Neuronal GLUT3 Research Articles

Food For Thought: Short-Term Fasting Upregulates Glucose Transporters in Neurons and Endothelial Cells, But Not in Astrocytes.

Our group previously reported that 6-h fasting increased both insulin II mRNA expression and insulin level in rat hypothalamus. Given that insulin effects on central glucose metabolism are insufficiently understood, we wanted to examine if the centrally produced insulin affects expression and/or regional distribution of glucose transporters, and glycogen stores in the hypothalamus during short-term fasting. In addition to determining the amount of total and activated insulin receptor, glucose transporters, and glycogen, we also studied distribution of insulin receptors and glucose transporters within the hypothalamus. We found that short-term fasting did not affect the astrocytic 45kDa GLUT1 isoform, but it significantly increased the amount of endothelial 55kDa GLUT1, and neuronal GLUT3 in the membrane fractions of hypothalamic proteins. The level of GLUT2 whose presence was detected in neurons, ependymocytes and tanycytes was also elevated. Unlike hepatic glycogen which was decreased, hypothalamic glycogen content was not changed after 6-h fasting. Our findings suggest that neurons may be given a priority over astrocytes in terms of glucose supply even during the initial phase of metabolic response to fasting. Namely, increase in glucose influx into the brain extracellular fluid and neurons by increasing the translocation of GLUT1, and GLUT3 in the cell membrane may represent the first line of defense in times of scarcity. The absence of co-localization of these membrane transporters with the activated insulin receptor suggests this process takes place in an insulin-independent manner.

WZB117 (2-Fluoro-6-(m-hydroxybenzoyloxy) Phenyl m-Hydroxybenzoate) Inhibits GLUT1-mediated Sugar Transport by Binding Reversibly at the Exofacial Sugar Binding Site

WZB117 (2-fluoro-6-(m-hydroxybenzoyloxy) phenyl m-hydroxybenzoate) inhibits passive sugar transport in human erythrocytes and cancer cell lines and, by limiting glycolysis, inhibits tumor growth in mice. This study explores how WZB117 inhibits the erythrocyte sugar transporter glucose transport protein 1 (GLUT1) and examines the transporter isoform specificity of inhibition. WZB117 reversibly and competitively inhibits erythrocyte 3-O-methylglucose (3MG) uptake with Ki(app) = 6 μm but is a noncompetitive inhibitor of sugar exit. Cytochalasin B (CB) is a reversible, noncompetitive inhibitor of 3MG uptake with Ki(app) = 0.3 μm but is a competitive inhibitor of sugar exit indicating that WZB117 and CB bind at exofacial and endofacial sugar binding sites, respectively. WZB117 inhibition of GLUTs expressed in HEK293 cells follows the order of potency: insulin-regulated GLUT4 ≫ GLUT1 ≈ neuronal GLUT3. This may explain WZB117-induced murine lipodystrophy. Molecular docking suggests the following. 1) The WZB117 binding envelopes of exofacial GLUT1 and GLUT4 conformers differ significantly. 2) GLUT1 and GLUT4 exofacial conformers present multiple, adjacent glucose binding sites that overlap with WZB117 binding envelopes. 3) The GLUT1 exofacial conformer lacks a CB binding site. 4) The inward GLUT1 conformer presents overlapping endofacial WZB117, d-glucose, and CB binding envelopes. Interrogating the GLUT1 mechanism using WZB117 reveals that subsaturating WZB117 and CB stimulate erythrocyte 3MG uptake. Extracellular WZB117 does not affect CB binding to GLUT1, but intracellular WZB117 inhibits CB binding. These findings are incompatible with the alternating conformer carrier for glucose transport but are consistent with either a multisubunit, allosteric transporter, or a transporter in which each subunit presents multiple, interacting ligand binding sites.

Novel Roles for the Insulin-Regulated Glucose Transporter-4 in Hippocampally Dependent Memory.

