Quantitative immunocytochemical study of blood-brain barrier glucose transporter (GLUT-1) in four regions of mouse brain.

Application of immunogold cytochemistry revealed polar (asymmetric) distribution of GLUT-1 in mouse brain microvascular endothelia, representing the anatomic site of the blood-brain barrier (BBB). This polarity was manifested by an approximately threefold higher immunolabeling density of the abluminal than the luminal plasma membrane of the endothelial cells. The immunoreaction for GLUT-1 in nonbarrier continuous (skeletal muscle) or fenestrated (brain circumventricular organs) microvascular endothelial cells was absent. In the choroid plexus, the basolateral plasmalemma of the epithelial cells was labeled more intensely than the vascular fenestrated endothelium. Addition of morphometry to the applied immunogold technique makes it possible for even subtle differences to be revealed in the density of immunolabeling for GLUT-1 in blood microvessels located in four brain regions. We found that the density of immunosignals in the microvessels supplying the cerebral cortex, hippocampus, and cerebellum was essentially similar, whereas in the olfactory bulb it was significantly lower. Asymmetric distribution of GLUT-1 in the endothelial plasma membranes presumably leads to a reduced concentration of glucose molecules in the endothelial cells compared to blood plasma and also secures their more rapid transport across the abluminal plasmalemma to the brain parenchyma.     

Subcellular distribution of glucose transporter (GLUT-1) during development of the blood-brain barrier in rats

Electron microscopy was used to quantify the subcellular distribution of the GLUT-1 isoform of the glucose transporter in developing microvessels of the brain of embryonic rats from E (embryonic stage) 13 to E19 and in adult rats. Gold-conjugated secondary antibodies were used to localize, on ultrathin sections of brain, a rabbit polyclonal antiserum (anti-GLUT-1) raised against a synthetic peptide encoding 13 amino acids of the C-terminus of the human glucose transporter. Staining was weak at E13 but increased in density during development into adulthood. The increase represented an increase in the absolute amount of transporter per vessel profile, with a concomitant decrease in vessel size with the narrowing of the wall. At early stages, the percentages of total particles per profile of lumenal membrane, ablumenal membrane, and cytoplasm were approximately equivalent. The ratio of lumenal to ablumenal particle density then shifted from below 1 at E13 to above 2 at E19 and to 4 in the adult. In contrast, vessels of the choroid plexus were devoid of labeling, but the choroid plexus epithelium stained as early as E15. In the brain, no astrocytes, neurons, or pericytes were stained at any stage examined. Developmental upregulation of the GLUT-1 glucose transporter therefore seems to occur at the blood-brain barrier, and the modulation of the subcellular distribution of the transporter can be correlated with other observed changes in the microvessels as they develop the blood-brain barrier phenotype. Received: 18 November 1995 / Accepted: 12 January 1996     

Immunogold study of regional differences in the distribution of glucose transporter (GLUT-1) in mouse brain associated with physiological and accelerated aging and scrapie infection

Distribution of glucose transporter (GLUT-1) in brain microvascular endothelia, representing the anatomic site of the blood-brain barrier (BBB), was studied in adult, physiologically aged, senescence-accelerated prone (SAMP8) and in scrapie-infected mice. Sections of tissue samples obtained from four brain regions (cerebral cortex, hippocampus, cerebellum, and olfactory bulb) and embedded in Lowicryl K4M were exposed to anti-GLUT-1 antiserum followed by gold-labeled secondary antibody. Labelling density was recorded over luminal and abluminal plasma membranes of the microvascular endothelial cells. We found that the density of immunosignals for GLUT-1 in the cerebral cortex showed a tendency toward insignificant diminution according to the following gradation-adult > SAMP8 > scrapie > aged mice-whereas in the hippocampus, this gradation was slightly different: adult > aged > scrapie > SAMP8 mice. In the cerebellum, immunolabelling was insignificantly diminished in aged mice, whereas it was significantly decreased in scrapie-infected and SAMP8 mice. The intensity of labelling of the vascular endothelium in the olfactory bulb was significantly lower than that in other brain regions, showing a slight decrease in the following sequence: adult > aged > scrapie > SAMP8 mice. These findings suggest that the process of aging as well as of related neurodegenerative disease affects unequally the distribution of GLUT-1 in the vasculature of different brain regions.     

