The uptake of glucose into most mammalian cells is mediated by a family of six facilitative glucose transporters (GLUTs). These proteins share a 60-70% homology but are kinetically distinct and are expressed in a tissue-specific manner. All six transporter isoforms have now been detected in mammalian brain. Of these, the two major isoforms are GLUT1, which is responsible the transport of glucose across the blood-brain-barrier (BBB) and into glia and GLUT3, which is the primary neuronal transporter. We have recently demonstrated the presence of GLUT4, the transporter normally confined to insulin- and contraction-sensitive tissues, i.e. muscle and adipose, in rodent brain. Using in situ hybridization and immunohistochemistry with laser confocal microscopy, we have found GLUT4 mRNA and protein to be primarily expressed in granule neurons in three brain regions: cerebellum, dentate gyrus and the olfactory bulb and this expression is enhanced in the cerebellum of genetically diabetic (db/db) mice. In addition, we have observed that the brains of mature (10 week old) db/db mice are 25% smaller and have significantly reduced rates of global cerebral glucose utilization, as compared with non-diabetic litter mates. GLUT3 mRNA expression is significantly reduced in the cortex which GLUT1 mRNA is significantly increased in both thalamus and hypothalamus. GLUT1 protein was not altered in total isolated cerebral microvessels, suggesting the reduced metabolism and growth was not a consequence of reduced glucose delivery to the brain in diabetes. In preliminary studies we demonstrated the presence of GLUT5, a putative fructose transporter, in microglia in both rat and human brain, as well as in human macrophages and neutrophils. More recently we have monitored the temporal changes in GLUT5 mRNA and protein expression in two animal models of stroke: middle cerebral artery occlusion, MCAO, in the spontaneously hypertensive, SH, rat and neonatal unilateral cerebral hypoxia-ischemia ( right common carotid artery ligation, 8% oxygen for 2.5 hours), as well as in human brain as a function of normal aging and the onset of Alzheimer's Disease (AD). In all situations GLUT5 provides an excellent marker for microglial migration and activation in both frozen and fixed tissue. In both stroke models there is an initial decrease in the GLUT5 mRNA in the hemisphere ipsilateral to the occlusion (ILH) 3-5 hours following the insult. In the SH- MCAO rat the number of microglia and the level of GLUT5 expression are elevated throughout the ILH at 48 hours whereas by 5 days the maximal expression is observed at the borders of the necrotic infarct, where it remains elevated for up to 30 days. In the neonatal model, which also produces the most damage in the are of middle cerebral artery distribution, enhanced GLUT5 mRNA expression around the infarct is apparent at 24 hr and at 3 days is intense in the necrotic hippocampus, the thalamus and the cortical infarct. The more rapid GLUT5 response is consistent with accelerated necrosis in the immature brain as well as with the known microglial response in this model. The young adult human brain is characterized by fewer microglia and less total GLUT5 protein levels as compared with either elderly control or AD brains. The significant observation in the latter brains was an elevation of GLUT5 in microglia surrounding neuritic plaques in the AD brains. As part of an ongoing multi-center clinical study, a protocol has been approved for EDMNS to screen blood from children with recurrent seizures who attend Child Neurology Clinics at the medical centers of Johns Hopkins, Yale, and Hershey for GLUT1 and GLUT3 deficiencies.