The role of insulin-regulated glucose transporter-4 (GluT4) in the brain is unclear. In the current study, we demonstrate that GluT4 is a critical component of hippocampal memory processes. Memory training increased hippocampal GluT4 translocation and memory acquisition was impaired by GluT4 blockade. Unexpectedly, whereas long-term inhibition of GluT4 impaired long-term memory, short-term memory was enhanced. These data further our understanding of the molecular mechanisms of memory and have particular significance for type 2 diabetes (in which GluT4 activity in the periphery is impaired) and Alzheimer's disease (which is linked to impaired brain insulin signaling and for which type 2 diabetes is a key risk factor). Both diseases cause marked impairment of hippocampal memory linked to hippocampal hypometabolism, suggesting the possibility that brain GluT4 dysregulation may be one cause of cognitive impairment in these disease states.

Brain glucose transporters

The normal adult brain constitutes approximately 2% of the body weight and consumes approximately 20% of glucose in the body (about 120 g of glucose per day). In the nervous system, the aerobic metabolism of glucose is the main source of energy in the form of adenosine triphosphate (ATP); most of this energy is utilized to sustain excitatory synaptic transmission. The interaction among neurons, astrocytes, and endothelial cells has a central role coupling energy supply with changes in neuronal activity. These cells express different subtypes of glucose transporters (GLUTs) that mediate the sodium-independent facilitated transport of glucose across membranes. GLUT1 and GLUT3 are expressed in the nervous system, GLUT1 in endothelial cells and astrocytes, and GLUT3 in neurons. Astrocytes take up glucose from the blood and metabolize it to lactate, which is then delivered to neurons. The relative contribution of this astrocyte-to-neuron lactate shuttle as a main source of energy to sustain neuronal physiology, compared to direct glucose uptake by neurons, is a matter of debate; nevertheless, the role of GLUT1 in astrocytes is underscored by the range of clinical phenotypes associated with GLUT1 deficiency. There are several reviews on the biochemistry and physiology of GLUTs, neurometabolic coupling between astrocytes and neurons, the clinical consequences of GLUT1 deficiency, and the involvement of GLUT1 in other neurologic disorders.(1-7.)

Creb1-Mecp2-mCpG Complex Transactivates Postnatal Murine Neuronal Glucose Transporter Isoform 3 Expression

The murine neuronal facilitative glucose transporter isoform 3 (Glut3) is developmentally regulated, peaking in expression at postnatal day (PN)14. In the present study, we characterized a canonical CpG island spanning the 5'-flanking region of the glut3 gene. Methylation-specific PCR and bisulfite sequencing identified methylation of this CpG ((m)CpG) island of the glut3 gene, frequency of methylation increasing 2.5-fold with a 1.6-fold increase in DNA methyl transferase 3a concentrations noted with advancing postnatal age (PN14 vs PN3). 5'-flanking region of glut3-luciferase reporter transient transfection in HT22 hippocampal neurons demonstrated that (m)CpGs inhibit glut3 transcription. Contrary to this biological function, glut3 expression rises synchronously with (m)CpGs in PN14 vs PN3 neurons. Chromatin immunoprecipitation (IP) revealed that methyl-CpG binding protein 2 (Mecp2) bound the glut3-(m)CpGs. Depending on association with specific coregulators, Mecp2, a dual regulator of gene transcription, may repress or activate a downstream gene. Sequential chromatin IP uncovered the glut3-(m)CpGs to bind Mecp2 exponentially upon recruitment of Creb1 rather than histone deacetylase 1. Co-IP and coimmunolocalization confirmed that Creb1 associated with Mecp2 and cotransfection with glut3-(m)CpG in HT22 cells enhanced glut3 transcription. Separate 5-aza-2'-deoxycytidine pretreatment or in combination with trichostatin A reduced (m)CpG and specific small interference RNAs targeting Mecp2 and Creb1 separately or together depleting Mecp2 and/or Creb1 binding of glut3-(m)CpGs reduced glut3 expression in HT22 cells. We conclude that Glut3 is a methylation-sensitive neuronal gene that recruits Mecp2. Recruitment of Creb1-Mecp2 by glut3-(m)CpG contributes towards transactivation, formulating an escape from (m)CpG-induced gene suppression, and thereby promoting developmental neuronal glut3 gene transcription and expression.