Expression of Class III facilitative glucose transporter genes (GLUT-10 and GLUT-12) in mouse and human adipose tissues

We have examined whether GLUT-10 and GLUT-12, members of the Class III group of the recently expanded family of facilitative glucose transporters, are expressed in adipose tissues. The mouse GLUT-12 gene, located on chromosome 10, comprises at least five exons and encodes a 622 amino acid protein exhibiting 83% sequence identity and 91% sequence similarity to human GLUT-12. Expression of the GLUT-12 gene was evident in all the major mouse adipose tissue depots (epididymal, perirenal, mesenteric, omental, and subcutaneous white; interscapular brown). The GLUT-10 gene is also expressed in mouse adipose tissues and as with GLUT-12 expression occurred in the mature adipocytes as well as the stromal vascular cells. 3T3-L1 adipocytes express GLUT-10, but not GLUT-12, and expression of GLUT-12 was not induced by insulin or glucose. Both GLUT-10 and GLUT-12 expression was also found in human adipose tissue (subcutaneous and omental) and SGBS adipocytes. It is concluded that white fat expresses a wide range of facilitative glucose transporters.     

Expression of glucose transporter isoform GLUT-2 and glucokinase genes in human brain

The glucose transporter isoform-2 (GLUT-2) and glucokinase are considered to be components of a glucose sensor system controlling several key processes, and hence may modulate feeding behaviour. We have found GLUT-2 and glucokinase mRNAs in several brain regions, including the ventromedial and arcuate nuclei of the hypothalamus. GLUT-2, glucokinase and glucokinase regulatory protein mRNAs and proteins were present in these areas as determined by biochemical approaches. In addition, glucose-phosphorylating activity with a high apparent Km for glucose that displayed no product inhibition by glucose-6-phosphate was observed. Increased glycaemia after meals may be recognized by specific hypothalamic neurones due to the high Km of GLUT-2 and glucokinase. This enzyme is considered to be the true glucose sensor because it catalyses the rate-limiting step of glucose catabolism its activity being regulated by interaction with glucokinase regulatory protein, that functions as a metabolic sensor.     

Cellular localization of facilitated glucose transporter 1 (GLUT-1) in the cochlear stria vascularis: its possible contribution to the transcellular glucose pathway

Immunoreactivity for the facilitated glucose transporter 1 (GLUT-1) has been found in the cochlear stria vascularis, but whether the strial marginal cells are immunopositive for GLUT-1 remains uncertain. To determine the cellular localization of GLUT-1 and to clarify the glucose pathway in the stria vascularis of rats and guinea pigs, immunohistochemistry was performed on sections, dissociated cells, and whole-tissue preparations. Immunoreactivity for GLUT-1 in sections was observed in the basal side of the strial tissue and in capillaries in both rats and guinea pigs. However, the distribution of the positive signals within the guinea pig strial tissue was more diffuse than that in rats. Immunostaining of dissociated guinea pig strial cells revealed GLUT-1 in the basal cells and capillary endothelial cells, but not in the marginal cells. These results indicated that GLUT-1 was not expressed in the marginal cells, and that another isoform of GLUT was probably expressed in these cells. Three-dimensional observation of whole-tissue preparations demonstrated that cytoplasmic prolongations from basal cells extended upward to the apical surface of the stria vascularis from rats and guinea pigs, and that the marginal cells were surrounded by these protrusions. We speculate that these upward extensions of basal cells have been interpreted as basal infoldings of marginal cells in previous reports from other groups. The three-dimensional relationship between marginal cells and basal cells might contribute to the transcellular glucose pathway from perilymph to intrastrial space. This study was supported by a grant-in-aid for scientific research (19570058) from The Ministry of Education, Culture, Sports, Science, and Technology of Japan.     

