Brain glucose transporter (Glut3) haploinsufficiency does not impair mouse brain glucose uptake

Abstract

Mouse brain expresses three principal glucose transporters. Glut1 is an endothelial marker and is the principal glucose transporter of the blood-brain barrier. Glut3 and Glut6 are expressed in glial cells and neural cells. A mouse line with a null allele for Glut3 has been developed. The Glut3(-/-) genotype is intrauterine lethal by 7days post-coitis, but the heterozygous (Glut3(+/-)) littermate survives, exhibiting rapid post-natal weight gain, but no seizures or other behavioral aberrations. At 12weeks of age, brain uptake of tail vein-injected ((3))H-2-deoxy glucose in Glut3(+/-) mice was not different from Glut3(+/+) littermates, despite 50% less Glut3 protein expression in the brain. The brain uptake of injected ((18))F-2-fluoro-2-deoxy glucose was similarly not different from Glut3(+/-) littermates in the total amount, time course, or brain imaging in the Glut3(+/-) mice. Glut1 and Glut6 protein expressions evaluated by immunoblots were not affected by the diminished Glut3 expression in the Glut3(+/-) mice. We conclude that a 50% decrease in Glut3 is not limiting for the uptake of glucose into the mouse brain, since Glut3 haploinsufficiency does not impair brain glucose uptake or utilization.

Copyright © 2011 Elsevier B.V. All rights reserved.

Figures

Figure 1. Expression of Glut1, Glut3, and Glut6 in brain homogenate from Glut3+/− mice

Shown here are representative immunoblots of brain homogenates subjected to PAGE and probing of resulting membranes with antibodies against mGlut3, hGLUT1, and hGLUT6. Digital image analysis quantified the mean expression of Glut3 at 52% of the Glut3+/+ littermates. The expressions of Glut1 and Glut6 in Glut3+/− brain were quantified at 100% and 100%, respectively.

Figure 2. Tolerance to prolonged fasting in mice with Glut3+/− genotype

Mice at 10-12 weeks of age were divided into four groups of four animals, male Glut3+/−, female Glut3+/−, male Glut3+/+, and female Glut3+/+. After ad lib feeding overnight, food was removed and glucose was determined in blood from a cheek puncture at times 0, 6 hr, 12 hr, 24 hr, and 48 hrs. Each mouse had single blood glucose determinations at five different time separated by one week before the next sequentially increased fasting time period. Ambient temperature was maintained at 29°C to minimize torpor that can occur with food restriction. At no time was there evidence of seizure activity or other change in behavior among the mice during the 48 hours of frequent observation. No significant difference in glucose concentrations was found comparing the groups at any of the durations of fasting.

Figure 3. Uptake of glucose into brains of mice with Glut3+/− genotype

Ten mice (5 males, 5 females) were injected with 1 μCi of 3H-2-deoxy glucose via tail vein and 45 minutes later were sacrificed by CO2 inhalation and brain removal. Eleven littermate controls (5 males, 6 females) were similarly evaluated. All animals were allowed free access to food overnight, but food was removed two hours prior to sacrifice. Each brain was removed, rinsed in saline, weighed, and dissolved in Scintigest (Fisher Scientific) at 50°C for at least 18 hours. Brain uptake of glucose was quantified as cpm 3H per brain. Male brains were slightly larger, but there was no difference between the Glut3+/− and Glut3+/+ groups, whether the data were expressed as cpm/brain or cpm/mg brain.

Figure 4. Representative PET/CT images from a Glut3+/− and Glut3+/+ mouse

Panel A shows examples of PET and PET/CT composite images for a heterozygous mouse (lower images) and a normal littermate. Panel B displays graphs of the time course of brain uptake and blood disappearance of 18F-FDG for a Glut3+/+ mouse and a Glut3+/− mouse. Graphs 1 and 3 also include points generated using the two-compartment model generated by the kinetic analysis. The kinetic data were indistinguishable between mice with the two Glut3 genotypes.