Brain glucose transporters, O-GlcNAcylation and phosphorylation of tau in diabetes and Alzheimer's disease

Abstract

Type 2 diabetes mellitus (T2DM) increases the risk for Alzheimer's disease (AD), but the underlying mechanism is unknown. In this study, we determined the levels of major brain glucose transporters, O-GlcNAcylation and phosphorylation of tau in the postmortem brain tissue from frontal cortices of 7 controls, 11 T2DM subjects, 10 AD subjects and 8 additional subjects who had both T2DM and AD. We found that the neuronal glucose transporter 3 was decreased to a bigger extent in T2DM brain than in AD brain. The O-GlcNAcylation levels of global proteins and of tau were also decreased in T2DM brain as seen in AD brain. Phosphorylation of tau at some of the AD abnormal hyperphosphorylation sites was increased in T2DM brain. These results suggest that T2DM may contribute to the increased risk for AD by impairing brain glucose uptake/metabolism and, consequently, down-regulation of O-GlcNAcylation, which facilitates abnormal hyperphosphorylation of tau.

Figures

Fig. 1

Levels of GLUT1, GLUT2, GLUT3 and HIF-1α in T2DM and control brains. (a) Crude extract of the frontal cerebral cortex from 11 T2DM and 7 control cases was analyzed by Western blots developed with antibodies to GLUT1, GLUT2, GLUT3 or HIF-1α. Actin blot was included for a loading control. (b and c) The blots were quantified densitometrically, and data are presented as percentage of control group (mean ± SD; *, p<0.05).

Fig. 2

Levels of GLUT1, GLUT2 and GLUT3 in the brains of controls and individuals with T2DM, AD and T2DM plus AD (T2DM-AD). The levels of GLUT1, GLUT2 and GLUT3 in the crude extract of the frontal cerebral cortex from 7 controls, 11 T2DM, 10 AD and 8 T2DM-AD cases was analyzed by quantitative Western blots. Data are presented as percentage of control group (mean ± SD; *, p<0.05 vs. controls; #, p<0.05 vs. T2DM group; &, p<0.05 vs. AD group).

Fig. 3

Levels of global protein O-GlcNAcylation and tau O-GlcNAcylation in the brains of individuals with T2DM, AD, T2DM plus AD (T2DM-AD) and controls. (a) The levels of protein O-GlcNAcylation in the crude extract of the frontal cerebral cortex from 7 controls, 11 T2DM, 10 AD and 8 T2DM-AD cases was analyzed by quantitative immuno-dot-blots and Western blots developed with monoclonal antibody RL2 against O-GlcNAc. Data are combined results from the two methods and are presented as percentage of the control group (mean ± SD; *, p<0.05 vs. control group). (b) Tau was first purified from human brains by using an immuno-affinity method with monoclonal tau antibody 43D, and the levels of O-GlcNAcylation of the purified tau was determined by quantitative immuno-dot-blots. Data are presented as percentage of the control group (mean ± SD; *, p<0.05 vs. control group).

Fig. 4

Comparison of site-specific tau phosphorylation among T2DM, AD, T2DM-AD and controls. The levels of tau phosphorylation at individual phosphorylation sites in the crude extract of the frontal cerebral cortex from 7 controls, 11 T2DM, 10 AD and 8 T2DM-AD cases was analyzed by quantitative Western blots developed with antibodies that recognize tau phosphorylated at the individual phosphorylation sites indicated in each panel. The blots were then quantified densitometrically and normalized by the total tau levels, as determined by using antibody R134d. Data are presented as percentage of control group (mean ± SD; *, p<0.05 vs. controls; #, p<0.05 vs. T2DM group; &, p<0.05 vs. AD group). pS, phospho-serine; pT, phospho-threonine.

Fig. 5

A proposed mechanism by which T2DM increases the risk for AD. In T2DM brain, decreased level of GLUT3 leads to a decrease in glucose uptake/metabolism. Decreased intraneuronal glucose metabolism results in decreased level of UDP-GlcNAc, which is a metabolic product of glucose via the hexosamine biosynthetic pathway (HBP). Because protein O-GlcNAcylation is mainly regulated by intracellular level of UDP-GlcNAc, a decrease of the latter then causes decreased tau O-GlcNAcylation. Because the latter regulates tau phosphorylation negatively, decreased O-GlcNAcylation thus facilitates hyperphosphorylation of tau, which, in turn, forms toxic tau oligomers and causes neurofibrillary degeneration and cognitive impairments. All these resultant consequences play active roles in the pathogenesis of AD. Abnormal hyperphosphorylation of tau also leads to the formation of neurofibrillary tangles (NFTs), a hallmark brain pathology of AD. In this figure, G on tau protein represents the O-GlcNAc group, and P represents the phosphate group. Abbreviations used in this figure: ADP, adenosine diphosphate; ATP, adenosine triphosphate; F-6-P, fructose-6-phosphate; Glc, glucose; GlcNAc, β-N-acetylglucosamine; Gln, glutamine; Glu, glutamate; GLUT, glucose transporter; OGA, O-GlcNAcase; OGT, O-GlcNAc transferase; TCA, tricarboxylic acid cycle; UDP, uridine diphosphate.