Developmental modulation of blood-brain barrier and choroid plexus GLUT1 glucose transporter messenger ribonucleic acid and immunoreactive protein in rabbits

Abstract

The transport of glucose across the brain capillary endothelium, which makes up the blood-brain barrier (BBB) in vivo, is developmentally up-regulated in the postnatal period, as the brain switches from combustion of circulating ketone bodies to glucose. The principle transporter mediating the uptake of circulating glucose across the BBB is the GLUT1 isoform. To further define molecular mechanisms underlying developmental modulation of the BBB GLUT1 transporter, the amounts of brain microvessel GLUT1 mRNA and immunoreactive protein were quantitated. In addition, an immunocytochemical analysis of GLUT1 expression at the choroid plexus in developing brain was performed, since this transporter isoform is selectively expressed at the choroid plexus epithelium basolateral membrane. Quantitative Western blotting employing purified human erythrocyte glucose transporter as an assay standard showed that the concentration of immunoreactive GLUT1 protein in 70-day-old rabbit brain microvessels (111 +/- 3 pmol/mg protein) was not significantly different from the concentration of D-glucose-displaceable cytochalasin-B-binding sites (102 +/- 25 pmol/mg protein). Thus, GLUT1 is the principle isoform mediating glucose transport across the developing BBB. Quantitative Western blotting was performed on microvessels isolated from brains of rabbits on postnatal days 1, 14, 28, and 70. The concentrations of immunoreactive microvessel GLUT1 at these four stages of development were 13 +/- 2, 4 +/- 1, 49 +/- 2, and 111 +/- 3 pmol/mg protein, respectively. Capillary depletion analysis showed that essentially all of brain GLUT1 mRNA arises from the microvascular fraction, and Northern analysis of 10 micrograms poly(A)+ RNA from brains of rabbits 1, 14, 28, and 70 days postnatally showed a preferential stabilization of the GLUT1 mRNA compared to mRNA for two cytoskeletal proteins, actin and tubulin. Immunocytochemical analysis of immunoreactive GLUT1 in choroid plexus epithelia showed the following developmental modulation of the transporter protein: 1 day < 14 days < 28 days > 70 days. The concentration of immunoreactive GLUT1 at the basolateral membrane of choroid plexus epithelium at 28 days was much greater than the immunostaining of rabbit brain microvessels at the corresponding age. In conclusion, these studies show that immunoreactive GLUT1 protein initially undergoes down-regulation between birth and 14 days and then undergoes marked up-regulation between 14 and 70 days. Conversely, the concentration of GLUT1 mRNA is virtually unchanged in brain of rabbits at 70 vs. 14 days postnatally. These combined data suggest that a principle mechanism underlying the developmental regulation of GLUT1 at the BBB may be posttranscriptional.