Glucose Transporters Regulation on Ischemic Brain: Possible Role as Therapeutic Target

Ischemic stroke is a major cause of death worldwide that provokes a high society cost. Deprivation of blood supply, with the subsequent deficiency of glucose and oxygen, triggers an important number of mechanisms (e.g. excitotoxicity, oxidative stress and inflammation) leading to irreversible neuronal injury. Consequently, ischemia increases the energy demand which is associated with profound changes in brain energy metabolism. Glucose transport activity may adapt to ensure the delivery of glucose to maintain normal cellular function, even at the low glucose levels observed in plasma during ischemia. In the brain, the main glucose transporters (GLUTs) are GLUT3 in neurons and GLUT1 in the microvascular endothelial cells of the blood brain barrier and glia. The intracellular signaling pathways involved in GLUT regulation in cerebral ischemia remain unclear; however, it has been established that ischemia induces changes in their expression. In this review, we describe the effect of glutamate-induced excitotoxicity, mitochondrial damage, glucose deprivation, and hypoxia on GLUTs expression in the brain. Additionally, we discuss the possible role of GLUTs as therapeutic target for ischemia. Despite of the intense research, current therapeutics options for stroke are very limited, therefore it is especially important to find new options. Few studies have examined the neuroprotective potential of GLUT up-regulation in ischemic stroke; however, evidence suggests that augmented GLUTs could be related to a protective mechanism. Increased understanding of the beneficial effects of GLUTs activation provides the rationale for targeting GLUT in the development of new therapeutic strategies.

Role of β-Adrenoceptors in Memory Consolidation: β3-Adrenoceptors Act on Glucose Uptake and β2-Adrenoceptors on Glycogenolysis

Noradrenaline, acting via beta(2)- and beta(3)-adrenoceptors (AR), enhances memory formation in single trial-discriminated avoidance learning in day-old chicks by mechanisms involving changes in metabolism of glucose and/or glycogen. Earlier studies of memory consolidation in chicks implicated beta(3)- rather than beta(2)-ARs in enhancement of memory consolidation by glucose, but did not elucidate whether stimulation of glucose uptake or of glycolysis was responsible. This study examines the role of glucose transport in memory formation using central injection of the nonselective facilitative glucose transporter (GLUT) inhibitor cytochalasin B, the endothelial/astrocytic GLUT-1 inhibitor phloretin and the Na(+)/energy-dependent endothelial glucose transporter (SGLT) inhibitor phlorizin. Cytochalasin B inhibited memory when injected into the mesopallium (avian cortex) either close to or between 25 and 45 min after training, whereas phloretin and phlorizin only inhibited memory at 30 min. This suggested that astrocytic/endothelial (GLUT-1) transport is critical at the time of consolidation, whereas a different transporter, probably the neuronal glucose transporter (GLUT-3), is important at the time of training. Inhibition of glucose transport by cytochalasin B, phloretin, or phlorizin also interfered with beta(3)-AR-mediated memory enhancement 20 min posttraining, whereas inhibition of glycogenolysis interfered with beta(2)-AR agonist enhancement of memory. We conclude that in astrocytes (1) activities of both GLUT-1 and SGLT are essential for memory consolidation 30 min posttraining; (2) neuronal GLUT-3 is essential at the time of training; and (3) beta(2)- and beta(3)-ARs consolidate memory by different mechanisms; beta(3)-ARs stimulate central glucose transport, whereas beta(2)-ARs stimulate central glycogenolysis.

