genes that impair monosaccharide uptake, which becomes rate-limiting in tissues where the transporters serve as the main glucose carrier systems. We focus in greater detail on De Vivo disease as a prototype of a brain energy failure syndrome, for which the greatest pathophysiological detail is known, but which presents the most therapeutic challenges. The study of these diseases illustrates fundamental aspects of energetic metabolism, while providing the basis for their diagnosis by simple metabolic screening and for their treatment by dietary modification.

[Type 1 glucose transporter (Glut1) deficiency: manifestations of a hereditary neurological syndrome]

AIM

To define this genetic syndrome.

DEVELOPMENT

The constellation of infantile epilepsy, acquired microcephaly and hypoglychorrachia is characteristic of glucose transporter type 1 (Glut1) deficiency syndrome, a prototype neurometabolic disorder caused by inheritable mutations in the gene SLC2A1. All known mutations reduce the function of Glut1 in the blood brain barrier and thus limit brain glucose availability. As the cerebral metabolic rate for glucose increases during infancy, patients become gradually symptomatic, a phenomenon that underscores the importance of early diagnosis via lumbar puncture and treatment, which has meet with some success in ameliorating several --but not all-- features of the disease.

CONCLUSION

The increasing number of mild phenotypic variants being described, owing to the improved awareness of the disease, has led to the consideration of Glut1 deficiency in the diagnosis of infantile seizures, mental retardation, familial epilepsy and movement disorders.

Impaired glucose transport into the brain: the expanding spectrum of glucose transporter type 1 deficiency syndrome

PURPOSE OF REVIEW

Glucose transporter type 1 deficiency syndrome (OMIM 606777) is a treatable epileptic encephalopathy resulting from impaired glucose transport into the brain. In recent years, the increasing number of patients has generated substantial insights into the manifestations and mechanisms of this disease. Current understanding of this novel disorder is reviewed, and recent advances in diagnosis and treatment are highlighted.

RECENT FINDINGS

The syndrome is now understood to be a complex neurological disorder. The clinical spectrum has recently been extended by infants with 'benign' transient hypoglycorrhachia, glucose transporter type 1 deficiency syndrome without seizures, and by adult cases. Other key findings in the last couple of years include (1) the description of electroencephalogram abnormalities, (2) a characteristic cerebral metabolic footprint in positron emission tomography imaging, and (3) the definition of molecular mechanisms and functional domains within the glucose transporter type 1 protein by in-vitro mutagenesis. The disease has also shed a new light on the mechanisms and the effectiveness of the ketogenic diet for seizure control.

SUMMARY

The syndrome is now well characterized in children and should be considered in any patient with intractable epilepsy. An effective therapy is available. The clinical spectrum and the molecular basis of the disease are increasingly heterogeneous and indicate complex pathogenic mechanisms that will ultimately lead to a classification on clinical, biochemical, and molecular grounds.

Effects of the ketogenic diet in the glucose transporter 1 deficiency syndrome

The ketogenic diet (KD), established to treat intractable childhood epilepsy, has emerged as the principal treatment of GLUT1 deficiency syndrome (OMIM 606777). This defect of glucose transport into the brain results in hypoglycorrhachia causing epilepsy, developmental delay, and a complex motor disorder in early childhood. Ketones provided by a high-fat, low-carbohydrate diet serve as an alternative fuel to the brain. Glucose, lactate, lipids, and ketones in blood and cerebrospinal fluid were investigated in five GLUT1-deficient patients before and on the KD. Hypoglycorrhachia was detected in the non-ketotic and ketotic state. In ketosis, lactate concentrations in the cerebrospinal fluid increased moderately. The CSF/blood ratio for acetoacetate was higher compared to beta-hydroxybutyrate. Free fatty acids did not enter the brain in significant amounts. Blood concentrations of essential fatty acids determined in 18 GLUT1-deficient patients on the KD were sufficient in all age groups. The effects of the KD in GLUT1 deficiency syndrome, particularly the course of blood lipids, are discussed in an illustrative case. In this syndrome, the KD effectively restores brain energy metabolism. Ketosis does not influence impaired GLUT1-mediated glucose transport into brain: hypoglycorrhachia, the biochemical hallmark of the disease, can be identified in GLUT1-deficient patients on a KD. The effects of ketosis on the concentrations of glucose, lactate, ketones, and fatty acids in blood and cerebrospinal fluid in this entity are discussed in view of previous data on ketosis in man.

GLUT4 overexpression or deficiency in adipocytes of transgenic mice alters the composition of GLUT4 vesicles and the subcellular localization of GLUT4 and insulin-responsive aminopeptidase

The majority of GLUT4 is sequestered in unique intracellular vesicles in the absence of insulin. Upon insulin stimulation GLUT4 vesicles translocate to, and fuse with, the plasma membrane. To determine the effect of GLUT4 content on the distribution and subcellular trafficking of GLUT4 and other vesicle proteins, adipocytes of adipose-specific, GLUT4-deficient (aP2-GLUT4-/-) mice and adipose-specific, GLUT4-overexpressing (aP2-GLUT4-Tg) mice were studied. GLUT4 amount was reduced by 80-95% in aP2-GLUT4-/- adipocytes and increased approximately 10-fold in aP2-GLUT4-Tg adipocytes compared with controls. Insulin-responsive aminopeptidase (IRAP) protein amount was decreased 35% in aP2-GLUT4-/- adipocytes and increased 45% in aP2-GLUT4-Tg adipocytes. VAMP2 protein was also decreased by 60% in aP2-GLUT4-/- adipocytes and increased 2-fold in aP2-GLUT4-Tg adipocytes. IRAP and VAMP2 mRNA levels were unaffected in aP2-GLUT4-Tg, suggesting that overexpression of GLUT4 affects IRAP and VAMP2 protein stability. The amount and subcellular distribution of syntaxin4, SNAP23, Munc-18c, and GLUT1 were unchanged in either aP2-GLUT4-/- or aP2-GLUT4-Tg adipocytes, but transferrin receptor was partially redistributed to the plasma membrane in aP2-GLUT4-Tg adipocytes. Immunogold electron microscopy revealed that overexpression of GLUT4 in adipocytes increased the number of GLUT4 molecules per vesicle nearly 2-fold and the number of GLUT4 and IRAP-containing vesicles per cell 3-fold. In addition, the proportion of cellular GLUT4 and IRAP at the plasma membrane in unstimulated aP2-GLUT4-Tg adipocytes was increased 4- and 2-fold, respectively, suggesting that sequestration of GLUT4 and IRAP is saturable. Our results show that GLUT4 overexpression or deficiency affects the amount of other GLUT4-vesicle proteins including IRAP and VAMP2 and that GLUT4 sequestration is saturable.