DIFFERENTIAL GLYCOSYLATION OF THE GLUT1 GLUCOSE-TRANSPORTER IN BRAIN CAPILLARIES AND CHOROID-PLEXUS

The sodium-independent GLUT1 glucose transporter is expressed in high density in human erythrocytes and in tissues which serve a barrier fun ction. In the polarized endothelial cells of the brain capillaries, wh ich comprise the blood-brain barrier (BBB), GLUT1 is expressed on both apical and basolateral membranes; however, in the epithelium of the c horoid plexus, GLUT1 expression is restricted to the basolateral surfa ce. The present study examined whether these differences in subcellula r localization of GLUT1 at the BBB and choroid plexus could be correla ted with differential N-linked or O-linked glycosylation of the protei n. Western blot analysis of solubilized brain capillaries (BC) and cho roid plexus (CP) revealed that while the BC GLUT1 had an average molec ular mass identical to that of the purified human erythrocyte transpor ter (54 kDa), the CP GLUT1 was of lower molecular mass (47 kDa). Treat ment of brain capillaries and choroid plexus with N-glycanase resulted in a shift in the mobility of the GLUT1 of both samples to a lower mo lecular mass of 42 kDa; however, in contrast, treatment with O-glycana se produced no change in the mobility patterns of GLUT1, but did resul t in O-linked deglycosylation of another BBB marker, gamma-glutamyl tr anspeptidase. In conclusion, BBB and choroid plexus GLUT1 are subject to differential N-linked glycosylation with the protein having an N-Ii nked carbohydrate side chain of higher molecular mass at the BBB in co mparison to the choroid plexus.