Supply and Demand in Cerebral Energy Metabolism: The Role of Nutrient Transporters

Glucose is the obligate energetic fuel for the mammalian brain, and most studies of cerebral energy metabolism assume that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state. Glucose transporter (GLUT) proteins deliver glucose from the circulation to the brain: GLUT1 in the microvascular endothelial cells of the blood-brain barrier (BBB) and glia; GLUT3 in neurons. Lactate, the glycolytic product of glucose metabolism, is transported into and out of neural cells by the monocarboxylate transporters (MCT): MCT1 in the BBB and astrocytes and MCT2 in neurons. The proposal of the astrocyte-neuron lactate shuttle hypothesis suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energetics, especially during activation. Since the identification of the GLUTs and MCTs in brain, much has been learned about their transport properties, that is capacity and affinity for substrate, which must be considered in any model of cerebral glucose uptake and utilization. Using concentrations and kinetic parameters of GLUT1 and -3 in BBB endothelial cells, astrocytes, and neurons, along with the corresponding kinetic properties of the MCTs, we have successfully modeled brain glucose and lactate levels as well as lactate transients in response to neuronal stimulation. Simulations based on these parameters suggest that glucose readily diffuses through the basal lamina and interstitium to neurons, which are primarily responsible for glucose uptake, metabolism, and the generation of the lactate transients observed on neuronal activation.

Omega-3 fatty acids and brain energy metabolism: Impact on the expression of glucose transporters and glucose transport activity in endothelial cells in culture

Alterations in cognitive performance and neuronal activity in animals defi cient in n-3 polyunsaturated fatty acids (PUFA) have been linked to the decrease in docosahexaenoic acid (22:6n-3, DHA) level in brain membranes. These alterations could partly result from changes in brain energy metabolism and namely in glucose transport activity. Indeed, we reported that several brain areas of rats raised on a diet defi cient in n-3 PUFAs have below-normal rates of glucose utilization [1]. We postulated that the quantity of glucose transporter GLUT1 in defi cient rats was functionally abnormal as its immunoreactivity was low. We present experimental data that evaluated: 1) the in vivo impact of an n-3 PUFA-defi cient diet on expression of the main glucose transporters of rat cerebral cortex (endothelial 55-kDa GLUT1, astrocytic 45-kDa GLUT1, and neuronal GLUT3); and 2) the in vitro impact of the three main long-chain n-6 and n-3 PUFAs, arachidonic acid (20:4n-6), eicosapentaenoic acid (20:5n-3) and DHA, on the fatty acid composition of membrane phospholipids and glucose uptake in two models of rat brain endothelial cells (primary cultures (RBEC), and immortalized cell line RBE4). In vivo study Rats raised with a n-3 PUFA defi cient diet throughout one generation were compared with rats fed a diet with adequate amounts of n-6 and n-3 PUFA. The fatty acid profi le of brain microvessels was determined in order to identify the alterations in n-6 and n-3 PUFA in endothelial cells compared to their profi le in neural cells from the cerebral cortex. The main glucose transporters were quantifi ed by western immunoblotting and by real time quantitative RT-PCR. Specifi c binding of cytochalasin B to GLUT1 of isolated microvessels was also determined. In vitro study The three PUFA studied were added to the culture medium (15μM) as sodium salts on the fi rst day of culture and until the cells were used. Glucose uptake was measured on monolayers of RBEC and RBE4 cells using the non-metabolisable radiolabelled glucose analogue 3-o-methylglucose ([3H]-3-o-MG). In vivo data showed that the 55-kDa GLUT1 protein quantity decreased by 25% in the n-3 PUFA-defi cient microvessels, as did 45 kDa-GLUT1 in the homogenate (30% decrease). Results from cytochalasin B binding confi rmed these data, showing a lower level of its maximum binding to GLUT1. However, GLUT3 protein quantity did not change as GLUT1 and GLUT3 mRNA level. In vitro data demonstrated that both cerebral endothelial cells had low n-6 (mainly 20:4n-6) and n-3 (mainly 22:6n-3) PUFA concentrations under standard conditions of culture. RBEC and RBE4 cells grown in medium supplemented with 20:4n-6, 20:5n-3 or 22:6n-3 (15 μM, physiological concentrations), avidly incorporated these PUFAs into their membrane phospholipids to approach the contents of cerebral microvessels of control rats (considered as physiological references). However RBE4 cells did not incorporate suffi cient 20:4n-6 compared to physiological concentrations. Supplementation with 20:5n-3 and 22:6n-3 increased the basal glucose transport in RBEC ([3H]-3-O-methylglucose uptake) by 50% and 35% respectively, while 20:4n-6 had no effect. On the other hand, none of the PUFA supplements had a signifi cant effect on glucose uptake by RBE4. We demonstrated that incorporation of n-3 long-chain PUFAs in RBEC membranes acts on glucose transport activity. We suggest that the decreased glucose utilization in the cerebral cortex of n-3 PUFA-defi cient rats is in part due to reduced amounts of the two GLUT1 isoforms, possibly issuing from post-transcriptional down-regulation of GLUT1 protein synthesis. In vitro studies suggest that n-3 PUFA can directly modulate the activity of glucose transport in endothelial cells by altering their membrane phospholipid content.