Regional distribution of glucose in mouse brain

After rapid inactivation of the enzymes responsible for glucose metabolism by microwave irradiation, concentrations of glucose in 20 regions of the mouse brain were estimated with combined gas chromatography-mass spectrometry (GC-MS). The highest concentrations of glucose were found in the periventricular nuclei of the hypothalamus and nucleus preopticus (P<0.05). The septum and nucleus amygdaloideus showed significantly higher glucose concentration compared with the cerebral neocortex, olfactory bulb, corpus striatum, cingulum, fornix, colliculus inferior, cerebellar cortex, corpus geniculatum laterale, substantia nigra, and nucleus ruber (P<0.05). The glucose concentration in the substantia nigra and nucleus ruber was significantly lower than in the other regions (P<0.01).     

Resistin modulates glucose uptake and glucose transporter-1 (GLUT-1) expression in trophoblast cells

The adipocytokine resistin impairs glucose tolerance and insulin sensitivity. Here, we examine the effect of resistin on glucose uptake in human trophoblast cells and we demonstrate that transplacental glucose transport is mediated by glucose transporter (GLUT)-1. Furthermore, we evaluate the type of signal transduction induced by resistin in GLUT-1 regulation. BeWo choriocarcinoma cells and primary cytotrophoblast cells were cultured with increasing resistin concentrations for 24 hrs. The main outcome measures include glucose transport assay using [³H]-2-deoxy glucose, GLUT-1 protein expression by Western blot analysis and GLUT-1 mRNA detection by quantitative real-time RT-PCR. Quantitative determination of phospho(p)-ERK1/2 in cell lysates was performed by an Enzyme Immunometric Assay and Western blot analysis. Our data demonstrate a direct effect of resistin on normal cytotrophoblastic and on BeWo cells: resistin modulates glucose uptake, GLUT-1 messenger ribonucleic acid (mRNA) and protein expression in placental cells. We suggest that ERK1/2 phosphorylation is involved in the GLUT-1 regulation induced by resistin. In conclusion, resistin causes activation of both the ERK1 and 2 pathway in trophoblast cells. ERK1 and 2 activation stimulated GLUT-1 synthesis and resulted in increase of placental glucose uptake. High resistin levels (50–100 ng/ml) seem able to affect glucose-uptake, presumably by decreasing the cell surface glucose transporter.     

Expression and function of GLUT-1 and GLUT-2 glucose transporter isoforms in cells of cultured rat pancreatic islets.

We have previously investigated glucose induction of glucokinase, glucose usage and insulin release in isolated cultured rat pancreatic islets (Liang, Y., Najafit, H., Smith, R. M., Zimmerman, E. C., Magnuson, M. A., Tal, M., and Mastchinsky, F. M. (1992) Diabetes (1992) 41, 792-806). Here we studied the expression and function of GLUT-1 and GLUT-2 glucose transporter isoforms, using the same system, i.e. isolated pancreatic rat islets immediately after isolation or cultured in the presence of 3 or 30 mM glucose for as long as 10 days. We found by immunofluorescence microscopy and Western and Northern blot analysis of islet extracts that GLUT-1 expression was induced in islet beta-cells in tissue culture both with low or high glucose present. The induction of GLUT-1 was specific to beta-cells but was not present in all beta-cells and was not detected in alpha-cells. GLUT-2 expression was also specific for beta-cells and was not observed in all beta-cells. Some beta-cells in culture coexpressed GLUT-1 and GLUT-2. The expression of the two glucose transporters was regulated in the opposite direction in response to glucose concentration in the culture medium. GLUT-1 was more effectively induced when glucose was low, and GLUT-2 expression was more pronounced when glucose was high in the culture media. Another difference between the two glucose transporters was that GLUT-2 expression was increased while GLUT-1 expression was decreased as culturing continued as long as 7 days. Thus, after 7 days of culture GLUT-2 expression in beta-cells was nearly the same at low and high glucose, whereas GLUT-1 was practically absent no matter what the glucose level was. In attempts to correlate GLUT-1 and GLUT-2 expression to beta-cell function glucose uptake and glucose-stimulated insulin release in fresh and cultured islets were measured. In freshly isolated islet glucose uptake was estimated to be 100-fold in excess of actual glucose use. Glucose uptake was reduced by 7-day culture to about one-third of that observed in freshly isolated islets no matter what the glucose concentration of the culture media. We conclude that in the present experimental system GLUT-1 and GLUT-2 expression and function are not closely associated with glucose usage rates or the secretory function of beta-cells.     