A ketogenic diet increases brain insulin-like growth factor receptor and glucose transporter gene expression.

A ketogenic diet suppresses seizure activity in children and in juvenile rats. To investigate whether alteration in brain IGF activity could be involved in the beneficial effects of the ketogenic diet, we examined the effects of this diet on IGF system gene expression in the rat brain. Juvenile rats were fed one of three different diets for 7 d: ad libitum standard rat chow (AL-Std), calorie-restricted standard chow (CR-Std), or a calorie-restricted ketogenic diet (CR-Ket). The calorie-restricted diets contained 90% of the rats' calculated energy requirements. The AL-Std diet group increased in weight, whereas the two CR groups merely maintained their weight during the 7-d diet. Glucose levels were significantly reduced in both CR groups compared with the AL-Std group, but only the CR-Ket group developed ketonemia. IGF1 mRNA levels were reduced by 30-50% in most brain regions in both CR groups. IGF1 receptor (IGF1R) mRNA levels were decreased in the CR-Std group but were increased in the CR-Ket diet group. Brain IGF binding protein (IGFBP)-2 and -5 mRNA levels were not altered by diet, but IGFBP-3 mRNA levels were markedly increased by the ketogenic diet while not altered by calorie restriction alone. Brain glucose transporter expression was also investigated in this study. Glucose transporter (GLUT) 4 mRNA levels were quite low and not appreciably altered by the different diets. Parenchymal GLUT1 mRNA levels were increased by the CR-Ket diet, but endothelial GLUT1 mRNA levels were not affected. Neuronal GLUT3 expression was decreased with the CR-Std diet and increased with the CR-Ket diet, in parallel with the IGF1R pattern. These observations reveal divergent effects of dietary caloric content and macronutrient composition on brain IGF system and GLUT expression. In addition, the data may be consistent with a role for enhanced IGF1R and GLUT expression in ketogenic diet-induced seizure suppression.

Increased expression of neuronal glucose transporter 3 but not glial glucose transporter 1 following severe diffuse traumatic brain injury in rats.

Traumatic brain injury results in an increased brain energy demand that is associated with profound changes in brain glycolysis and energy metabolism. Increased glycolysis must be met by increasing glucose supply that, in brain, is primarily mediated by two members of the facilitative glucose transporter family, Glut1 and Glut3. Glut1 is expressed in endothelial cells of the blood-brain barrier (BBB) and also in glia, while Glut3 is the primary glucose transporter expressed in neurons. However, few studies have investigated the changes in glucose transporter expression following traumatic brain injury, and in particular, the neuronal and glial glucose transporter responses to injury. This study has therefore focussed on investigating the expression of the glial specific 45-kDa isoform of Glut1 and neuronal specific Glut3 following severe diffuse traumatic brain injury in rats. Following impact-acceleration injury, Glut3 expression was found to increase by at least 300% as early as 4 h after induction of injury and remained elevated for at least 48 h postinjury. The increase in Glut3 expression was clearly evident in both the cerebral cortex and cerebellum. In contrast, expression of the glial specific 45-kDa isoform of Glut1 did not significantly change in either the cerebral cortex or cerebellum following traumatic injury. We conclude that increased glucose uptake after traumatic brain injury is primarily accounted for by increased neuronal Glut 3 glucose transporter expression and that this increased expression after trauma is part of a neuronal stress response that may be involved in increasing neuronal glycolysis and associated energy metabolism to fuel repair processes.

Astrocytic demise precedes delayed neuronal death in focal ischemic rat brain

Active neuronal–glial interaction is important in the maintenance of brain homeostasis and is vital for neuronal survival following brain injury. The time course of post-ischemic astroglial dysfunction and neuronal death was studied in the spontaneously hypertensive rat (SHR) brain following permanent middle cerebral artery occlusion (MCAO). In situ hybridization with 35 S -labeled riboprobes for GFAP and GLUT3 was used to monitor mRNA expression in glia and neurons. Astrocytic proteins GFAP, vimentin, S100, Glutathione- S-Transferase Y b (GST Y b) and neuronal protein TG2 were detected by immunofluorescence. Cells were co-stained with in situ end labeling (ISEL) to detect DNA fragmentation, a hallmark of cell death. GFAP mRNA expression declined rapidly in the ischemic region of the cortex and was almost absent by 12 h. Immunohistochemical studies revealed a parallel decline in the corresponding protein: a reduction in GFAP staining was apparent in the infarct after 3 h and by 24 h, there was essentially no remaining GFAP. Three other glial proteins (vimentin, S100 and GST Y b) disappeared from infarct over a similar time course. A few ISEL positive cells were observed in the infarct at 6 h, but maximal detection was not seen until 24–48 h. Most of the ISEL-positive cells were neurons, identified by co-staining with the neuronal marker TG2. Few cells expressing GFAP or other glial markers were positive at any time point. Neuronal GLUT3 mRNA declined more slowly than GFAP mRNA in the ischemic core and disappeared during the period of neuronal death. Concurrent with the loss of GFAP mRNA and protein expression in the infarct, there was a rapid rise in GFAP mRNA in the peri-infarct region of ipsilateral hemisphere and proximal region of the contralateral hemisphere. This was followed by the enhanced GFAP protein expression characteristic of reactive astrocytes, but over a significantly slower time course. These studies show that MCAO leads to a rapid decline of GFAP mRNA and glial proteins, which appears to precede the decline in neuronal mRNA and neuronal death within the infarct. Early astroglial dysfunction may play a critical role in determining the outcome of acute hypoxic–ischemic injury by compromising neuronal–glial interactions.

Glucose transporter proteins in brain: delivery of glucose to neurons and glia.

Glucose is the principle energy source for the mammalian brain. Delivery of glucose from the blood to the brain requires transport across the endothelial cells of the blood-brain barrier and into the neurons and glia. The facilitative glucose transporter proteins mediate these processes. The primary isoforms in brain are GLUT1, detected at high concentrations as a highly glycosylated form, (55 kDa) in blood-brain barrier, and also as a less glycosylated, 45 kDa form, present in parenchyma, predominantly glia; GLUT3 in neurons; and GLUT5 in microglia. The rest of the transporter family, GLUTs 2, 4, and 7, have also been detected in brain but at lower levels of expression and confined to more discrete regions. All of the transporters probably contribute to cerebral glucose utilization, as part of overall metabolism and metabolic interactions among cells. We discuss the properties, regulation, cell-specific location, and kinetic characteristics of the isoforms, their potential contributions to cerebral metabolism, and several experimental paradigms in which alterations in energetic demand and/or substrate supply affect glucose transporter expression.