Brain-type glucose transporter (GLUT-1) is selectively localized to the blood-brain barrier. Studies with quantitative western blotting and in situ hybridization

The hypothesis that the GLUT-1 glucose transporter isoform is expressed selectively in brain at the capillary endothelium, i.e. the blood-brain barrier (BBB), was tested by using quantitative Western blotting, cytochalasin B binding, and in situ hybridization in bovine brain cortex. Purified human red cell glucose transporter was used as the standard for quantitative Western blots, because the mobility of the human erythrocyte and BBB glucose transporters in electrophoretic gels was identical. The concentration of immunoreactive glucose transporter in bovine BBB plasma membranes was 10.8 +/- 0.9 pmol/mgp (mean +/- S.E., n = 6). This value was not statistically different from the estimate of the maximal binding sites of D-glucose-displaceable [3H]cytochalasin B binding in the BBB membrane preparations, 11.7 +/- 3.5 pmol/mgp. In situ hybridization experiments using 35S-labeled antisense and sense riboprobes corresponding to nucleotides 385-932 of the GLUT-1 cDNA showed prominent hybridization of the antisense probe over brain microvascular endothelium, but no hybridization over neuropil greater than that found with the 35S-labeled sense probe. These studies are consistent with the following conclusion: (a) essentially 100% of the glucose transporter binding sites at the BBB can be accounted for by the GLUT-1 isoform; (b) in situ hybridization studies confirm previous Northern blot analysis and indicate the GLUT-1 gene is expressed selectively in microvascular endothelium in brain with minimal, if any, expression of this gene in neurons or glial cells in vivo.     

Glycolytic enzymes and a GLUT-1 glucose transporter in the outer segments of rod and cone photoreceptor cells

The presence of glycolytic enzymes and a GLUT-1-type glucose transporter in rod and cone outer segments was determined by enzyme activity assays, glucose uptake measurements, Western blotting, and immunofluorescence microscopy. Enzyme activities of six glycolytic enzymes including hexokinase, phosphofructokinase, aldolase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, pyruvate kinase, and lactate dehydrogenase, were found to be present in purified rod outer segment (ROS) preparations. Immunofluorescence microscopy of bovine and chicken retina sections labeled with monoclonal antibodies against glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, and lactate dehydrogenase have confirmed that these enzymes are present in rod and cone outer segments and not simply contaminants from the inner segments or other cells. Rod outer segments were also found to contain glucose transport activity as detected by 3-O-[14C]methylglucose uptake and exchange. The glucose transporter had a Km of 6.3 mM and a Vmax of 0.15 nmol of 3-O-methylglucose/s/mg of ROS membrane protein for net uptake and a Km of 29 mM and a Vmax of 1.06 nmol of 3-O-methylglucose/s/mg of ROS membrane protein for equilibrium exchange. These Km values for net uptake and equilibrium exchange are similar to values obtained for human red blood cells and are characteristic of GLUT-1-type glucose transporter. The transport was inhibited by both cytochalasin B and phloretin. Western blot analysis and immunofluorescence microscopy using type-specific glucose transporter antibodies indicated that both rod and cone outer segment plasma membranes have a GLUT-1 glucose transporter of Mr 45K as found in red blood cells and brain microsomal membranes. Solid-phase radioimmune competitive inhibition studies indicated that rod outer segment plasma membranes contained 15% the number of glucose transporters found in human red blood cell membranes and had an estimated density of 400 glucose transporter per micron2 of plasma membrane. These studies support the view that outer segments can generate energy in the form of ATP and GTP by anaerobic glycolysis to supply at least some of the energy requirements for phototransduction and other metabolic processes.     