Effects of hypoxia-ischemia on GLUT1 and GLUT3 glucose transporters in immature rat brain.

Cerebral hypoxia-ischemia produces major alterations in energy metabolism and glucose utilization in brain. The facilitative glucose transporter proteins mediate the transport of glucose across the blood-brain barrier (BBB) (55 kDa GLUT1) and into the neurons and glia (GLUT3 and 45 kDa GLUT1). Glucose uptake and utilization are low in the immature rat brain, as are the levels of the glucose transporter proteins. This study investigated the effect of cerebral hypoxia-ischemia in a model of unilateral brain damage on the expression of GLUT1 and GLUT3 in the ipsilateral (damaged, hypoxic-ischemic) and contralateral (undamaged, hypoxic) hemispheres of perinatal rat brain. Early in the recovery period, both hemispheres exhibited increased expression of BBB GLUT1 and GLUT3, consistent with increased glucose transport and utilization. Further into recovery, BBB GLUT1 increased and neuronal GLUT3 decreased in the damaged hemisphere only, commensurate with neuronal loss.

Modulation of Expression of Glucose Transporters GLUT3 and GLUT1 by Potassium and N-Methyl-d-aspartate in Cultured Cerebellar Granule Neurons

Depolarization is known to stimulate neuronal oxidative metabolism. As glucose is the primary fuel for oxidative metabolism in the brain, the entry of glucose into neural cells is a potential control point for any regulatory events in brain metabolism. Therefore, the effects of depolarizing stimuli, high K+ and N-methyl-d-aspartate (NMDA), were examined on the functional expression of glucose transporter isoforms GLUT1 and GLUT3 in primary cultured cerebellar granule neurons. Higher levels of glucose transport activity were observed in neurons cultured in 25 mMKCl (K25) compared to those in 5 and 15 mM KCl (K5 and K15). The elevated glucose transport activity correlated with increased levels of GLUT3 protein and, to a lesser extent, GLUT1. Both GLUT3 and GLUT 1 were regulated at the level of mRNA expression. Addition of NMDA to K5 and K15 cultures increased both glucose uptake and GLUT3 protein levels, with smaller changes in GLUT1. NMDA effects were not additive with K25 effects. All these changes were observed only with chronic exposure of neurons to high K+ or NMDA; no acute effects on glucose uptake or transporter expression were found. Thus, chronic depolarization of primary cerebellar granule neurons acts as a stimulus for the expression of the neuronal GLUT3 glucose transporter isoform.

Gene expression of GLUT3 glucose transporter regulated by glucose in vivo in mouse brain and in vitro in neuronal cell cultures from rat embryos.

This study was designed to determine whether glucose regulates the gene expression of glucose transporter GLUT3 in neurons. We examined the regulation of GLUT3 mRNA by glucose in vivo in mouse brain and in vitro by using neuronal cultures from rat embryos. Hypoglycaemia (< 30 mg/dl), produced by 72 h of starvation, increased GLUT3 mRNA in mouse brain by 2-fold. Hybridization studies in situ demonstrated that hypoglycaemia-induced increases in GLUT3 mRNA expression were observed selectively in brain regions including the hippocampus, dentate gyrus, cerebral cortex and piriform cortex, but not the cerebellum. Primary neuronal cultures from rat embryos deprived of glucose for 48 h also showed an increase (4-fold over control) in GLUT3 mRNA, indicating that glucose can directly regulate expression of GLUT3 mRNA. In contrast with hypoglycaemia, hyperglycaemia produced by streptozotocin did not alter the expression of GLUT3 mRNA. We also confirmed previous findings that hypoglycaemia increases GLUT1 mRNA expression in brain. The increase in GLUT1 expression was probably limited to the blood-brain barrier in vivo, since GLUT1 mRNA could not be detected in neurons of the mouse cerebrum. Thus we conclude that up-regulation of neuronal GLUT3 in response to glucose starvation represents a protective mechanism against energy depletion in neurons.