Exercise training increases glucose transporter protein GLUT-4 in skeletal muscle of obese Zucker (fa/fa) rats

The present study examined the level of GLUT-4 glucose transporter protein in gastrocnemius muscles of 36 week old genetically obese Zucker (fa/fa) rats and their lean (Fa/-) littermates, and in obese Zucker rats following 18 or 30 weeks of treadmill exercise training. Despite skeletal muscle insulin resistance, the level of GLUT-4 glucose transporter protein was similar in lean and obese Zucker rats. In contrast, exercise training increased GLUT-4 protein levels by 1.7 and 2.3 fold above sedentary obese rats. These findings suggest endurance training stimulates expression of skeletal muscle GLUT-4 protein which may be responsible for the previously observed increase in insulin sensitivity with training.     

Cryopreservation of mouse embryos affects later embryonic development possibly through reduced expression of the glucose transporter GLUT1

In order to study the effects of cryopreservation on later embryonic development, two-cell mouse embryos were frozen, thawed, and then allowed to develop into blastocysts. The percentage of cryopreserved embryos which developed into blastocysts was significantly lower than that of fresh two-cell embryos. The amount of glucose incorporation in terms of ³H-2-deoxyglucose uptake in blastocysts developed in vivo, and in vitro from fresh or frozen-thawed two-cell embryos, was 473 ± 108, 105 ± 75, and 43.0 ± 28.3 fmol per embryo per hour, respectively. Quantification of glucose transporter GLUT1 in these embryos by Western blotting was reflective of the degree of glucose incorporation. The implantation rate of blastocysts developed in vitro from frozen-thawed two-cell embryos (22.0%) was significantly lower than that developed in vivo (41.1%). These data suggest that cryopreservation may have later consequences on embryonic development through a mechanism that involves altered GLUT1 expression. Mol. Reprod. Dev. 48:496–500, 1997.© 1997 Wiley-Liss, Inc.     

Decreased glucose transporter 1 gene expression and glucose uptake in fetal brain exposed to ethanol.

Using pregnant rats fed equicaloric liquid diets (AF, and libitum-fed controls; PF, pair-fed controls; EF, ethanol-fed), we have previously shown that maternal alcoholism produces a specific and significant decrease of glucose in the fetal brain, which is accompanied by growth retardation. To further define the mechanisms of ethanol-induced perturbations in fetal fuel supply, we have examined (i) the uptake of 2-deoxyglucose (2-DG) by dissociated brain cells from fetal rats that were exposed to ethanol in utero and (ii) the steady-state levels of the glucose transporter-1 (GT-1) mRNA. A 9% decrease in brain weight (P less than 0.001) and a 54.8% reduction in 2-DG uptake into brain cells (P less than 0.02) were found in offspring of EF mothers compared to the AF group. Brain weight correlated with the rate of 2-DG uptake (P less than 0.05). Northern blot analysis showed a 50% reduction of GT-1 mRNA in EF brain relative to that in the AF and PF groups. We conclude that glucose transport into the brain is an important parameter altered by maternal ethanol ingestion.     

Spatial distribution of mDLG6 mRNA in embryonic and adult mouse brain

We have isolated mouse DLG6 (mDLG6) cDNA clones by RT-PCR and then by using the RT-PCR products to screen a mouse brain cDNA library. The deduced amino acid sequence of mDLG6 shows 79.2% and 82.7% overall identity to human (hDLG6) and rat DLG6 (rDLG6), respectively. In situ hybridization revealed that mDLG6 mRNA is predominantly expressed in embryonic and adult brain.